ISSN 1063-7745, Crystallography Reports, 2008, Vol. 53, No. 4, pp. 708–712. © Pleiades Publishing, Inc., 2008. Original Russian Text © A.P. Voronov, G.N. Babenko, V.M. Puzikov, A.D. Roshal’, V.I. Salo, 2008, published in Kristallografiya, 2008, Vol. 53, No. 4, pp. 747–751.
CRYSTAL GROWTH
Doping of KDP Single Crystals with Cerium: Growth and Optical Properties A. P. Voronova, G. N. Babenkoa, V. M. Puzikova, A. D. Roshal’b, and V. I. Saloa a
Institute for Single Crystals, National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkov, 61001 Ukraine b Kharkov National University, Kharkiv, 61077 Ukraine e-mail:
[email protected] Received August 2, 2007
Abstract—The features of doping of KDP crystals with cerium ions and organocerium complexes with alizarin complexon and arsenazo III have been investigated. It is established that “direct” doping by introducing cerium salts into the initial solution cannot be implemented. The effect of organometallic complexes of cerium on the crystal growth has been studied. Organocerium complexes predominantly enter the prismatic or pyramidal growth sectors. It is shown that the complex arsenazo III + Ce blocks the growth of the prismatic sector. Ceriumdoped KDP crystals exhibit a photoluminescence band peaking at the wavelength λmax = 350 nm. PACS numbers: 81.10.Dn, 78.55.-m DOI: 10.1134/S1063774508040251
INTRODUCTION
3–
This study is devoted to the search for new crystalline media for scintillators. Previously, we have obtained KH2PO4 (KDP) scintillation crystals doped with Tl [1]. Having a low density and small effective atomic number (ρ = 2.34 g/cm3 and Zeff = 14), KDP crystals almost do not absorb γ rays; hence, doped KDP:Tl crystals can be used as selective scintillators for detecting fast neutrons in mixed neutron/gamma fields. Detection of neutrons with energies from 0.1 to 10 MeV by a KDP:Tl crystal, containing hydrogen bonds, is performed using recoil protons upon elastic neutron scattering from hydrogen nuclei. The luminescence decay kinetics at room temperature obeys a monoexponential law with a fairly large time constant: 300 ns [2]. At the same time, it is known that, for example, cerium ions Ce3+ are effective activators for “fast” (τ < 100 ns) oxide scintillators [3]. Here, we investigated the possibility of doping KDP crystals with cerium ions both through direct introduction of inorganic salts and using organometallic compounds: alizarin complexon + Ce and arsenazo III + Ce. Chemical analysis of the crystals has been performed and their optical and spectral properties have been studied. EXPERIMENTAL KDP single crystals were doped with cerium ions by introducing the salt Ce(NO3)3 into the initial solution. It is established that in a stoichiometric (pH = 4) phosphate solution of KDP, Ce3+ ions react with phosphate
ions PO 4 with the formation of insoluble tertiary cerium orthophosphate CePO4, which has the solubility product SP = 10–23 and almost completely precipitates. Chemical analysis (with a sensitivity to Ce3+ not lower than 10–6 wt %) showed absence of Ce3+ ions in KDP crystals and solutions after precipitate separation. Attempts of doping crystals with Ce3+ ions under conditions of low pH, which provides formation of acid lanthanide phosphates [4], did not lead to the formation of soluble acid cerium phosphate, and KDP crystals could not be doped in this way. An attempt was made to introduce not Ce3+ ions but their complexes with organic ligands into crystals. Cerium complexes should satisfy at least two requirements: (i) have a high strength (in order not to be destroyed by phosphate ions) and (ii) correspond to the experimentally ascertained criteria of incorporation of molecules of organic phosphors into KDP crystals [5]. To form a stable complex organometallic compound with cerium, a ligand should contain certain functional groups: carboxyl, arsonic, hydroxyl, etc. Especially strong bonds arise if the functional groups of a ligand are in the ortho-position. In accordance with these conditions, we chose such ligands as alizarin complexon (AC) and arsenazo III (ÄIII). The KDP single crystals doped with organocerium complexes were grown by the method of decreasing temperature in the range 40–60°ë in the kinetic mode. The supersaturation of the solutions was no less than 5%. The cerium concentration in the initial solutions was 10–2 wt %. The activator content in the crystals was controlled by atomic emission spectral analysis, with
708
DOPING OF KDP SINGLE CRYSTALS WITH CERIUM
