Electronic Structure and Optical Spectra of Tetrahedral

Russian Journal of Inorganic Chemistry, Vol. 48, No. 4, 2003, pp. 537–555. Translated from Zhurnal Neorganicheskoi Khimii, Vol. 48, No. 4, 2003, pp. 620–638. Original Russian Text Copyright © 2003 by Burkov, Egorysheva, Kargin. English Translation Copyright © 2003 by MAIK “Nauka /Interperiodica” (Russia).

PHYSICAL METHODS OF INVESTIGATION

Electronic Structure and Optical Spectra of Tetrahedral [MO4]n– Complexes of 3d Metals V. I. Burkov*, A. V. Egorysheva**, and Yu. F. Kargin** * Moscow Physicotechnical Institute, Institutskii proezd 9, Dolgoprudnyi, Moscow oblast, 141700 Russia ** Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia Received September 30, 2002

Abstract—The electronic absorption spectra of tetrahedral 3d-metal complexes [MO4]n– have been surveyed, and their typical features as applied to crystals containing [MO4]n– ions with a definite electronic configuration have been considered. The local symmetry of an [MO4]n– ion determines the character of changes in the spectra of existing crystals. Comparison of the energy positions of absorption bands, their splitting and intensity in the spectra of these ions in different crystal matrices makes it possible to determine the oxidation state and the type of coordination polyhedron of a 3d metal ion.

Electronic absorption spectroscopy, along with other methods (IR and Raman spectroscopy, EPR, NMR, etc.), occupies a prominent place among the physical methods used for studying inorganic coordination compounds. The theoretical background for these investigations—ligand crystal-field theory—has been formulated in classical works on atomic and molecular spectroscopy by Bethe, Kramers, Van Vleck, Ballhausen, and others. A great body of experimental spectroscopic information on coordination compounds has been gathered. These data have been generalized in reviews and monographs, in particular, in [1–3]. The spectra of inorganic complexes with different symmetries and electronic structures have been comprehensively described and interpreted. Nevertheless, the spectroscopic database for 3d-metal oxide complexes is far from being complete. In this group, the spectra of octahedrally coordinated ions are the best understood. Information on the tetrahedral [MO4]n– complexes of many 3d metals is absent even in special monographs, because most of them are focused on tetrahedrally coordinated ions in halide crystals. The discovery of stimulated emission at d–d transition wavelengths of tetrahedrally coordinated ions has generated interest among researchers. Thus, many works referring to the spectroscopy of these ions have been published within the past few years. In particular, the feasibility of using these ions as activators for tunable solid-state lasers has been surveyed by Kück [4]. Note that d-metal-doped photorefractive crystals often exhibit much better properties than pristine crystals. However, relevant works mainly refer to tetrahedrally coordinated 3d1 and 3d2 ions. The major objective of this work is to generalize currently available data on the spectroscopic properties

of 3d metal ions in a tetrahedral oxygen environment based on commonly accepted conclusions of ligand field theory. In some instances, information on the electronic transitions in d-metal complexes can be extracted only from the optical spectra of doped crystals. The d-metal atoms isomorphously substituted for atoms of a crystal matrix are located in positions whose symmetry is, as a rule, lower than Td . This makes the assignment of absorption bands observed in the spectra to definite electronic transitions more difficult, on the one hand, but allows one to reveal weak transitions, which are forbidden by the selection rules for the Td symmetry, on the other hand. When gyrotropic crystals are used as a host, additional information on the electronic transitions of doped 3d-metal complexes can be obtained from circular dichroism (CD) spectra [5]. Spectra of Tetrahedral 3d-Metal Complexes According to the ligand field (LF) theory [1], the relative energies of the molecular orbitals (MOs) in tetrahedral [MO4]n– complexes (Fig. 1) increase in the following order: 1e < 3t2 < 1t1 < 2e < 4t2 . The 1e, 3t2 , and 1t1 MOs are localized on an oxygen atom, and the 2e and 4t2 MOs are d orbitals localized on the metal atom. Six ranges of transitions can be observed when electrons are promoted from the 1e, 3t2 , and 1t1 MOs to the 2e and 4t2 orbitals. d0 Configuration. For a tetrahedral d0 complex of 6

symmetry Td , the ground state configuration is 1 t 1 (A1) and the lowest excited state has the configuration 5 1 t 1 2e1 . The following singlet–singlet transitions are

537

538

BURKOV et al. ε, 102 cm–1 Mn 140

Mné–4 3a1

O

100

80

20 5t2 –20 4t2

4p –60

t2σp t1π

2e 3t2 t 1

4s –100 3d –140

a1σp eπ t2π

1e 2a1 2t2

–180

–220

t2σs a1σs

1t2 1a1

–260 Fig. 1. Schematic of molecular orbitals for the [MnO4]– ion [16].

possible: (1) 1t1 2e (1T1)1T2, (2) 1t1 4t2 1 1 1 1 ( A2)( E)( T1) T2, (3) 3t2 2e (1T1)1T2, (4) 3t2 4t2 (1A1)(1E)(1T1)1T2, (5) 1e 2e (1A1)(1A2)(1E), and (6) 1e 4t2 (1T1)1T2; only the transitions to the 1T2 states are electric dipole-allowed ones. Among tetrahedral d 0 complexes, classic ions [MnO4]–, [CrO4]2–, [VO4]3–, and [TiO4]4– are primarily of interest. Spectroscopic information on the first two complexes is extensive [1]. Crystal field-induced CD spectra in the region of charge transfer (CT) transitions have been studied [5]. The spectral properties of [VO4]3–, unlike those of [TiO4]4–, have been studied in sufficient detail [6, 7]. No information on CT absorption bands is available. In these cases, the position of the lowest-energy transition (1t1 2e) on the energy scale is estimated from the plot of the 1t1 2e transition energy versus the optical electronegativity of the central metal ion. This plot is linear, and the energies for [MnO4]–, [CrO4]2–, and [VO4]3– are known (Table 1);

thus, the 1t1 2e transition energy for [TiO4]4– was estimated to be above 50000 cm–1 [1, 8]. Weak and, most likely, magnetic dipole-allowed 1T ) in the 2e or 3t2–2e (1A1 transitions 1t1 1 –1 range 14000–17000 cm are observed in the absorption spectra of LiClO4 · 3H2O/LiMnO4 · 3H2O and Ba(ClO4)2 · 3H2O/Ba(MnO4)2 · 3H2O crystals [9, 10]. d1 Configuration. The 2E ground state is derived 6 6 0 from the 3 t 2 1 t 1 2e14 t 2 configuration. The first excited 6

6

1

configuration 3 t 2 1 t 1 2e04 t 2 generates the 2T2 state. The 6

5

0

next lowest configuration 3 t 2 1 t 1 2e24 t 2 gives the 2(2T1) + 2(2T2) + 4T2 states. These states are arranged on the energy scale in the following order: 4 4T2 < 2T2(a) < 2T (b) ≈ 2T (a) < 2T (b) [11, 12]. The transitions from the 2 1 1 2E state to the 2T and 2T states are electric dipole2 1 allowed transitions. Transitions from the 2E state to the

