The preparation and characterization of the solid solution series CuFexGe1−xS2 (0.5 < x < 1.0)

JOURNAL OF SOLID STATE CHEMISTRY 19,75-80 (1976) The Preparation and Characterization CuFe,Ge,-,S, (0.5 < x < 1.0) J. ACKERMANN, S. SOLED, of the Solid Solution Series A. WOLD,* AND E. KOSTINERf * Department qf Chemistry, Brown University, Providence, Rhode Island 02912, and the TDepartment of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268 Received March 26, 1976 Single crystals of the solid solution series CuFe,Gel-,Sz (0.5 < x i 1.0) have been prepared by the chemical vapor transport technique. X-ray diffraction analysis and density measurements have indicated that all members of this system crystallize with the chalcopyrite structure. Mijssbauer spectra show that these crystals contain both iron(H) and iron(II1) on tetrahedral sites and that the iron concentration agrees with that determined by chemical analysis. Magnetic susceptibilities for x = 0.53 display antiferromagnetic behavior. The Nkel temperature of 12°K and an effective moment of 5.0 BM is observed, which approaches the calculated spin-only moment of 4.92 BM. As x increases, deviations from spin-only behavior occur, indicating complex magnetic interactions. prepare members of the intermediate solid solution series CuFe,Ge,& (0.5 < x < 1.0) either as polycrystalline powders or, preferably as single crystals. The members of this solution series would contain both Fe”’ and Fe” according to the stoichiometry CuFe,Ge,-,S, = CuFe~‘_,Fe~:‘_,Ge,-,S, (0.5 < x < 1.O). The growth, crystallographic, and magnetic properties of crystals which contain d5 and d6 iron on tetrahedral sites are reported. Introduction Chalcopyrite, CuFeS,, crystallizes with an ordered zincblende structure, space group 132d, in which all the atoms occupy slightly distorted tetrahedral sites (I). The metal atoms are located at the centers of sulfur tetrahedra, each corner of which is shared by two neighboring tetrahedra (Fig. 1). Both MGssbauer (2, 3) and neutron diffraction (4) studies indicate that chalcopyrite is antiferromagnetic with a Ntel temperature of 853°K. These studies also confirm that iron is present in the high spin d5 electron configuration (Fe”‘).’ Briartite, CuFeo.sGeo.&, has been reported to crystallize with a stannite-related structure in the space group Id2m (Fig. 2) (5). Wintenberger et a/. (6, 7) have shown that briartite orders antiferromagnetically at 12°K and contains iron in the high spin d6 state (Fe”). It would be of interest to see if it is possible to Experimental Single crystals of the solid solution series CuFe,Ge,-,S, (0.5 < x < 1.O) were grown by the chemical vapor transport technique (8). The elements were purified before use. High purity copper (Matthey-Johnson 99.999 %) was reduced in a 15 y0 HZ/85 ‘A Ar atmosphere at 600°C for 4 hr. High purity iron (Leico 99.9999%) was reduced in a similar atmosphere at 950°C for 20 hr. Sulfur (Atomergic 99.9999 %) was vacuum sublimed at 120°C and iodine (Baker 99.95 7;) at 50” to remove trace impurities. Electronic grade germanium (Gal- * To whom all correspondence should be addressed. 