ITSE/CNR, P.O. Box 10, 1-00016 Monterotondo Stazione, Italy. The growth of CoGa2O4 and ZnCr2O4 spine! single crystals by means of the chemical vapour ...
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Journal of Crystal Growth 79 (1986) 410—416 North-Holland. Amsterdam
CRYSTAL GROWTH, THERMODYNAMICAL AND STRUCTURAL STUDY OF CoGa2O4 AND ZnCr2O4 SINGLE CRYSTALS F. LECCABUE and C. PELOSI MASPEC/CNR Institute, Via Chiavari 18/A, 1-43100 Parma, Italy
and E. AGOSTINELLI, V. FARES, D. FIORANI and E. PAPARAZZO ITSE/CNR, P.O. Box 10, 1-00016 Monterotondo Stazione, Italy
The growth of CoGa2O4 and ZnCr2O4 spine! single crystals by means of the chemical vapour transport technique using chlorine as a transport agent is reported here. A thermodynamical study of the CoGa2O4—Cl2 system has been performed in order to define the growth conditions, and structural investigations using X-ray diffraction are carried Out. Moreover, the magnetic and electronic properties of the polycrystalline powdered samples were studied by means of magnetic susceptibility measurements, X-ray photoelectron spectroscopy and X-ray induced Auger spectroscopy.
1. Introduction It is well known that the flexibility of the spine! lattice with respect to the cation distribution is responsible for the large variety of physical properties observed in spinels. From the point of view of the cation distribution the CoGa2O4 and ZnCr2O4 compounds of two different 2 + N~are ±04examples spine! system, partially types of the M inverse M 1 XNX(N2 M~)X4 spine!, and normal spine! M.N2 X4, respectively (the octahedra! sites are within brackets and the tetrahedral ones outside). This distribution is responsible for cornpletely different kinds of magnetic behaviour, Up to date, few works have reported on the crystal growth of ternary oxides and in particular on the chemical vapour transport technique (CVT) of ternary spinel oxides [1—61.In general the crystal growth of the oxide compounds by the CVT method presents additional difficulties compared to su!phides, selemdes and tellurides [1]: i.e., (i) a higher growth temperature is needed, (ii) chemical reactions with quartz walls of the ampoule may occur producing a high density of nucleation points and the formation of silicates, and (iii) difficulties, —
—
connected with the use of gaseous transport agents such as chlorine or hydrochloric acid, are encountered. In this paper we report the growth of CoGa 204 and ZnCr2O4 spine! single crystals by means of the CVT technique in a closed tube, using chlorine as a transport agent. A thermodynamical study has to define the growth conditionsbeen and undertaken structural investigations performed by X-ray diffraction are reported. Moreover, the magnetic and electronic properties were studied by means of magnetic susceptibility measurements, X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger spectroscopy, performed on polycrystalline powdered samples, used as starting material to grow the crystals.