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539

α

E || b

E || b

E || a

E || a 11

12

13

20

25

30

35 40 ν, 103 cm–1

Fig. 2. Absorption spectrum of [MnO4]2– at T = 4.2 K [13]. 5

6

0

states of other excited configurations—3 t 2 1 t 1 2e24 t 2 6

5

1

(2(2T1) + 2(2T2)), 3 t 2 1 t 1 2e14 t 2 (4(2T1) + 4(2T2)), and 5

6

1

3 t 2 1 t 1 2e14 t 2 (4(2T1) + 4(2T2))—are possible [11, 13]. 2T ) transition is a ligand-field The 2e 4t2 (2E 2 (LF) transition, and the rest are CT transitions [8, 14, 15]. The electronic absorption spectra of [MnO4]2– [11, 13, 16–18], [CrO4]3– [15, 19, 20], and [VO4]4– [21, 22] in different crystal matrices are typical of the spectra of d1 ions. The interpretation of the spectra of [MnO4]2– (Fig. 2) is presented in Table 2. The bands in the region of 34000 cm–1 are difficult to assign to particular transitions, since the components of the one-electron 2e and 4t2 states have similar energies [1]. Inasmuch as the energies of the 2T2(b) and 2T1(a) (t1 2e) transitions are close to each other, the bands corresponding to these transitions are, as a rule, spectrally unresolved (Table 2). A low positional symmetry of [MO4]n– ions in some crystal matrices (Table 2) determines a considerable splitting (∆ν ≥ 7000 cm–1) of the lowest excited 4t2) (e.g., [MnO4]2– in BaSO4 , state 2T2(a) (2e

[CrO4]3– in Ca2VO4Cl, [VO4]4– in Mg2SiO4 or CaGeO4). This fact is the reason why Kosky and McGarvey [18] erroneously assigned the shortest wavelength component of this transition to the first CT 2e) state. A different intertransition to the 2T2 (1t1 pretation of the spectra of [MnO4]2– in BaSO4 has been suggested in [23]: the second and third components of the band in the range 10000–16500 cm–1 (Table 2) are 3T , 3T d–d transitions of the assigned to the 3A2 2 1 5+ Mn ion bound to a tetrahedral array of oxygen atoms, and the band at 34000 cm–1 corresponds to the first CT transition; however, the existence of Mn5+ in the crystal has not been proven experimentally. Table 2 shows that the crystal-field transitions 2T (2e 4t2) for the [MnO4]2–, [CrO4]3–, and 2 [VO4]4– complexes fall within the same spectral region, although the first CT transition energies are significantly different. For [VO4]4– in Mg2SiO4 , the first CT transition is observed at ν > 40000 cm–1, and the weak band at 37500 cm–1 is assigned to the spin-forbidden 2E 4T (1t 2e) transition [22]. 2 1 2E

Table 1. CT transition energies (ν, cm–1) in d 0 tetrahedral complexes [MO4]n– [8] Complex [MnO4]–

1t1

2e

3t1

2e

15100

24700

17700

30300

1t1

4t2

29500

18000 [CrO4]2– [VO4]3– [TiO4]4–

26800

39200

36600

36900

45000

45000

~50000

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3t2

4t2

44400

540

BURKOV et al.

Table 2. Transition energies (cm–1) in the spectra of d1 tetrahedral [MO4]n– complexes (assignment as in [11, 13]) 6 6

1

0

Transition 2E( 3t 2 t 1 2e 4t 2 ) Crystal (2e BaSO4 : [MnO4]2–

2T 2

4t2) 2

10500–13500 A'1 (2T2)

(t1

2T 2

2e) (t1

2T 2

2e) (t1

2T 1

2T 1

2e) (t1

2e)

(3t2 (t1

2T 2 2T 2

Positional sym- Reference 2e) metry 4t2)

18400

24700

–

28300

34700

Cs

[13, 17, 18]

17000

22900

26600

30200

34600

C2v

[11, 12, 13]

28000

–

–

–

–

C2

[12, 15]

27000

35000

–

–

–

Td or D2d

[15]

–

–

–

–

Cs

[22]

–

–

–

–

Cs

[22]

6

2

2

10000–15500 A''1 (2T2) 2

13500–16500 A'1 (2T) K2SO4 : [MnO4]2–

10 800–13500

Ca2VO4Cl : [CrO4]3–

10000 2E (2T2)

Li3PO4 : [CrO4]3–

11 500

Mg2SiO4 : [VO4]4–

10000 A'1 (2T2)

17100

2B (2T ) 2 2

2

2

12100 A''1 (2T2) 2

14000 A'1 (2T2) CaGeO4 : [VO4]4–

2

8800 A'1 (2T2)

>40000 (37500 2E 4T ) 2

–

2

10600 A''1 (2T2) 2

12200 A'1 (2T2)

Note that the contours of all the absorption bands in the spectra of d1 ions show vibrational progressions originating from vibronic coupling with excitation of local vibrations of [MO4]n– tetrahedra. The luminescence of crystals containing [MnO4]2– and [CrO4]3– ions have been studied in [17, 23, 24]. The luminescence spectra of these ions show one or several (for a low positional symmetry of [MO4]n–) overlapping broad bands in the region of LF transitions. The luminescence of [VO4]4– has not been detected [22]. d2 Configuration. The electronic configuration d2 is inherent in the [FeO4]2–, [MnO4]3–, [CrO4]4–, and [VO4]5– ions. Taking into account the above interactions [11], the ground configuration of these tetrahedral 6

complexes (…1 t 1 2e2) gives the 3A2, 1E, and 1A1 states, 3A being the lowest lying. Excited configurations give 2 6

1

a number of triplet and singlet states: …1 t 1 2e14 t 2

(3T2, 3T1, 1T2, 1T1) and …1 t 1 2e04 t 2 (3T1, 1T2, 1E1, 1A1). The Td symmetry implies the 3A2(2e2)

3T (2e14 t 1 ) 1 2

2

and 3T1(2e04 t 2 ) dipole moment-allowed transitions, the probability of the second two-electron transition being low [1]. The magnetic dipole-allowed transition to the 3T (2e14 t 1 ) states manifests itself in the CD spectra of 2 2 these ions in optically active crystal matrices. Transitions to the 1E(2e2) and 1A1(2e2) states are spin-forbidden. Among the CT transitions, the 1t1 2e transition is the lowest lying [25, 26]. An interpretation of the spectra of d 2 ions based on the Tanabe–Sugano diagram is presented in Table 3. The energies of the first excited states can differ depending on the crystal field strength. [MnO4]3–. A representative absorption spectrum of [MnO4]3– (in Li3PO4:Mn5+) is shown in Fig. 3. The lowering of symmetry from Td to Cs and C2 , caused by the distortion of [MnO4]3– tetrahedra in the lattices of

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ELECTRONIC STRUCTURE AND OPTICAL SPECTRA

Sr5(PO4)3Cl and Ca2PO4Cl, respectively, results in the splitting of electronic states and in the removal of symmetry restrictions (Table 3). Two sharp bands corre1E(2e2) transition between spond to the 3A2(2e2) the states of the ground configuration; these bands are the beginnings of vibrational progressions with the frequencies corresponding to stretching vibrations of [MnO4]3–. As was shown in [29], the splitting of the 1E state increases and the buildup of the vibronic bands in the spectra of LiPO4 , apatite, and spodiosite correlates with the increase in the angular distortion of the [MnO4]3– tetrahedron in the series of these compounds. Incomplete experimental data in [39, 40] have led to misinterpretation of the absorption spectra of [MnO4]3–.