complete evaporation of the material in an ac arc discharge (initiated by an IVS-28 generator) and detection of radiation with a DFS-8 spectrograph. An alizarin complexon solution was prepared using a weight, according to the technique reported in [6]. An AIII solution was prepared by dissolving an exact weight in distilled water. A solution of the (AC + Ce) complex was obtained as a result of interaction of Ce(NO3)3 and AC aqueous solutions, taken in the ratio 1 : 1, and an (AIII + Ce) solution was obtained through interaction of Ce(NO3)3 and AIII aqueous solutions taken in the ratios 1 : 1 and 1 : 2. The structural formulas of organometallic complexes with cerium are given in Fig. 1. The absorption and photoluminescence spectra were investigated on optically polished samples of KDP crystals, 10 × 10 × 10 mm3 in size, oriented along the X[100], Y[010], and Z[001] crystallographic axes. The absorption (Lambda 35 spectrophotometer, Perkin Elmer) and photoluminescence (Hitachi F4010 spectrofluorimeter) spectra of the initial growth solutions and doped crystals were measured in the wavelength range from 220 to 800 nm at room temperature, with a light beam transmitted along the optical axis of the crystals. RESULTS AND DISCUSSION Crystal Growth KDP crystals belong to the symmetry space group I42d; during crystallization, they are faceted with the planes of a tetragonal prism {100} and a tetragonal pyramid {101}. Prismatic planes {100} are formed by phosphate–potassium packets; during growth, they do not have an electric charge. Pyramidal planes {101} are 3– formed alternately by phosphate tetrahedra PO 4 and ä+ ions. Accordingly, negative and positive charges alternately arise on the growing (101) plane [7]. It has been established [8] that impurities of organic phosphors, dissociating in a solution with the formation of anions, are selectively incorporated into the pyramidal growth sector, and neutral molecules are incorporated into the prismatic sector. Such selective distribution is due to the charge state of both the growing crystal face and the products of dissociation of organic molecules in the solution [6, 9]. 1 AC exists in a solution in the form of a zwitterion (has distributed negative and positive charges) [10]. In sum, the neutral prismatic plane {100} has locally negative and positive charges, which are related to the anion and cation lattice sites, and AC interacts with the growth plane {100}, incorporating into the prismatic sector of the crystal. Replacement of a hydrogen atom in AC with a Ce3+ ion during complex formation does not lead to significant changes in the ligand structure [10]. Apparently, this fact explains incorporation of an (AC + Ce) comCRYSTALLOGRAPHY REPORTS
Vol. 53
No. 4
2008
O
OH
709 (a) O
Ce
O OC CO H2C CH2 H2C N
O
O
(b) AsO3H2
O OH O Ce As
OH O N N
N– N
–O
SO3
–
3S
Fig. 1. Structural formulas of the organometallic complexes of cerium: (a) alizarin complexon + cerium (AC + Ce) and (b) arsenazo III + cerium (AIII + Ce).
plex into the prismatic sector, as a result of which the latter acquires a characteristic red-orange color. A small number of (AC + Ce) complexes enter the pyramidal 2 sector, which acquires a weak orangish hue. When a complex compound with (AIII + Ce) (1 : 1) is formed, two hydrogen ions of arsonic and hydroxyl groups are substituted with the formation of a singly charged anion, which interacts with the growth plane {101} and should be incorporated mainly into the pyramidal sector of the crystal. However, it was found that a KDP crystal is not colored during growth in the presence of AIII + Ce complexes. The use of a binuclear complex (AIII + Ce) (1 : 2) leads to an increase in the cerium concentration in the pyramidal sector of the crystal. The lower cerium content in the prismatic sector in the case of (AIII + Ce) (1 : 2), in comparison with (AIII + Ce) (1 : 1), may be related to blocking of the prismatic sector growth. Table 1 contains the results of analysis of the Ce content in the growth sector of KDP crystals doped with different organocerium complexes. It can be seen that, when (AC + Ce) and (AIII + Ce) complexes are used, there are a large number of cerium ions in the prismatic and pyramidal sectors, respectively. Table 1. Cerium concentration in the prismatic and pyramidal growth sectors of KDP crystals doped with organometallic complexes of cerium Growth sector of a KDP crystal Prism Pyramid
Cerium concentration, wt % (AC + Ce)
(AIII + Ce) (1 : 1)
(AIII + Ce) (1 : 2)
1 × 10–2 5 × 10–3
5 × 10–3 1 × 10–2
1 × 10–3 3 × 10–2
VORONOV et al.