2. Crystal growth and thermodynamical study The chemical vapour transport technique has proved to be the most suitable method for growing oxide and calchogenide spinel single crystals of high purity and perfection. Although the transport efficiency is higher at higher growth tempera-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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ture, in our experiments we use intermediate temperatures since both the chlorides and oxides of the III and VI group metals may react with the quartz wall of the ampoule. The vapour phase reaction of gaseous metal chlorides with oxygen in an open system [5] and several transport agents containing chlorine such as CrC!3, ZnC12 [7] may be used. However, in the latter case there could be some drawbacks since a deviation in the compound stoichiometry can affect both the growth rate and the solid phase stability. Moreover, if metal chlorides different from those which form the compound are introduced, effects on the physical properties are expected due to the heavy contamination. The source materials were prepared by solid state reaction of binary oxides in air. These were annealed at 800°Cfor one day, then ground and annealed again at 1200°Cfor two days. The starting oxides had the following purities: ZnO (99.99%), Cr203 (99.999%), Ga203 (99.9997%), CoO (99.99%). The polycrystalline materials were sealed in a previously evacuated (10—6 Torr) quartz ampoule under a chlorine pressure of about 200 Torr. The experimental growth conditions were the following: chlorine pressure at room temperature 200 Torr, average temperature 925 °Cand temperature difference 50—70°C, for the CoGa2O4 chlorine pressure at room temperature 250—300. Torr, average temperature 975°C and temperature difference 50—70°C, for the ZnCr1O4. The growth time was two weeks. The quartz ampoule dimensions were: length 18 cm and internal diameter 1.5 cm. The furnace had eight heating zones thus different temperature gradients and profiles were possible. The distance between source and grown crystals was 9—10 cm. The ZnCr2O4 2and in CoGa2O4 crystals were generally I >< 1 mm size, black metallic in colour and had an octahedral shape. Some crystals, after aluminium coating, were examined by scanning electron mlcroscopy (SEM) and the micrographs are shown in figs. la and lb. The ZnCr2O4—Cl2 system has already been studied [6], but for our growth we have used temperatures and chlorine pressures lower than those reported in [6]. The CoGa204—C12 system
4 and ZnCr2O4 single crystals
ig. I.
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SEM
niicrographs of single crystals obtained by (a) (‘O(id 204 and (b) ZnCr2O4 spinels.
CVr:
has never been studied. In order to predict the optimal range of the growth conditions, i.e. temperature and chlorine concentration, we carried out a thermodynamical study of the CoGa204—Cl2 system; we have taken into account the following set of reactions: CoGa 2O4 + 2 Cl = CoC1 + 2 GaC1 + 2 0 (1) 2
CoC12 + 4C12 = CoC13, 2 CoCl2 = Co2Cl4, G Cl + Cl G Cl a 2 a Cl2 + ~02 C120, ~Cl + 0 = ClO
2
2’
(2) (3)
—
2
2
2
(4) (5) 6
2’
which resulted in a set of non-linear equations of
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and ZnCr2O4 single crystals
the type: (7) (,J
where i = 1, 6 according to the reaction considered; f is extended to all species present in the ith reaction and q~is the reaction coefficient of the jth species. The equation system is completed by the following linear equations:
5 6
. . . ,
P(CoCl2) + P(CoC13) + P(Co2C14) + P(GaC1) +P(GaCI3) + P(02) + P(C12)
+
P(C120)
+P(Cl02)=P10~, P(GaC1)
+
P(GaCl3)
(8) =
P(02)
+~
P(C120)
+P(C102), 2[P(C0C12) =
+
P(GaC1)
P(CoCl3)
+
+
(9)
2 P(Co2Cl4)}
P(GaCI3),
(10)
which represent Dalton’s law (eq. (8)) and the stoichiometric constraints existing in the solid phase (eqs. (9) and (10)). The enthalpy and entropy values used to calculate the K, are reported in table 1. Emmenegger [1] discussed the mass transport efficiency of these ternary oxides as a function of spinel formation energy values obtamed from its own oxides. This author pointed out that the mass transport can be successfully
T (‘C)
Fig. 2. Partial resulting fromasthe heterogeneous equilibrium in the pressures CoGa2O4—C12 system a function of temperature, when the total pressure is setted at 1 atm: (1) P(CoCI2); (2) P(C0CI3); (3) P(C02CI4); (4) P(GaCI); (5) P(GaCI3); (6) P(02); (7) P(C12); (8) P(C120); (9) P(C102).
Table 1 Thermodynamical data used in the present3T+ calculations: c X105/T2 the+formation ax 10_6 enthalpies T2 (in cal/mo!) (z~H1)and entropies (LtS~)and the heat capacity (Cr) given by the expression: a + b x i0~ Species ZtHf Z~Sf C~ Refs. (kcal/mol) (u.e.) a b c d CoCI 3 CoCI2 Co2Cl4 GaCI GaC13 Cl2 02 CI~0 C102 Ga203 CoO CoGa2O4
—39.1 —22.575 —83.8 —19.54 —108.4 0 0 19.2 24.5 —260.3 —57.1 —321.4
~ Estimated (see text).