α, cm–1 140

The luminescence of [MnO4]3– has been comprehensively studied [27, 29, 41–43]. The luminescence spectrum of this ion shows a set of sharp vibronic bands forming a vibrational progression, which begins in the range 8300–9000 cm–1 and corresponds to the spin-for3A transition. bidden 1E 2 The assignment of the CT transition bands in the spectra of [MnO4]3– was reported in [11] (Table 4). Note that the transition energies listed in the first column correspond to the positions of the maxima of the Gauss components obtained by decomposition of the spectrum in [32, 44]. [FeO4]2–. Different variants of the interpretation of the absorption spectra of [FeO4]2– have been suggested [11, 14, 30–32, 44–46] (Fig. 4). Weak bands observed in the IR region of the spectra of K2SO4:[FeO4]2– and K2CrO4:[FeO4]2– are attributed to the transitions from the 3A2 state into the 1E(2e2)- and 1A1(2e2) states split in the field of symmetry Cs (Table 3). The band with a maximum at 13300 cm–1 is assigned to the transition into the 3T2 state; this state is slightly split (~150 cm–1), which points to an insignificant Cs distortion of the [FeO4]2– tetrahedron. The large intensity of this band (the transition is a magnetic dipole-allowed one) is rationalized by mixing of the 3T2 and 3T1 states due to spin–orbit coupling [31]. The next two overlapping broad bands with maxima at 18200 and 20000 cm–1 are assigned to the transitions to the 3T1 components of the state that originates from strong mixing of the 3T (2e14 t 1 ) and 3T (2e04 t 2 ) states [31], which accounts 1 1 2 2 for the virtually equal intensities of these bands. In [30], the band at 18200 cm–1 is assigned to the 3A2

1E

transition, and the band with ν > 19000 cm–1 is assigned to a CT transition, which accounts for the complicated structure of the contour of this band. In addition to routine absorption spectra of K2SO4:[FeO4]2–, the same authors [30] studied the spectrum of this sample while continuously pumping the excited 1E(2e2) state. This made it possible to deter3T (2e14 t 1 ) 1 2

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

541

LMCT 120 3T (F) 1

100 80 1A 1

60 40 3T 2

20 0

3T (P) 1

15000

10000

20000

25000

30000 ν, cm–1

Fig. 3. Absorption spectra of Li3PO4:[MnO4]3– crystals at T = 300 K (the ligand–metal charge transfer (LMCT) band is shown).

ε, l mol–1 cm–1 1500 (b) E1

(a) LMCT 1

E2

1000

1

LMCT 2

νs

νs

E

6220 6230 ν, cm–1 3

500 1

A1

3

T1

E || b E⊥b

T2

0 10000

20000

30000 ν, cm–1

Fig. 4. (a) Absorption spectra of K2SO4:[FeO4]2– at T = 15 K and (b) luminescence spectrum at T = 30 K [46].

mine the energy position of the upper singlet states. According to these data, the forbidden transitions to the 1T (2e14 t 1 ) and 1T (2e14 t 1 ) states lie at ν > 19300 cm–1 2 1 2 2 and ν > 21300 cm–1, respectively. The 3A2 3T (2e04 t 2 ) transition 1 2 30000 cm–1.

manifests itself as a shoulder at

At the same time, as assumed in [45], the high oxidation state of the central Fe6+ ion and the instability of the [FeO4]2– complex result in degeneracy of the Fe(3d)–O(2p) levels. In this case, the crystal field the-

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Transition 3A2(2e2) Crystal (structural type) Li3PO4:[MnO4]3– Sr5(PO4)3Cl:[MnO4]3– (apatite)

K2SO4:[FeO4]2– K2CrO4:[FeO4]2– [FeO4]2– 9M KOH Bi12TiO20:[CrO4]4– (sillenite) Ca2GeO4:[CrO4]4– (olivine)

3T (2e4t ) 2 2

1A (2e ) 1 2

8940 8560 (1A') 8710 (1A'') [785] 8613

12200 10000–12500

13840 13450

3T (2e4t ) 1 2

15380 [800] 14250–15000 (3E) 16250 (3A2) [750] 15075

1T (2e4t ) 2 2

1T (2e4t ) 1 2

3T ( 4t 2 ) 1 2

22200 –

Positional symmetry

Reference

~Td Cs

[15, 27] [26]

Cs

[28]

20500 –

21600 –

19030

21771

19200

21340 [750]

–

C2

[26]

20620 [700] (1.2 × 10–2)

–

26700 (1 × 10–3)

C2

[29]

30000

Cs

[30]

Vol. 48 No. 4 2003

10811

13333

11500

–

10500 (9 × 10–4)

–

12000–15500 (3E) 16000–20000 (3A2) 13000 (3E) 16700 (3A2) (6.8 × 10–3)

13330 (8 × 10–3)

9176 (6 × 10–6)

18200 (3.6 × 10–2)

~19300

~21500

17900 [730] 17800 12157

– – 14700

– – -

20400 [730] – 18600

Cs Td T

[31] [31, 32] [33]

–

–

20500 (6 × 10–5)

Cs

[34]

–

–

22700

Cs

[34]

–

12320 (3A'') 13550 (3A') 15790 (3A'') (9 × 10–4) 13600 (3A'') 15100 (3A') 17460 (3A'') 15207

–

–

24383

C2

[35]

9500

–

15150

19841

–

24390

S4

[36]

8800

7600

–

12200

–

–

16530

S4

[37]

9740

7500

–

11965

–

–

18500

C1

[38]

–

–

–

11843

–

–

19180

C2

[38]

8400 (1A) 8700 (1A) 8320 (1A) 8685 (1A) [710] (1.3 × 10–5)* 6219 (1A') 6230 (1A'') (1.5 × 10–6) 6209 – 7856 –

Mg2SiO4:[CrO4]4– (olivine)

–

LiAlO2:[CrO4]4– (wurtzite) Gd3Sc2Ga3O12: [CrO4]4– (garnet) Y3Al5O12:[VO4]5– (garnet) LiGaO2:[VO4]5– (wurtzite) LiAlO2:[VO4]5– (wurtzite)

–

* The f values for E ⊥ z are given.

13365 [730] 12670 8670 8340 (3A'') 8620 (3A') 10300 (3A'') (5 × 10–5) 9150 (3A'') 9820 (3A') 11500 (3A'') 10505

9112 9060 – 14730 (9 × 10–6)

15240

BURKOV et al.

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Sr5(VO4)3F:[MnO4]3– (apatite) Ca2PO4Cl:[MnO4]3– (spodiosite) Ca2VO4Cl:[MnO4]3– (spodiosite)

1E(2e2)

542

Table 3. LF transition energies (cm–1) in the spectra of tetrahedral d2 complexes [MO4]n– (vibrational progression frequencies, cm–1, are given in brackets; transition oscillator forces f are given in parentheses)

ELECTRONIC STRUCTURE AND OPTICAL SPECTRA

3T (2e14 t 1 ) 1 2

states. Therefore, it is a challenge to identify the absorption bands corresponding to the spin-forbidden transitions to the 1E and 1A1 states and, especially, to determine their energy positions. This seems to be one reason behind the considerable differences in Racah parameters B and C reported in the literature (Table 5). The luminescence spectrum of [CrO4]4– shows one 3A transition broad band corresponding to the 3T2 2 [34, 35, 41, 50]. The position of the luminescence band maximum changes in a wide range from 8800 cm–1 in Mg2SiO4 [34] to 6250 cm–1 in Gd3Sc2Al5O12 [59], as a function of the positional symmetry of the Cr4+ ion in the host crystal lattice and the Dq value, which dictates the splitting of the excited 3T2 state. These factors determine the luminescence band half-width in a particular matrix. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