710
Table 2. Effect of organometallic complexes of cerium on the growth rate of KDP crystals Growth rate, mm/day Growth sector of a KDP crystal
Complex (AC + Ce)
(AIII + Ce) (1 : 1)
(AIII + Ce) (1 : 2)
3.75 2.75 1.36
2.27 1.14 2.0
1.7 0.13 13.1
Pyramid (Vz) Prism (Vxy) Vz /Vxy
The effect of organocerium complexes on the crystal growth rate is shown in Table 2. As follows from these data, the (AC + Ce) complex does not affect the growth of the prismatic and pyramidal sectors, and the crystal grows uniformly in the X[100], Y[010], and Z[001] directions. The (AIII + Ce) (1 : 1) complex blocks the growth of the prismatic sector, and the crystal becomes elongated, growing in the Z[001] direction. The pyramid growth rate, in comparison with the case of (AC + Ce), is also retarded. The (AIII + Ce) (1 : 2) complex almost stops the prism growth and retards even more the pyramid growth. Apparently, the blocking effect of the (AIII + Ce) complex on the crystal growth is related to the spatial structure of the arsenazo III molecule. The location of such a large molecule in the interplane space is likely to lead to the formation of a growth step echelon, whose advance requires to increase the solution supersaturation; the latter cannot always be done due to the suppression of the solution stability and occurrence of spontaneous crystallization.
Optical and Luminescence Properties of Crystals The absorption spectra of the initial solutions of AC and Ce(NO3)3 in distilled water, as well as AC and its complex with Ce3+ in a KDP solution, are shown in Fig. 2. The spectrum of AC in water has two absorption bands with the wavelengths λmax = 528 and 326 nm. In the presence of KDP, a hypsochromic shift of the bands of this dye to λmax = 424 and 278 nm is observed. The Ce(NO3)3 solution has absorption peaks at 294 and –
252 nm, which are characteristic of Ce3+ and NO 3 ions [11]. The position of the absorption bands in the spectra of the (AC + Ce) complex in a KDP solution does not differ from such for free AC. This fact is in good agreement with the data reported in [12], where it was shown that the formation of complexes of AC with Ni2+, Zn2+, Gd3+, and Th4+ ions does not lead to a change in the spectral properties of the ligand. The absorption spectra of the KDP crystals doped with the (AC + Ce) complex (Fig. 3) exhibit two absorption bands with λmax = 470 and 290 nm, which are shifted to longer wavelengths with respect to that for the (AC + Ce) complex in a KDP solution. The absorption spectra of the prismatic and pyramidal sectors of a crystal have different intensities, whereas the band peaks coincide. The absorption band in the spectrum of KDP doped with the (AC + Ce) complex (λmax = 470 nm) is related to the ligand absorption. The band with λmax = 290 nm is due to the 4f 5d electronic transitions in Ce3+ ions. A, rel. units
A, rel. units 1.6
1.0 1 0.8
1.2
3
0.6 0.8
2
0.4 0.4
0.2
3
4
1
2 300
400
500
600
4
0 200
0 200
5
700
800 λ, nm
Fig. 2. Absorption spectra of (1) AC and (2)Ce(NO3)3 in water and (3) AC and (4) the (AC + Ce) complex in a KDP solution.
300
400
500
600
700
800 λ, nm
Fig. 3. Absorption spectra of the KDP crystals doped with (AC + Ce) and (AIII + Ce) complexes: (1, 2) (AIII + Ce) (1 : 1), prismatic (1) and pyramidal (2) faces; (3) (AIII + Ce) (1 : 2), a pyramidal face; and (4, 5) (AC + Ce), prismatic (4) and pyramidal (5) faces. CRYSTALLOGRAPHY REPORTS
Vol. 53
No. 4
2008
DOPING OF KDP SINGLE CRYSTALS WITH CERIUM
Nevertheless, the fluorescence spectra indicate the presence of incorporated Ce3+ ions. Apparently, the organocerium complexes at the crystallization front (on a growth step or a terrace), when the (101) plane is negatively charged, decompose into a ligand and a Ce3+ ion, and the latter interacts with the phosphate ions forming this plane and is incorporated into the crystal.
I, rel. units 1.0 0.8 0.6
Thus, doping of KDP crystals with cerium through introduction of organometallic complexes into a crystal leads (depending on the ligand nature) to incorporation of both complex ions as a whole and individual cerium ions, formed as a result of decomposition of complexes. Free cerium ions have a fluorescence band with λmax = 350 nm, and the fluorescence band at λmax = 650 nm is related to the (AC + Ce) complex.
2
0.4 1
0.2
4 0
3 300
711
400
500
600
700
800 λ, nm
Fig. 4. Photoluminescence spectra of the KDP crystals doped with (AC + Ce) and (AIII + Ce) complexes: (1, 2) (AC + Ce), a prismatic face, excitation with light at λ = (1) 300 and (2) 470 nm; (3) (AIII + Ce) (1 : 1); and (4) (AIII + Ce) (1 : 2), a pyramidal face, excitation with light at λ = 290 nm. The spectra are normalized to the maximum value.