79.8 71.3 107.6 57.36 79.7 53.288 49 63.6 61.36 20.31 12.65 32.96
21.458 13.961 31.754 9.082 17.21 8.76 7.016 10.85 10.03 26.98
—1.539 1.789 0.384 0 2.47 0.26
—2.499 —0.174 —1.308 —0.481
3.69
—5.02
26.98
3.69
—5.02
—0.65
0.49 —0.511
[9]
191 [9] [9] 1101 [101 [11] [111 [11] [91 [121 Est. ~
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CoGa,O
4 and ZnCr2O4 single crystals
performed only when the formation energy of spinel compounds assumes large negative values, When the formation energy obtained from the reaction between the binary oxides is positive, one has to consider the complex system resulting from a randomly disordered alloy [13]. In the case of CoGa 204, on the basis of the relationship existing between the cation preference energy and the formation enthalpy [14], one can expect a formation enthalpy value as low as about 4 kcal/mol. As a consequence, it is necessary to grow the crystals at a relatively low temperature order to reduceand the mixing of metallic cations onin the tetrahedral octahedral sublattices. In fig. 2 the partial pressures inside the growth system, under a total pressure of 1 atm, are shown. The main contribution to the mass transport of the elemental components is given by CoCI 2, GaCI3 and 02. More complex gaseous species may be present, i.e. CoGa2Cl3 [15], but their contribution to the mass transport is expected only at lower temperatures. In fig. 3 the normalized transport rate function, dr/d T, calculated by means of numerical methods, is shown. The mass transport rate, neglecting the geometrical and the diffusion constants, was calculated according to the formula given in ref.
/
j
/
// ~
__________________________________________ 800
Trc)
1100
1200
Fig. 3. The normalized transport rate function (dr/dT) is plotted against the temperature. A larger transport rate is obtained at temperature as high as 1000°C.
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[16]. This plot shows that a larger transport rate is expected to occur at a higher temperature than that used for the experiments.
3. Characterization 3.1. Structural data CoGa2O4 (molecular weight 232.37) crystallizes in the cubic space group Fd3m; lattice parameter 3, D~ = 6.01 g cm3, a = 8.339(4) A, V= u579.8 A anionic parameter = 0.2588(10) A. A Nicolet automatic four-circle diffractometer using a graphite-monochromated Mo Ka radiation (A = 0.71069 A) was utilized for data collection at room temperature. The unit cell dimensions were obtamed by a least-squares refinement of setting angle for 15 reflections with 29> 60°.The intensity of 75 independent reflections with I> 3a(I) were used to refine the parameters u, but not the cation site occupancy (CSO), owing to the proximity of the atomic number of cobalt and gallium. The experimental parameters a and u are in good agreement with those obtained from powder neutron diffraction [17],which allowed the determination of the cation distribution, showing a partial 2~ions on the tetrainversion with 40% of the Co hedral sites. By assuming this value to be the equilibrium one at the annealing temperature of 1473 K, the approximated equation given in ref. [14] yields a preference energy difference, ~% P, of —
ZnCr
2O4 (molecular weight = 233.360): space group Fd3m, 3. a = 8.359(5) A, V= 584.1 A~,D~= 5.31 g cm 3.2. XPS and Auger spectroscopy analysis X-ray photoelectron spectroscopy and X-ray induced Auger spectroscopy were chosen for studying the electronic structure of the above compounds. XPS results on ZnCr 2O4 have already been discussed [8]. Fig. 4 shows a survey of photoelectron and Auger lines obtained by exposing CoGa2O4 to AIKa12 radiation. Data obtained from detailed measurements in the broad spectral region are
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CoGa
2O4 and ZnCr2O4 single crystals
Ga2p
Ga Auger L3M45M45
Co2p
L2M45M45~~ )s
fOAuger
~
)
xl]KL2/
Valence band _______1____ 1500 1400
I 1100
1000~
700 -If 500
400
I 300
KINETIC ENERGY (eV) Fig. 4. Survey spectrum of CoGa2O4 showing photoelectron and Auger peaks. XPS and Auger spectra have been taken with a VG ESCA 3 Mk II spectrometer, equipped with an Al Ku12 source (hi’ = 1486.6 eV).