α, cm–1 150

580

18600

12157

100

14700

600

125

75 50 580

25 0

8670

ory does not adequately describe the electronic structure of [FeO4]2 , since it is considerably contributed to by the metal–ligand interactions, which should be considered not only in terms of first- and second-order, but also in terms of higher order perturbation theory. Thus, to adequately describe the spectra of [FeO4]2–, not only the transitions to singlet and triplet excited states, but also the transitions to quintet states that arise from the degeneracy of the Fe(3d)–O(2p) levels should be taken into account. In [45], the bands below 24000 cm–1 are assigned to LF transitions and the bands above 24000 cm–1 are assigned to CT transitions. The luminescence spectra of [FeO4]2– are similar to those of [MnO4]3–; the spectra show a number of sharp vibronic bands, which form a vibrational progression 3A beginning at 6200 cm–1 and arise from the 1E 2 transition [30, 46]. [CrO4]4–. Until recently, all information on the spectroscopic properties of [CrO4]4– was extracted from the absorption spectrum of Cr(OBut)4 (But is C(CH3)3) [47]. The advent of the tunable laser with an Mg2SiO4:[CrO4]4– active medium [48] has stimulated studies of the spectroscopic characteristics of this complex in different host crystals with olivine [34, 49–51], garnet [36, 52], sillenite [33, 53], wurtzite [35], and Y2SiO5 [54–57] structures. Figure 5 shows the region of LF transitions in the absorption spectrum of [CrO4]4– in the Bi12TiO20 host crystal; the positions of absorption band maxima in the spectra of host crystals containing [CrO4]4– ions and their assignment are listed in Table 3. The strength of the ligand crystal field (Dq) in [CrO4]4– dictates the specific features of the spectra of this ion. As follows from the Tanabe–Sugano diagram [1], the energies of the transitions to the 1E(2e2) and 3T (2e14 t 1 ) states are very close to each other, as well 2 2 as the energies of the transitions to the 1A1(2e2) and

543

8

10

12

14

16

18

20 ν,

22

24

103

cm–1

Fig. 5. Absorption spectrum of the Bi12TiO20〈Cr〉 at T = 10 K [33].

In [34, 47], strong absorption bands (ε ≈ 104 l mol−1 cm–11) at 37000 and 43000 cm–1 are attributed to the CT transitions of [CrO4]4–, without assigning them to definite electronic transitions. [VO4]5–. The spectroscopic properties of [VO4]5– have been studied in [37, 38, 60, 61]. As a result of a small Dq value, the 3T2 state lies beneath the 1E state on the energy scale. The most comprehensive information on the arrangement of energy levels for [VO4]5– has been extracted from the spectra of an LiAlO2〈V〉 crystal [38, 60] (Table 3). The bands corresponding to the tran1 sitions from the ground state to the 3T2, 3T1(2e14 t 2 ), and 3T (2e04 t 2 ) 1 2

states have a complicated structure, which reflects a severe distortion of the [VO4]5– tetrahedron in the crystal lattice of LiAlO2 . The luminescence spectrum of the [VO4]5– ion shows a broad band, as a result of the small Dq value; as in the spectrum of [CrO4]4–, this band is caused by 3A transition [60]. This spectrum differs the 3T2 2 from the corresponding spectra of [MnO4]3– and [FeO4]2– ions. The higher Dq values for [MnO4]3– and [FeO4]2– account for the fact that the 1E state appears to be the lowest excited state, and the luminescence spectra of these complexes show a sharp band correspond3A transition [27, 29, 30, 35, 41–43, ing to the 1E 2 46, 50]. Table 5 shows that the Dq value increases in the series [VO4]5–, [CrO4]4–, [MnO4]3–, and [FeO4]2–, which can be associated with a decrease in the metal–oxygen distances in these complexes. In contrast, the Racah 1 Hereafter,

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No. 4 2003

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BURKOV et al.

Table 4. CT transition energies (ν, cm–1) in [MnO4]3– and [FeO4]2– (the vibrational progression frequencies, cm–1, are given in brackets) [MnO4]3– [11] ν

ε,

l

28400 31600 33600

mol–1

[FeO4]2– [30] Transition

ν*

cm–1

1200 3500 800

[38], 0.54 for [CrO4]4– [34], 0.42 for [MnO4]3– [11, 29], and 0.27 for [FeO4]2– [30]. The enhancement of the nepheloauxetic effect with an increase in the oxidation state of the central ion in [åO4]n– points to an increase in the covalence of the M–O bond, which results in a decrease in the energy of the first CT transition. As a result, the difference between the energies of the transi1 tions to the 3T2, 3T1(2e14 t 2 ) states and the first CT transition decreases; this accounts for an increase in the intensity of these transitions in the crystal field in the series of ions [VO4]5–, [CrO4]4–, [MnO4]3–, and [FeO4]2– due to the intensity borrowing mechanism. d3 Configuration. The ground configuration of a d3 6 1 tetrahedral ion, …1 t 1 2e24 t 2 , gives several states, the energy of the 4T1 state being the lowest. As for the

2e 3T1 (2E) 4t2 3T1 (4T1) 2e 3T1 (2E)

20040 [750] 1t1 26370 [780] 1t1 – 3t2

* Position of the 0–0 band.

parameter B for these complexes tends to decrease, with respect to the parameter of a free 3d ion β = Bcomplex/Bfree ion , in the same series: 0.59 for [VO4]5– Table 5. Crystal-field parameters for d2 [MO4]n– ions Ion

Dq

Ca2PO4Cl [CrO4]4– Bi12TiO20 LiNbGeO5 Y2SiO5 Y3Al5O12 Gd3Sc2Ga3O12 Mg2SiO4 Ca2GeO4 LiGaO2 LiAlO2 [VO4]5– LiGaO2 LiAlO2 Y3Al5O12

C

cm–1

Host [FeO4]2– K2SO4 K2CrO4 [MnO4]3– Li3MO4 (M = V, P, As) Li3PO4 Sr5(PO4)3Cl

B

Dq/B

C/B

Reference

1330 1300 1294 1100 1100

420 373 375 430 500

1790 1380 1390 1800 2250

3.17 3.51 3.45 2.56 2.44

4.26 3.7 3.7 4.19 4.5

[11] [30] [46] [11] [29]

1130 1100 1065 1150

510 500 430 550

2167 2000 2540 2200

2.2 2.20 2.48 2.09

4.25 4.0 4.25 4.0

[27] [26] [58] [26]

800 900 810–910 910 960 1037 950 1010 1010 895 962 1055 1065

400 450 750 506 660 590 730 860 555 540 480 428 450

1880 2025 3375 2423 2500 2301 3134 4220 2331 2268 2016 2667 2417

2.0 2.0 1.1–1.2 1.8 1.45 1.76 1.3 1.2 1.8 1.66 2.0 2.5 2.4

4.7 4.5 4.5 4.8 3.8 3.9 4.3 5.0 4.2 4.2 4.2 6.2 5.4

[33] [56] [57] [54] [55] [36] [36] [51] [34] [34] [49] [35] [35]

755 862 750

520 504 427

2898 2822 2780

1.45 1.71 1.75

5.6 5.6 6.5

[38] [38] [37]

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ELECTRONIC STRUCTURE AND OPTICAL SPECTRA α, cm–1

545

∆εc × 103, cm–1 6

(b)

(a)

4

4

T1(4P)

2

10

0 4

–2 16000 4

1

20000

24000 ν, cm–1

T2(4F) 2

2

A2(4F)

E(2G) 2

T2(2G)

T1(2G)

0.1

6000

8000

10000 12000 14000 16000 18000 20000 22000 24000 ν, cm–1

Fig. 6. (a) Absorption and (b) CD spectra of the Bi12SiO20〈Mn4+,P〉 crystal at T = 300 K [62].