1 In this paper, Fen and Fe”’ will be used to denote the d6 and d5 electron configurations of iron, respectively. Copyright Q 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain 75 76 ACKERMAN ET AL. 0 cu 0 Fe OS @ Sn -Cl- Chalcopyr Stannite FIG. 2. The stannite structure, space group Ii12m. Note the ordering of the iron and tin atoms. ite FIG. 1. The structure of chalcopyrite, space group md. lard-Schlesinger 99.9999%) was used without further treatment. Nominal stoichiometries of Cu, Fe, Ge, and S were introduced into an II-mm i.d. silica tube and evacuated to 1 FmHg. After iodine (5 mg/cm”) was sublimed inside (9), the silica tube was sealed. The sealed tube was wrapped with a tightly wound Kanthal wire (0.057-in. diam.) coil to even out extraneous temperature gradients within the transport furnace. The tube was heated for 12 hr, during which the growth zone was held 100°C higher than the charge zone. This process back-transported all nucleation sites from the growth zone. After thermal equilibration of the growth and charge zones, the temperature of the growth zone was then reduced over a period of 5 days to the values given in Table I. After the growth period, the tube was cooled slowly to room temperature. The crystals were removed and washed with absolute ethanol in order to extract the iodine. Using this method, crystals were produced with dimensions 10 x 4 x 1 mm and weights up to 60 mg. The crystals grew as platelets, with colors varying from black, for those with low iron content, to gold, for those with high iron content. TABLE I CONDITIONS OF PREPARATION OF CuFe,Ge,-,S, Charge stoichiometry CuFeO&eO.St CuFeo.&ed32 CuF%.7Geo.3S2 CuRdkb.& CuJkd3eo.A Growth stoichiometry Transport agent CuFeo.53Ge0.47SZ 12 CuFeO.&eO.& 12 Cul%.&%& 12 CuFeo.s4Geo.l.& CuFeo.dhdS 12 12 Temperature gradient (charge zone-growth zone) Duration (weeks) (“C) 850-790 840-780 825-750 830-770 850-790 3 3 3t 3 3 SOLID SOLUTION SERIES Chemical Analysis In chemical vapor transport reactions of mixed solution series there is the possibility that the crystal growth stoichiometry differs from the charge (nominal) stoichiometry. To determine the true stoichiometry of the single crystals, iron analysis was performed using the photometric thiocyanate method (10, II). Known weights of the unknowns were dissolved in 50 ml of aqua regia. Oxides of nitrogen and sulfur were dispelled from the solution by boiling. Precipitated sulfur was removed by filtration. After 400 ml of water and 25 ml of 0.5 M potassium thiocyanate were added, the solution was diluted volumetrically to 500 ml with water. The absorbance at 480 pm of the red (Fe(H,O),SCN)‘+ complex was measured at once on a Cary 14 spectrophotometer. Weighed amounts of freshly reducedhigh-purity iron were dissolved and used as references.A 0.025 M potassium thiocyanate solution was used as the blank. The charge and growth stoichiometries are listed in Table I. 77 CuFe,Ge,-,S, Chicago Corp.). Isomer shifts are reported with respect to the zero position of a crystal of sodium iron(U) nitropentacyanide dihydratc, Na,(Fe(CN),NO)*2H,O (National Bureau of Standards, Standard Reference Material No. 725). The quadrupole splitting of the standard was taken as 1.7048mm/set (12). Results, X-Ray Measurements, and Densities X-ray powder diffraction patterns were obtained for ground single crystals of the members of the solution series CuFe,Ge,-,S, (0.5 < x < 1.0) with a Norelco diffractometer using monochromatic radiation (AMR Focusing Monochromator) and a high-intensity copper source (CuKor, = 1.5405A). Scanning rates of 1a 20/min and *” 28/min were used. High purity silicon (a, = 5.4301 A) was added as an internal standard in order to obtain precise lattice dimensions. Both stepcounting on ground crystals and long-exposure precession techniques on single crystals were used to search for reflections causedby the ordering of the iron and germanium atoms. Observation of the (101) reflection and the absence of the (110) and (002) reflections for all members MSssbauer Measurements of this seriesshows that the members adopt the The iron-57 Mossbauer spectra of ground I32a’ chalcopyrite space group in which the singlecrystals of CuFe,Ge,_,S, (0.5 < x < 1.O) Fe”, Fe”‘, and Ge atoms are distributed were measuredwith a model NS-1 Mossbauer randomly upon the Fe sites of chalcopyrite. spectrometer (Nuclear Science and Engineer- The precision lattice constants for the meming Corp.) operating in the constant accelera- bers of this series are given in Table II. The tion mode. The 14.4-keV radiation emitted introduction of iron into briartite, CuFe,.,from 20 mCi of 57Co diffused into Pd was Ge0.5SZ,replaces germanium with the larger detected with a gas proportional counter and FelI1 atoms. In order to maintain electroneucollected with a 400-channel analyzer (Nuclear trality, some of the Fe” is converted into TABLE II CRYSTALLOGRAPHIC PARAMETERSAND DENSITIESFOR CuFe,Ge,-,S, x 0.53 0.65 0.75 0.84 0.95 1.0 c (4 v (A”) P&S PEdC 10.531(l) 10.536(l) 10.531(l) 10.517(l) 10.488(l) 10.409(l) 299.4 299.7 298.8 295.8 293.2 290.2 4.25(l) 4.20(l) 4.18(l) 4.19(l) 4.17(l) 4.18(l) 4.25 4.20 4.17 4.18 4.17 4.18 a (4 -__ ~__ 5.332(l) 5.334(l) 5.327(l) 5.304(l) 5.287(l) 5.280(l) :8 ACKERMAN Il.5 E.hE 6.K FIG. 3. The variation composition. x 8.72 R.ES ET AL. changes in the a,- and c,,-axeswith stoichiometry are shown in Figs. 3 and 4 and disp!ay a nonlinear behavior. Densities of single crystals of the solid solution series CuFe,Ge,-,S, (0.5 < x < 1.0) were obtained by the hydrostatic technique (13) using pertluoro-(1-methyldecalin) as the fluid medium. Prior to each measurement,the fluid was calibrated against a single crystal of silicon with a density of 2.328 g/cm”. All measurements were made at 24 f 0.2”C and 31 + 3 % relative humidity. The observed densities are given in Table II with the values calculated from the analyzed stoichiometries and observed lattice constants. 8.9s of the a0 parameter with Miissbauer Spectra - Il.%+ Mossbauer spectra of the members of the solid solution series CuFe,Ge,-,S, where x = 0.53, 0.65, and 0.84 have been obtained. The spectra display two quadrupole sp!it absorptions. From the quadrupole splittings it was possible to assign one of the doublets to Fe” (QS 2.52-2.57 mm/set) and the other doublet to Fen’ (QS 0.46-0.56 mm/set). The ratio of the area under the Fen’ doublet to the area under the Fen’ + Fe” doublets (Fe”‘/ Fe,,,,‘) is presented in Table III, together with the ratios calculated from chemical analysis. Observation of the two distinct doublets displays the presenceof both Fen and Fe”’ in the chalcopyrite structure. Furthermore, the agreement between observed and calculated Fel’VkotaI ratios shows the concentrations of Fe” and Fe”’ to be consistent with chemical D D ll.SZ.. D D 18.99.. .z .z ”” 00 18.96.. 18.9.. ID D 18.W. Il.92 B.SS 8.62 x 1.72 FIG. 4. The variation composition. 8.82 I.% of the cg parameter with smaller Fe”‘. In such double substitutions the change in lattice dimension with stoichiometry may be expected to be complex. The TABLE III THE ISOMER SHIFTS (IS) AND QUADRUPOLE SPLITTINGS (QS) x = 0.53,0.65, FOR MEMBERS OF THE SERIES AND 0.84” CuFe,Ge,-,Sz WHERE Ratio of Fe”‘/Fe tota, Inner peaks (Fe”‘) Outer peaks (Fe”) x IS Qs IS QS Mijssbauer Analysis 0.53 0.65 0.84 0.568 0.546 0.567 0.55 0.47 0.49 0.884 0.857 0.858 2.52 2.57 2.48 0.13 0.42 0.70 0.11 0.46 0.81 a Ail spectra were recorded at 22 f 2°C. CuFe,Ge,-,S, SOLID SOLUTIONSERIES analysis. For members of the solution series with x 2 0.84, a six-line magnetic hyperfine spectrum is seen; however, the lack of resolution of the peaks in this spectrum prevents an accurate determination of the area ratios and the Fe” and Fe”’ assignments. Magnetic Measurements Magnetic measurements were made using the Faraday balance described by Morris and Wold (14). For every sample the bulk magnetic susceptibility was measured as a function of field strength at 300 and 77°K (HondaOwens method; 15, 16). The results are shown in Figs. 5 and 6. The lack of any field dependence indicates the absence of ferromagnetic impurities or bulk ferromagnetic order in these 79 Z-,’ : giY 21.. _I,,’.