listed 2P in table 2. Both the binding energy of Co 3/2 and the presence of a broad, intense shake-up feature, about 6 eV distant from the 2± principal lead environment us to the identification Co ions in anline, oxidic [18]. The of binding energies relevant to gallium indicate the presence of Ga3~bond to 02_ ions [8,19]. Auger data for gallium and oxygen lines confirm the above conclusions and are useful for characterizing in more detail the electronic structure of CoGa 2O4. Unfortunately, we were unable
to collect Auger data for cobalt because of the low peak intensity of the Co L3MM series. Combination of Auger photoelectron kinetic energies (EKE),energy and (A theKE), characteristic energy of the radiation used to excite the sample (hv), leads to the definition of the Auger parameters [19]: a = AKE + ~KE + hv. a values of gallium and oxygen measured in CoGa 204 show differences of + 1.9 and + 2.0 eV, respectively, when compared with the values obtained in reference measurements on Ga203, while the analogous differences
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CoGa
2O4 and ZnCr2O4 single crysta!s
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Table 2 Binding energies, Auger energies and Auger parameters; the data are in eV; measurements of peak maxima are accurate to ±0.1 eV; binding and Auger energies have been corrected for static charging by taking the contaminant C Is line as lying at a binding energy equal to 285 eV Binding energies Cr2p3/l 577.4
ZnCr2O4 °~
CoGa 2O4
Cr2p1/2 587.3
Binding 2P3/2energies Co 781.3
Cr3p 44.6
Ols 531.4
Ga 2P3/2
Ga 2P3/2
Ga 3p
Ga 3d
0 Is
1118.5
1145.5
106.5
20.0
531.3
Auger energies CoGa2O4
GaL3M45M45 1063.1
CoGa 2O4 °~
GaL2M45M45 1090.2
OKVV 511.3
Auger parameters, a 2P3/2
0 KVV, 1s
Ga L3M45M45, 2181.6
1042.6
OKL1V 489.4
From ref. [81.
in binding energy of the core lines were only +1.4 and + 1.3 eV, respectively. We have also calculated surface atomic ratios by applying a “First Principles Model” [20] to photoelectron peak intensities in CoGa2O4. We have considered the shake-up satellite intensity as part ofWhile the true 2P3/2 transition. the intensity of the Coatomic ratio, 1.87, is in agreemeasured [0]/[GaI ment with the theoretical value of 2.00 within a relative error of less than 10%, i.e. within the typical uncertainty of the quantitative method, the [0]/[CoJ atomic ratio, 6.12, shows a significant Co2 ± depletion on the surface of the sample, as high as about 50%. 3.3. Magnetic Properties 2 + ions in both CoGa 2°4’ The presence sites of Co tetrahedral and octahedral implies the coexistence of different magnetic interactions: A—A, B—B and A—B. Recent neutron diffraction experiments [17] indicate that the nearest neighbour B—B interactions are ferromagnetic, while the A—A and A—B interactions axe antiferromagnetic. The competition between these interactions prevents the establishment of a long range magnetic order (neutron
diffraction data do not show magnetic reflections down to 1.5 K), while it stabilizes a spin-glass stage at a low temperature, as shown by a peak in the AC susceptibility curve at a frequency dependent temperature (TF = 9.77 K at v = 11.2 Hz; TF = 9.91 at a’by= 112 Hz; TF = 10.53 K at (the a’ 1120 Hz) Kand irreversibility properties difference between field-cooled and zero fieldcooled DC susceptibility curves). This is in agreement with the general magnetic phase diagram theoretically predicted for spinels [21]. ZnCr 3 + ions occupy octahedral sites 2O4.ZnCr The Cr only. The 2O4 spine! is an antiferromagnet showing a first order phase transition at TN = 13 K. The results of our low temperature susceptibility measurements are reported elsewhere [22,23]. 4. Conclusion Single crystals of CoGa 2O4 and ZnCr2O4 spinels have been grown by means of chemical vapour transport technique, using chlorine as a transport agent. Because of the low value of formation enthalpy of CoGa2O4, estimated from Xray data, from sintered powder, it is necessary to grow this oxide at low mass-transport regime, i.e.