excited configurations, transitions into both the 2e and 4t2 AOs of the metal are possible [1]. The electronic configuration d3 is inherent in Mn4+, Cr3+, and V2+ ions. The tetrahedral coordination of these ions is uncommon. Nevertheless, information on the spectroscopic properties of these ions introduced as a substitutional impurity into some host crystals is available in the literature. To the best of our knowledge, experimental data on the CT transitions in these complexes are lacking. Thus, we do not consider excited configurations and possible CT transitions. The spectra of d3 ions in the region of LF transitions are interpreted in the weak-field approximation. According to the Orgel diagram, the transitions from the 4T1 (F) state to the 4T2 (F), 4A2 (F), and 4T1 (P) states are possible. The transition to the 4A2 (F) state is an electric dipole-forbidden transition. The transitions to the 2E, 2T1 , and 2T2 states are spin-forbidden. The absorption and induced CD spectra of the [MnO4]4– ion in the gyrotropic Bi12SiO20 crystal were studied in [62]. In the crystal lattice of Bi12MO20 (M = Si, Ge, Ti), manganese atoms in two oxidation states, +4 and +5, are in the center of regular tetrahedra (positional symmetry T). Doping this crystal with phosphoRUSSIAN JOURNAL OF INORGANIC CHEMISTRY

rus allows one to obtain a crystal containing exclusively the [MnO4]4– tetrahedra. The absorption and CD spectra of a Bi12SiO20〈Mn4+,P〉 crystal are shown in Fig. 6. If the symmetry of a complex is lowered to T, as in doped sillenite crystals, the difference between the irreducible representations T1 and T2 vanishes; i.e., these states coalesce into one state with the transformation properties of the T state in the framework of the T group. However, for interpretation of the spectroscopic characteristics of crystals, the tetrahedral [MO4]n– groups are more conveniently considered to have the higher symmetry Td . It should be remembered that the wave functions transformed by the A1, A2 and T1, T2 irreducible representations in symmetry group Td , have the same transformation properties of A and T, respectively, in the framework of symmetry T. The spectra of Bi12SiO20〈Mn4+,P〉 show bands at 7500–9000, 9500–13000, 16130, 17700, 20800, and 23600 cm–1 (Fig. 6). In symmetry Td , the strongest and broadest bands with maxima at 11000, 20800, and 23600 cm–1 should be assigned to the 4T1(4F) 4T (4F), 4T (4F) 4A (4F), and 4T (4F) 4T (4P) 2 1 2 1 1 transitions, respectively. A relatively low intensity of 4A (4F) transition (ν ≈ 20800 cm–1), as the 4T1(4F) 2 compared to the intensities of the other two transitions,

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is due to the fact that it is electric dipole-forbidden in contrast to the transitions to the 4T2(4F) and 4T1(4P) states. In the CD spectra of Bi12SiO20〈Mn4+,P〉, the 4T (4F) 4A (4F), 4T (4P) transitions give rise to a 1 2 1 positive band consisting of several components (Fig. 6). 4T (4P) transition is The CD band of the 4T1(4F) 1 4A (4F) weaker than the CD band of the 4T1(4F) 2 transition, due to a considerable negative contribution of the CD band at 25000 cm–1 associated with the bandto-band transition of the crystal matrix. The best fit of 4T (4F), 4T (4F) 4A (4F), and the 4T1(4F) 2 1 2 4T (4F) 4T (4P) transition energies calculated in 1 1 terms of crystal field theory to the experimental values is attained at Dq = –1150 cm–1 and B = 960 cm–1 [62]. According to the Tanabe–Sugano diagram, the weak band beginning at 7348 cm–1 should be assigned to the 2E(2G) of the spin-forbidden transition 4T1(4F) 4+ Mn ion. The two weakest bands at 16130 and 17700 cm–1 in the absorption spectrum, which are correlated with the complicated negative band in the CD spectrum (Fig. 6), are caused by spin-forbidden 4T (4F) 2T (2G) and 4T (4F) 2T (2G) transi1 1 1 2 tions, respectively [62]. According to structural data [63], the tetrahedral Mn4+ ion also exists in a heteropoly acid (12-WMnIV); the spectrum of this acid shows a strong band (ε ~ 9.9 × 104) at ν0 ≈ 38400 cm–1 and a broad weak band (ε ≈ 18) with a maximum at ν1 ≈ 7000 cm–1. Comparison of this spectrum to the spectrum of 12-WCrIII, which shows a band at ν0 ≈ 38400 cm–1 and two bands at ν1 ≈ 8300 cm–1 and ν2 ≈ 16000 cm–1 [64], permits the conclusion that the strong band at ν ≈ 38400 cm–1 is caused by electronic transitions of the polytungstate anion. The band at ν1 ≈ 7000 cm–1 in the spectrum of the compound containing [MnO4]4– and the band at ν1 ≈ 8300 cm–1 in the spectrum of the compound containing [CrO4]5– are 4T (F) transition, and the assigned to the 4T1(F) 2 –1 band at ν2 ≈ 16000 cm is attributed to the 4T1(F) 4A (F) transition [63, 64]. The third LF transition 2 4T (F) 4T (P) is unobservable in the spectra 1 1 because of a large half-width of the band at ν2 and the long-wavelength wing of the transition at ν0 . Note that this assignment is inconsistent with the conclusions in [62]. No information on the spectra of [VO4]6– is available. d4 Configuration. This electronic configuration is inherent in Cr2+, Mn3+, and Fe4+ ions. The ground term 6 2 is 5T2( t 1 2e24 t 2 ). At Dq/B < 1.7, the spectra should show one broad band, corresponding to the transition to 3 the 5E(2e14 t 2 ) state, and several sharp bands caused by

3

3

3

the 3T1(2e4 t 2 ), 3T2(2e4 t 2 ), and 1A1(2e4 t 2 ) transitions. Experimental data on the spectra of d4 ions in the tetrahedral oxygen environment are limited (in the spectra of magnesium chromium spinels, the band at 6670 cm–1 is 5E transition of [CrO ]6– [65]). assigned to the 5T2 4 Therefore, in analysis we used the data on the spectroscopic properties of these complexes in crystals of isoelectronic oxide analogues, chalcogenides. The absorption spectra of tetrahedral Cr2+ complexes in host crystals of CdS [66], ZnS [67], ZnTe [68], CdTe [69], and CdSe [70–73] have been studied. The maximum of the absorption band corresponding to 5E transition of the Cr2+ ion in these crystals the 5T2 falls within the range 5000–6000 cm–1. A broad lumi5T ) is observed at 3500– nescence band (5E 2 –1 5500 cm . Inasmuch as the crystal field in tetrahedral Mn3+ complexes is stronger [66] than in the corresponding 5E tranCr2+ complexes, the band caused by the 5T2 2+ sition of the Cr ion in the CdS crystal lies at 5300 cm–1, whereas the corresponding band in the spectrum of Mn3+ is observed at 7800 cm–1. The spectra of the [FeO4]4– ion have been poorly studied. The bands observed in the absorption spectrum of α-quartz crystals with an amethyst color are believed to arise from absorption of the [FeO4]4– ion [74, 75]. 5E LF The band at 10600 cm–1 is assigned to the 5T2 transition. The broad band composed of three components with maxima at 16700, 18600, and 19000 cm–1 is 5T (1t 2e). assigned to the CT transition 5T2 1 1 The oscillator forces f for these bands are ≈0.01 [76]. To the best of our knowledge, no information on CT transitions for d 4 complexes is available. d5 Configuration. The ground state of d5 ions (Mn2+, Fe3+) is 6S. For the 3d5 configuration, the energylevel diagrams in tetrahedral and octahedral ligand fields coincide; however, due to different crystal field strengths—Dqtetr = –4/9Dqoct , absorption spectra of tetrahedral complexes are shifted toward lower frequencies. In the limiting case of a weak field (Dq/B < 2.3), 3 the ground state is 2e24 t 2 ; in a strong tetrahedral field, 1

the 2e44 t 2 state with an unpaired electron is the ground one. However, the latter has not been identified thus far [1]. 3