*.*: ,l > ,, -- .* E Iq.’ ,‘ ..* ,’ ,’ z ,1.’ *.- Do 7.. ,,I’ **.. __-‘*.... .a.-/ e... ‘_ _:..:.__ I ~Lb,,nl~bdLn~~noDI~oD~~nn~ .,AfAA~~Yh X18 ~ @a0 4b04” ; 68.. $ -no 0” FIGS. 7 and 8. x,’ versus temperature for members series Cu(Fe,Ge,-,)S,. 0.0, Cu(Feo.&ecdSZ; ---, Cu (Feo.&eo.&h; 0 0 0, Cu(Feo.75Ge~.&2; 000, Cu(Feo.&eo.ldS2; A A A, Cu(Feo.95Ge0.0s)S2. of FIGS. 5 and 6. The variation of the molar susceptibility with field strength at 300‘K (Fig. 5) and at 77’K (Fig.6)forCu(FexGe,-,)S,.(A)x =0.53;(B)x = 0.65; (C) .Y= 0.75; (D) x = 0.84; (E) s = 0.95. the solution samples. The susceptibility was also measured as a function of temperature from 3 to 575°K. The plot of inverse susceptibility vs temperature for CuFe0.53 Ge0.47S2 is shown in Fig. 7. This material shows antiferromagnetic order with a N&l temperature of 12°K and a paramagnetic moment of 5.0 BM, which is consistent with a spin-only moment due to iron in the high spin d6 configuration. For materials with a greater Fe concentration, complex magnetic behavior involving Fe” and Fe”’ situated randomly in a chalcopyrite structure vs appears. In the inverse susceptibility temperature curves (Figs. 7 and S), this complex magnetic behavior is observed by a broadening of the Neel points as well as a reduction of the paramagnetic moment from the spin-only values. The magnetic parameters for these materials are summarized in Table IV. 80 ACKERMAN TABLE ET AL. IV MAGNETIC PARAMETERS OF CuFe,Gei-,S2 x e (“K) TN (“W Perr(obs)(BM) Perr<eaic) (BM) Temperature of paramagnetic behavior (“K) 0.53 0.65 0.75 0.84 0.95 -12 33 80 112 12 16 16 35 4.99(2) 4.30(2) 3.56(2) 2.81(2) 5.00 5.36 5.57 5.72 40-575 100-575 185-575 300-575 1.0 NCel temperature and paramagnetic region above decomposition temperature 853 (3, 18) Bull. Sot. Acknowledgment 6. J. ALLEMANLI The assistance of the Army Research Office, Durham, North Carolina, the Materials Research Laboratory, Brown University, and the University of Connecticut Research Foundation is acknowledged. The authors would like to thank Mr. Don Schleich for communicating the technique of using wire coils to improve crystal growth by chemical vapor transport. 7. P. IMBERT,F. VARRET, AND M. WINTENBERGER, References I. N. W. BUERGER AND M. J. BUERGER, Amer. Mineral. 19, 289 (1934). 2. E. FRANK, II, Nuovo Cim. B 58,407 (1968). 3. M. DIGUISEPPE, J. STEGER, A. WOLD, AND E. KOSTINER, Znorg. Chem. 13,182s (1974). 4. CORLISS, J. ELLIOT, AND J. HASTINGS, P&x G. DONNAY, N. L. M. D. H. DONNAY, Rev. 112,1917 (1958). 5. W. SCHAEFER AND R. NIETSCHE, -645 (1974). Mat. Res. Bull. 9, AND M. Fr. Mineral. Cristallogr. WINTENBERGER, 93, 14 (1970). J. Phys. Chem. Solids 34,1675 (1973). 8. H. SCHAEFER,“Chemical Transport Reactions,” Academic Press, New York (1964). 9. A. FINLEY, D. SCHLEICH, J. ACKERMANN, S. SOLED, AND A. WOLD, Mat. Res. Bull. 9, 1665 (1974). 10. G. LUNDELL, H. BRIGHT, AND J. HOFFMAN, “Applied Inorganic Analysis,” Wiley, New York (1953). II. “Standard Methods of Chemical Analysis,” Vol. 1, Van Nostrand Princeton, N.J. (1962). 12. R. W. GRANT, R. M. HOUSELEY,AND U. GONSER, Phys. Rev. 178, 523 (1969). 13. R. L. ADAMS, Ph.D. thesis, Brown University (1973). 14. B. MORRIS AND A. WOLD, Rev. Sci. Znstrum. 39, 1937 (1968). IS. K. HONDA, Ann. Phys. (N. Y.) 32,1048 (1910). 16. M. OWENS, Ann. Phys. (N. Y.) 37, 657 (1912). 17. T. TERANISHI, J. Phys. Sot. Japan 16,188l (1961).