CONCLUSIONS
Figure 3 shows the absorption spectra of the KDP crystals doped with (AIII + Ce) (1 : 1) and (AIII + Ce) (1 : 2) complexes. These spectra contain only one absorption band (λmax = 255 nm), which is due to the absorption of Ce3+ ions. The absorption bands of the AIII ligand and (AIII + Ce) complexes were not found. Figure 4 shows the fluorescence spectra of the KDP crystals doped with (AC + Ce), (AIII + Ce) (1 : 1), and (AIII + Ce) (1 : 2) complexes. It can be seen that the KDP crystals doped with the (AC + Ce) complex have dichroism. Exposure of a crystal to light with λ = 300 nm leads to the fluorescence at λ = 340 nm, and excitation with light at λ = 470 nm causes long-wavelength fluorescence at λ = 650 nm. The emission band peaking at λmax = 340 nm is due to the interconfiguration transitions from the lower level of the 5d shell of Ce3+ ions to the levels of the ground state of the 4f configuration. The long-wavelength band (650 nm) is the fluorescence band of the bound ligand [12]; hence, it can be attributed to the emission of the (AC + Ce) complex. The crystals doped with the (AIII + Ce) complex demonstrate different spectral behavior. The fluorescence spectrum of such crystals contains one luminescence band peaking at λmax = 350 nm, which is excited by light with a wavelength of 250 nm. Since (AIII + Ce) complexes and the ligand are brightly colored (the complexes color a KDP solution dark cherry), the absence of crystal coloring and absorption bands of bound or unbound ligands in the spectra indicates that AIII and its complexes are not incorporated into the crystal. CRYSTALLOGRAPHY REPORTS
Vol. 53
No. 4
The preliminary experiments on irradiation of KDP:Ce3+ crystals with 252Cf neutrons and 60Co γ rays showed that the γ-ray and neutron spectra are not overlapped. The scintillator proposed performs effective separation of neutron/γ rays; hence, doped crystals can be used in scintillation technique.
2008
The conditions for growing KDP crystals in the presence of organometallic complexes with cerium (AC + Ce) and (AIII + Ce) have been determined. It is shown that, depending on the charge state of ligands in the solution, cerium ions are predominantly incorporated into prismatic (AC) or pyramidal (AIII) growth sectors. The effect of organometallic complexes on the crystal growth rate has been investigated. It is found that (AC + Ce) complexes do not block the crystal growth, whereas (AIII + Ce) complexes block the prismatic sector growth and retard the pyramidal sector growth. In the latter case, the formation of binuclear AIII complexes enhances the blocking effect. Apparently, this situation is related to the spatial structure of the AIII molecule and formation of a growth step echelon upon incorporation of a molecule into the interplane space of the crystal. The KDP crystals doped with cerium complexes exhibit photoluminescence with λmax = 350 nm, which is caused by the interconfiguration transitions from the lower level of the 5d shell of Ce3+ ions to the levels of the ground state of the 4f configuration. REFERENCES 1. A. P. Voronov, V. I. Salo, V. M. Puzikov, et al., Kristallografiya 51 (4), 742 (2006) [Crystallogr. Rep. 51, 696 (2006)]. 2. I. N. Ogorodnikov, V. A. Pustovarov, V. M. Puzikov, et al., HASYLAB Annual Report (HASYLAB, Hamburg, 2007), p. 439.
712
VORONOV et al.
3. M. E. Globus and B. V. Grinev, Inorganic Scintillators. New and Conventional Materials (Akta, Kharkov, 2001) [in Russian]. 4. I. A. Bondar’, Chemistry of Rare-Earth Elements. Compounds of Rare-Earth Elements: Silicates, Germanates, Phosphates, Arsenates, and Vanadates (Nauka, Moscow, 1983) [in Russian]. 5. A. P. Voronov, V. I. Salo, V. M. Puzikov, et al., Kristallografiya 51 (1), 169 (2006) [Crystallogr. Rep. 51, 150 (2006)]. 6. R. Pribil, Analytical Applications of EDTA and Related Compounds (Pergamon, Oxford, 1972; Mir, Moscow, 1975) [in Russian]. 7. S. A. de Vries, P. Goedtkindt, S. L. Bennett, et al., Rhys. Rev. Lett. 80, 2229 (1998).
8. B. Kahr and W. Gurney, Chem. Rev. 101, 893 (2001). 9. S. Hirota, H. Miki, K. Fukui, et al., J. Cryst. Growth 235 (1–4), 541 (2002). 10. N. M. Dyatlova, V. Ya. Temkina, and K. I. Popov, Complexons and Complexonates of Metals (Khimiya, Moscow, 1988) [in Russian]. 11. A. J. Gordon and R. A. Ford, The Chemist’s Companion: A Handbook of Practical Data, Techniques, and References (Wiley, New York, 1972; Mir, Moscow, 1976). 12. H. Kunkely and A. Vogler, Inorg. Chem. Commun. 10, 355 (2007).
Translated by Yu. Sin’kov
SPELL: 1. zwitterion, 2. orangish
CRYSTALLOGRAPHY REPORTS
Vol. 53
No. 4
2008