In high-spin complexes, the 2e24 t 2 configuration produces the ground state 6A1 . Inasmuch as this is the only sextet level, all absorption bands must be spin-forbidden, which manifests itself in the low extinction coefficients of the observed bands. The absorption spectra of [MnO4]6– show a complicated set of relatively narrow bands (Table 6). According to crystal field theory [79], the strongest and sharp-

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Table 6. Transition energies (cm–1) in the spectra of tetrahedral Mn2+ complexes [2, 77, 78] Transition 6A1(S)

Crystal

4T (G) 1

4T (G) 2

4A ; 4E(G) 1

4T (D) 2

4E(D)

4T (P), 4A 1 2

3K2O · 7SiO2

21200

22900

23600

26200

28000

–

Zn2SiO4〈Mn〉

21300

23000

23600

–

–

–

MnCO3

18350

22600

24580

27255

29170

31900 36500

MnZn(edta) · 6H2O

18700

22700

24800

27800

29700

Mn(NCO ) 4

20400

22400

23300

–

–

–

Mn(NCS) 4

2–

19600

21740

22730

25500

27030

–

MnS–ZnS

18850

20100

21280

–

23300

25700

2–

22045

Table 7. Transition energies (cm–1) in the spectra of [FeCl4]– [79–82] and [FeO4]5– [83–89] complexes Transition 6A1(S)

Ion

CT transition

4T (G) 1

4T (G) 2

4A (G), 1 4E(G)

4T (D) 2

4E(D)

4T (P) 1

13400– 14800

15300– 16900

18100– 18900

19480– 19930

21600

22200– 22910

27500

–

AlPO4

18400

21100

23100

24500

26580

–

45900

–

AlAsO4

18500

21300

23000

24400

26050

–

42300

–

GaPO4

18000

20900

23000

24300

26440

–

46000

–

α- SiO *2

18700

20200

22500

24800

27300

–

40700

51100

(6 × 10–5)

(11 × 10–5)

(0.085)

(0.13)

22026

25348

29940

–

–

Host [FeCl4]–

1t1

2e 1t1

4t2

[FeO4]5–

(2.4 × 10–5) (2.0 × 10–5) (10 × 10–5) Zn 2 SiO ** 4

K[AlSi3O8]

15974

18330

21500

17270

19305

16000

20400

22720

24260

26300

–

–

–

22852

17860 Y3Fe5O12

16400

20410

21050

26300

26670

–

–

–

LiGa 5 O ** 8

15790

18480

20740

21640

24630

31850

–

–

LiAl 5 O ** 8

15748

19080

21110

22222

25510

28169

–

–

β- NaAlO ** 2

15300

18750

21230

22300

25740

–

–

–

* Transition oscillator forces for an ordinary ray are given in parentheses. ** Transition energies were obtained from the luminescence excitation spectra of [FeO4]5–. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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Table 8. Crystal-field parameters for d5 and d7 [MO4]n– ions Dq Crystal Na2Mn2Si2O7

B

C

cm–1

α

Reference

410

806

2786

68

[85]

Y3Al5O12〈Fe3+〉

1036

554

3411

82

[85]

Y3Fe5O12

1117

606

2696

–

[90]

620

900

2400

–

[84]

560

705

2730

725

629

2935

86

[85]

700

668

3160

–

[83]

710

683

3245

–

[83]

743

535

3155

–

[86]

700

565

3000

–

[87]

770

605

3046

–

[88]

800

644

2960

–

[89]

390

775

3490

–

[92]

400

735

3230

–

[92]

450

650

2925

–

[93]

K[AlSi3O8]〈Fe3+〉 α-SiO2〈Fe3+〉 AlPO4〈Fe3+〉 Zn2SiO4〈Fe3+〉 LiGa5O8〈Fe3+〉 LiAl5O8〈Fe3+〉 β-NaAlO2〈Fe3+〉 ZnO〈Co2+〉 MgAl2O4〈Co2+〉 Bi12TiO20〈Co2+〉

est bands in the spectrum correspond to the 6A1(S) 4E(G),4A(G) and 6A (S) 4E(D) transitions. The typi1 cal Dq values for the [MnO4]6– ion are 300–400 cm–1. For many compounds containing tetrahedrally coordinated Mn2+ ions, green luminescence is observed, in contrast to the octahedrally coordinated Mn2+ ion for which red and orange luminescence is typical [3]. The spectra of a tetrahedrally coordinated Fe3+ ion in host halide crystals have been comprehensively studied. As an example, Table 7 lists the positions of absorption band maxima and their assignment to LF transitions for the [FeCl4]– ion [79–82]. It is generally agreed that the first CT transition (1t1 2e) of this ion gives rise to the band at ν ≈ 27500 cm–1 [1]. For the oxygen environment of Fe3+, the energies of all transitions should increase. Unfortunately, the lattices of most oxide crystals contain both tetrahedrally coordinated Fe3+ ions and octahedral [FeO6]9– ions [3]. This leads to ambiguity in the interpretation of the absorption bands and, hence, to considerable differences in calculated crystal field parameters, even in the case of the same crystal matrix (Table 8). From this standpoint, decolorized amethyst (α-SiO2) crystals and doped crystals of the structural quartz analogues AlPO4 , AlAsO4 , and GaPO4 in which iron atoms are in quasi-tetrahedral positions, are the most convenient for studying the spectral properties of [FeO4]5– [83].

[91]

The absorption and CD spectra of AlPO4:[FeO4]5– crystals have been studied in [94]. As follows from Fig. 7, all spin-forbidden LF transitions are more clearly pronounced in the CD spectra. Their positions are consistent with those in the spectra of the [FeO4]5– ion in other quartz-like crystals (Table 7). All CD bands in the region of LF transitions for a dextrorotatory crystal (rotation of the light polarization plane) are negative, except the band at ν ≈ 21276 cm–1 whose intensity is low. The strongest and sharpest band in the absorption 4E(D) and CD spectra corresponds to the 6A1(S) transition, as in the case of the [MnO4]6– ion. The absorption spectrum of AlPO4 : [FeO4]5– shows two overlapping CT bands with maxima at 40 650 and 45 870 cm–1, and the CD spectrum shows three bands at 36 360, 39 682, and 44440 cm–1, whose intensity is one order of magnitude higher than the intensity of the LF transition bands. All these bands are assigned to the first 2e [94]. As is known, the lowest CT transition t1 3

lying state 5E of the excited configuration 2e34 t 2 interacts with the hole in the ligand orbital to produce two sextet and two quartet states: 6T1, 6T2, 4T1 , and 4T2 . The 6A 6T transition is electric dipole-allowed and 1 2 determines the band intensity in the absorption spectrum. The absorption band of the magnetic dipole6T transition should be significantly allowed 6A1 1 weaker. However, in the CD spectrum, the lowering of symmetry from Td to T can result in comparable inten-

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εc, cm–1 0.32

(a)

(a')

0.30

10

0.28 0.26 0.24

1

300

0.1 200

300

400

400

500

600

500

700

600

800

700

900

1000 λ, nm

∆εc × 10–3, cm–1 2

(b)

0 –2 –4 –6 –8 –10 –12 –14 –16 –18 300

350

400

450

500

550 λ, nm

Fig. 7. (a, a') Absorption and (b) CD spectra of iron-doped AlPO4 crystals in the region of LF transitions at T = 300 K [94].

sities of the transitions to the 6T2 and 6T1 states. Since all the iron atoms are in positions with symmetry C2 , all degenerate states of Fe3+ are not only split but also mixed, so that the symmetry of the resulting states is either A or B. In the C2 symmetry group, the transition from the ground state into any excited state will be symRUSSIAN JOURNAL OF INORGANIC CHEMISTRY

metry-allowed in both the electric and magnetic dipole approximations and, hence, will be active in the CD spectrum. Therefore, the presence of two bands in the absorption spectrum and three bands in the CD spectrum is associated with the splitting of tetrahedral states and mixing of split components under the conditions of