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at relatively low temperature and low transport agent concentration. The preference energy difference was calculated to be 1.3 kcal/mol from X-ray diffractometry measurements on samples quenched from 1200°C;these samples showed a 40% cobalt inversion on tetrahedral sites. Moreover, photoelectron peak intensity evidenced a surface depletion of cobalt as high as 50%. Finally, in the CoGa 204 spinel the spin-glass state was evidenced at low temperature (TF = 10 K) by the AC susceptibility measurements at a frequency dependent temperature and by the magnetic property irreversibility. —
Acknowledgements G. Cossu and G. Righini (Servizio ESCA, Area della Ricerca di Roma) are thanked for technical assistance with the XPS and Auger spectra. Finally, we thank R. Panizzieri and P. Filaci for technical assistance in crystal growth and in magnetic susceptibility measurements, respectively.
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and ZnCr2O4 single crystals [5] S. Ito, H. Miyashita and N. Yoneda, in: Proc. Intern. Conf. on Ferrites, Japan, 1980, p. 733. [61A. Pajaczkowska, W. Piekarczyk, P. Peshev and A. Toshev, Mater. Res. Bull. 16 (1981) 1091. [7] H Von Philipsborn, J. Crystal Growth 9 (1971) 296. [8] C. Battistoni, J.L. Dormann, D. Fiorani, E. Paparazzo and s. Viticoli, Solid State Commun. 39 (1981) 581. [9] I. Barin, 0. Knacke and 0. Kubaschewski, Thermodynamical Properties of Inorganic Substances, Suppi. (Spnnger, Berlin 1977). [10] D.W. Shaw, J. Phys. Chem. Solids 36 (1975) 111. [11] NBS, Selected Values of Thermodynamic Properties (Washington, DC, 1969). [121 0. Kubaschewski, E.Ll. Evans and C. Alcook, Metallurgical Thermochemistry (Pergamon, Oxford, 1967). [13] F. Leccabue, R. Panizzieri and C. Pelosi, Mater. Chem. Phys. 9 (1983) 71. [14] S. Krupicka and P. Novak, Oxide Spinels, in: Ferromagnetic Materials, Vol. 3, Ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1982).
[151H.
Schafter and J. Karbinski, Z. Anorg. Allg. Chem. 403 (1974) 116. [161 C. Paorici, C. Pelosi, G. Attolini and G. Zuccalli, J. Crystal Growth 28 (1875) 358. 1171 J.L. Soubeyroux, D. Fiorani and E. Agostinelli, J. Magnetism Magnetic Mater. 54—57 (1986) 83. 1181 N.S. McIntyre and MG. Cook, Anal. Chem. 47 (1975) 2208. [19) CD. Wagner, Faraday Disc. Chem. Soc. 60 (1975) 291; C.D. Wagner, D.A. Zatko and RH. Raymond, Anal. Chem. 52 (1980) 1445. 1201 C. Battistoni, G. Mattogno and E. Paparazzo, Surface Interface Anal. 7 (1985) 117. [21] C.P. Poole, Jr. and H.A. Farach, Z. Physik B 47 (1985) 55. [22] D. Fiorani and S. Viticoli, J. Magnetism Magnetic Mater. 49 (1985) 83. [231 D. Fiorani, I. Phys. C17 (1984) 4837.