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BURKOV et al. εc × 103, cm–1

6

30 (a)

20

∆εHc × 103, cm–1

10

5

0~ ~ ∆εc × 103, cm–1 15

0

(c)

–5

10

(b)

5

0

0.5

0.6

0.7

0.8 λ, µm

Fig. 8. (a) Absorption, (b) CD, and (c) MCD spectra of the Bi12TiO20〈Co〉 crystal at T = 300 K [93]. 4T transition is symmetry C2 . The fact that the 6A1 1 CD-active is presumably associated with the spin–orbit 4T mechanism of intensity borrowing from the 6A1 2 transition. The luminescence spectrum of [FeO4]5– shows one broad band in the range 12500–15000 cm–1, which cor6A (S) responds to the intersystem crossing 4T1(G) 1 [86]. In LiGa5O8 [87], LiAl5O8 [88, 89], and Mg2SiO4 [95] crystals, in which Fe3+ ions are in both the tetrahedral and octahedral environment, only tetrahedral ions exhibit luminescence. The luminescence excitation spectra of these crystals [87, 88, 95] coincide in band positions and intensities with the corresponding spectra of β-NaAlO2〈Fe〉 [89] and Zn2SiO4〈Fe〉 [86] crystals (Table 7) in which iron atoms are also in the tetrahedral environment. d6 Configuration. The ground electronic configuration for tetrahedrally coordinated Fe2+ and Co3+ ions is

2

t 1 2e44 t 2 . This configuration produces a series of states, the 5E state being the lowest lying. The following CT 4t2, 3t2 4t2 , and transitions are possible: 1t1 1e 4t2 . In the crystal field of symmetry Td, one tran5T , should be observed. Depending on sition, 5E 2 the crystal field strength and positional symmetry of an ion, one or several IR bands correspond to this transition. The spectroscopic properties of [FeO4]6– doped to the spinel MgAl2O4 host have been studied in [96, 97]. A broad band with a maximum at 4500–4900 cm–1 cor5T transition. The estimated responds to the 5E 2 oscillator force of this transition is f ≈ 5 × 10–4 [96]. The low-temperature contour of this band exhibits vibrational progressions beginning at 3595 and 3660 cm–1. These bands are associated with the splitting of the 5E and 5T2 states [96]. The ground state 5E is split due to second-order spin–orbit coupling with the constant ξ = 98 cm–1.The 5T2 state experiences a strong influence from the Jahn–Teller dynamic effect (EJT(5T2) = 945 cm–1) [96]. Recently [97], the emergence of sharp bands at 3595 and 3660 cm–1 was explained as due to OH vibrations. An increase in temperature (297–600 K) is accompanied by a broadening of absorption bands and a decrease in their intensities, which was rationalized as a result of the Jahn–Teller effect and vibronic coupling [97]. The displacement of the barycenter of the bands toward higher energies is accounted for by a considerable distortion of the [FeO4]6– tetrahedron upon heating [97]. In the absorption spectra of ZnSe〈Fe2+〉 and 5T transition energy is 3150 CdSe〈Fe2+〉, the 5E2 2 2+

and 2400 cm–1, respectively [98, 99]. For Fe(HMPA ) 4 (HMPA is hexamethylphosphoramide [(CH3)2N]3PO) in which the nearest environment of the metal atom is composed of oxygen atoms, this transition is observed at ~6950 cm–1 [1]. The absorption spectra of [CoO4]5– have been poorly studied. The band at 8300 cm–1 is assigned to the LF transition of this ion in the Y3Al5O12 host crystal [100]. d7 Configuration. The ground state of tetrahedral d7 3 Co2+ and Ni3+ complexes is the 4A2 state (the 2e44 t 2 configuration generates the 4A2, 2E, 2T2, and 2T1 states). In

turn, 5

6

the

excited

configurations

4

5

4

1 t 1 2e44 t 2 ,

4

3 t 2 1 t 1 2e44 t 2 , and 1e3…2e44 t 2 give a series of doublet and quartet states [1]. Among all possible transitions, 4T transitions are electric dipole-allowed, the 4A2 1 4 4T transition is magnetic dipoleand the A2 2 allowed.

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Table 9. Transition energies in the spectra of [CoO4]6– Transition 4A2(F) Crystal ZnO

Reference 4T (F) 2

4T (F) 1

4T (P) 1

4500

6170, 7150, 7690

15310, 16360, 17652

(7.8 × Y3Ga5O12

4514 (1 ×

α-SiO2

(5.2 ×

10–4)*

6309, 6596, 7340, 7812 (2.7 ×

10–6)

14225, 16515, 17421 (3.0 ×

10–4)

[92, 103]

10–3) [103]

10–4)

–

5700, 6700, 7700

15100, 17100, 18500

[104]

MgAl2O4

4500

6670, 7520, 8200

15870, 16940, 18180

[101, 102]

Bi12TiO20

4550

6060, 7230, 7350

14640, 15740, 16800

[93]

Ca3SnCoGe3O12

4200

6150, 6600

15400, 16450, 17600

[2]

ZnS

–

6640

13732, 14930

[92]

CdS

5220–5600

6135, 6496

13540, 13618, 13847, 14142

[2]

* Transition oscillator forces are given in parentheses.

Table 10. Transition energies in the spectra of [NiO4]6– Transition 3T1(F) Crystal ZnO

3T (F) 2

3A (F) 2

4600

8340

(3.9 × Bi12TiO20

10–5)*

(8.6 ×

1E,

T2

–

3T (P) 1

15260, 16180 (1.6 ×

10–5)

Reference [92]

10–3)

4200

8475, 8850, 9225

12900

14300, 15500, 16800

[107]

4100, 4400

8400

–

14700, 15650

[2]

5000

9000–10000

–

16000

[2]

SnO

4215, 4248, 4312, 4400

8340, 12404, 13365

–

15200, 15300, 15486, 15635

[2]

ZnS

4940

8550, 9390

–

12500, 13080

[92]

CdS

3902, 4413

7840, 9710, 10254

–

12199, 12294, 12509, 12623, 15100

[2]

Ca3TiNiGe3O12 NiMgAl2O4

* Transition oscillator forces are given in parentheses.

The absorption spectrum of [CoO4]6– shows two strong composite bands: the band in the visible corre4T (P) transition and the sponding to the 4A2 1 weaker band in the IR region arising from the 4A2 4T (F) transition (Fig. 8). The typical band positions are 1 listed in Table 9. The band at 4500 cm–1, corresponding 4T is to the magnetic dipole transition 4A2 2 extremely weak so that it is unobservable in the spectra of many compounds. The band at 7000 cm–1 consists, as a rule, of three triplets with intensities increasing in RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

ascending order of frequency [92, 100–104]. The band in the visible consists of two to four components. The structure of the broad band is evidently related to the splitting of the 4T1 state in a low-symmetry crystal field. The transition to the 4T1(P) states is active in the CD and magnetic circular dichroism (MCD) spectra [93]. The presence of the [CoO4]6– ion can be inferred by the characteristic shape of this band in the MCD spectrum [105]. The low-temperature absorption spectra of ZnO:Co crystals show, in addition to the aforementioned bands,

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1.5

1.0

0.8

0.6

to [1], CT transitions in the spectrum of [NiCl4]2– lie at ν > 30000 cm–1. An analogous transition in the spectrum of ZnO〈Ni〉 is attributed to the band at ν ~ 25000 cm–1 [92]. However, this assignment is doubtful: first, the intensity of this transition is very low; second, based on the general regularities of the nepheloauxetic series, this transition in the spectra of tetrahedral Ni(II) complexes with an oxygen environment should lie at ν > 30000 cm–1. The LF transitions of the [NiO4]6– ion are the transi-

0.5

(‡) 3

101

T1(F) → 3T1(P)

T1(F) → T2(D) T1(F) → 1E(D)

3

α, cm–1

3

3

T1(F) → 3T2(F) 3 T1(F) → 3A2(F)

4

100

3000

8000

13000 ν, cm–1

18000

23000

(b)

2.5

0

5

0.5

0.6

0.7 (c)

0.8 λ, µm

–2.5

0

∆εHc × 103, cm–1

∆εc × 103, cm–1

5.0

–5

Fig. 9. (a) Absorption, (b) CD, and (c) MCD spectra of the Bi12TiO20〈Ni〉 crystal at T = 300 K [107].

a series of bands assigned to spin-forbidden transitions because of their low intensity [66, 92]. The spectra were interpreted in the weak-field approximation. The Dq value for the [CoO4]6– ion varies from 390 to 450 cm–1 (Table 8). The CT transitions in the absorption spectra of [CoCl4]2– lie at ν > 42000 cm–1 [1]. No information on the absorption spectrum on the [CoO4]6– ion in the region of CT transitions is available. d8 Configuration. The tetrahedral coordination of 2+ Ni ions is uncommon. For tetrahedral Ni2+ com4 plexes, the ground electronic configuration 2e44 t 2 is typical, which generates the 3T1, 1T2, 1E, and 1A1 states. The CT transitions to the following excited states are 5 5 possible: 3A2, 3E, 3T1, 3T2 (1 t 1 2e44 t 2 ); 3A1, 3E, 3T1, 3T2 5

6

5

5

(3 t 2 1 t 1 2e44 t 2 ); and 3T1, 3T2 (1e3…2e44 t 2 ). According

5

tions between the states with the 2e44 t 2 and 2e34 t 2 configurations, which correlate with the states of a d8 ion in a weak field [1]. The spectrum of such an ion shows absorption bands corresponding to spin-allowed transi3T (F)), 8000– tions at 4000–5000 cm–1 (3T1 2 –1 3 3 A2(F)), and 15000–17000 cm–1 10000 cm ( T1 3T (P)) (Table 10). In the low-temperature (3T1 1 spectra of some crystals containing [NiO4]6– [92, 106], the band at 15000 cm–1 has a fine structure, which is attributed to splitting of terms in low-symmetry fields and vibronic couplings. In this case, the crystal-field parameters Dq = 420 cm–1, B = 770 cm–1, and C/B = 4.8 are typical [92]. The line-shape analysis of the absorption and CD 3T (P) transition in bands that arise from the 3T1(F) 1 6– the spectra of a Bi12TiO20:[NiO4] crystal shows [107] that the band component maxima are spaced at intervals ∆ν = 1200 cm–1. Only the first component of this band at 14300 cm–1 contributes to the MCD spectrum. As can be seen from Fig. 9, this negative contribution has the form of the B term [108]. According to [66, 92], the fine structure of the band contour is caused primarily by spin–orbit coupling; however, the manifestation of vibronic coupling cannot be ruled out [92]. The fact that the MCD band is observed only in the region of the first-component maximum of the absorption bands in the visible range points to spin–orbit coupling in both the ground and excited states. d9 Configuration. Only one term 2D corresponds to the d9 electronic configuration. In the crystal field of symmetry Td , this term is split into two states 2T (2e44 t 5 ) 2 2

6

and 2E(2e34 t 2 ), the first of them being the ground state. Therefore, the spectrum of Cu2+ should show only one broad band corresponding to the LF transition. As shown for copper-doped garnet and ZnO 2E (2e 4t2) transition crystals [92, 109], the 2T2 –1 lies at 6000 cm , and the oscillator force of this transition is f ≈ 10–4 [66]. At low temperatures, this spectral region shows a number of sharp bands forming vibrational progressions caused by spin–orbit coupling and the splitting of terms in low-symmetry fields [92]. The CT spectra of tetrahedral copper complexes are sufficiently simple and arise from the transitions from the 1e, 3t2 , and 1t1 orbitals of the ligands to the

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4t2 atomic orbital of the metal; the ground state is 6 4 5 2(1 t 1 2e 4 t 2 ), and the excited states, in order of increas6 5 6 5 6 ing energy, are 2T1(1 t 2 2e44 t 2 ), 2T2(3 t 2 1 t 1 2e44 t 2 ), and 2E(1e33 t 6 1 t 6 2e44 t 6 ). These transitions are electric 2 1 2

2T

dipole-allowed. The CT transitions for copper(II) tetrahalides have been comprehensively studied. The energy of the low-frequency CT transition for [CuCl4]2– is ν ≈ 24000 cm–1 [1]. It is believed that, for the [CuO4]6– ion or more specifically, for Cu(II) in a tetrahedral oxygen environment, this transition lies at ν > 25000 cm–1. Therefore, the above systematized data on the electronic absorption spectra of tetrahedral complexes [MO4]n– of 3d metals will be useful in studying the spectroscopic properties of new compounds of this type. It is pertinent to note that CD and MCD spectroscopy holds promise for assignment of the observed absorption bands to definite electronic transitions. REFERENCES 1. Lever, A.B.P., Inorganic Electronic Spectroscopy, Amsterdam: Elsevier, 1984. 2. Boksha, O.N. and Grum-Grzhimailo, S.V., Issledovaniya opticheskikh spektrov kristallov s ionami gruppy zheleza pri komnatnoi i nizkikh temperaturakh (RoomTemperature and Low-Temperature Optical Spectra of Crystals Doped with Iron-Group Metals), Moscow: Nauka, 1972. 3. Sviridov, D.T., Sviridova, R.K., and Smirnov, Yu.F., Opticheskie spektry ionov perekhodnykh metallov v kristallakh (Optical Spectra of Transition-Metal Ions in Crystals), Moscow: Nauka, 1976. 4. Kück, S., Appl. Phys. B, 2001, vol. 72, no. 5, p. 515. 5. Burkov, V.I., Neorg. Mater., 1994, vol. 30, no. 1, p. 12. 6. Müller, A., Diemann, E., and Ranade, A.C., Chem. Phys. Lett., 1969, vol. 3, no. 7, p. 467. 7. Duinker, J.C. and Ballhausen, C.J., Theor. Chim. Acta, 1968, vol. 12, p. 325. 8. Sanderson, R.T., Zh. Neorg. Khim., 1992, vol. 37, no. 7, p. 1666. 9. Johnson, L.W., Hughes, E., and McGlynn, S.P., J. Chem. Phys., 1971, vol. 55, no. 9, p. 4476. 10. Ballhausen, C.J. and Trabjerg, I.B., Mol. Phys., 1972, vol. 24, no. 4, p. 689. 11. Vanquickenborne, L.G. and VerDonck, E., Inorg. Chem., 1976, vol. 15, no. 2, p. 454. 12. DiSipio, L. and Oleari, L., J. Chem. Soc., Faraday Trans. 2, 1972, vol. 68, no. 5, p. 776. 13. Borromei, R., Oleari, L., and Day, P., Phys. Status Solidi B, 1984, vol. 124, no. 2, p. 707. 14. Carrington, A. and Symons, M.C.R., Chem. Rev., 1963, vol. 63, no. 5, p. 443. 15. Milestein, J.B., Ackerman, J., Halt, S.L., and McGarvey, B.R., Inorg. Chem., 1972, vol. 11, no. 6, p. 1178. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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