R. K. QUINN, R. SIMMONS, AND J. J. BANEWICZ
230
The Magnetic Susceptibility of Tantalum Diselenidel"
by Rod K. Quinn, Robert Simmons,lb and John J. Banewicz Department of chemistry, Southern Methodist University, Dallas, Texas 76222 (Received August 6, 1966)
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The magnetic susceptibilities of compositions around TaSez have been determined as functions of temperature from 80 to 1250°K. The results suggest that the selenium-rich limit of the solid solution region close to TaSe2 extends only to TaSe1.g2. The behavior of the selenides is antiferromagnetic with a NBel temperature of 130°K. Above 300"K., the susceptibilities obey the CurieWeiss equation. With diamagnetic corrections of 20 X 10-6 ~m.~/mole for Ta4+and 48 X 10" cm.S/mole for Se-2, the Curie constant is 0.7" cm.a/mole and the Weiss constants are between 2400 and 2800".
Introduction The selenides of tantalum have been the subject of much recent experimental investigation, in most cases confined to crystal structure determination. Ariya, Zaslavskii, and Matveeva2¬ed a region of homogeneity in the range TaSel.oto TaSe2.0with lattice parameters changing negligibly in the range. They also observed a rapid decomposition of TaSe2 close to 900'. Brixner2b reported TaSe2 to have metal-like conductivity and to exist in two hexagonal forms, (Y and p. He stated that TaSe was also hexagonal with lattice parameters very close to the (Y form of TaSez. Aslanov, Ukrainskii, and Simanova also investigated TaSe2 and noted two forms, one very close to Brixner's cy, the other of a different structure, Furthermore, they observed traces of TaSeain compositions exceeding TaSel.98 in selenium content. Kadijk, Huisman, and Jellinek,4 in the most thorough study, described four different hexagonal forms of TaSez identified as 2s, 3s, 453, and 6s, indicating that the repeating units in the c direction had thicknesses of 2,3,4, or 6 TaSezslabs. The 3s form occurred when the preparation was carried out at 5oQ-60O0 and the 2s a t temperatures between 900 and 1 O O O O . Mixtures of 2s and 3s were obtained at intermediate temperatures. The 4s was observed only once. The 6s appeared as a sublimate at the top of the reaction tube. Excess tantalum could be incorporated in the octahedral interstices to fonn lower selenides. The 2s and 3s forms of Kadijk, Huisman, and Jellinek corresponded to Brixner's (Y and 6 TaSet and the 6s to one of the forms represented The J o u d of Physical Chemistry
by Aslanov, Ukrainskii, and Simanov. The seleniumrich limit of the diselenide was indicated by Jellinek to be very close to TaSez.oo. Traces of TaSea were observed in TaSe2.0r but not in the stoichiometric diselenide. We have been investigating the magnetic susceptibilities of many of the chalkogenides of transition metals. In spite of the existence of much information on the structure of the tantalum selenides, there seemed to be a lack of data on the susceptibilities. Therefore, an investigation was undertaken beginning with the selenides of tantalum with compositions close to TaSez.
Experimental Section Sample Preparation. Samples were prepared by the direct reaction in vacuo of the calculated amounts of selenium and tantalum in Vycor or quartz reaction tubes at 900-1000°. The selenium was 99.99901, in purity and the tantalum was estimated to be better than 99.95%. After 24 hr. at the reaction temperature, a fine gray powder, homogeneous in appearance, resulted with compositions below TaSa .oo in selenium content. I n higher selenides smaller amounts of a (1) (a) This research waa supported by the Robert A. Welch Foundation of Houston, Texas, under Grant N-056; (b) Welch Research Fellow, 1963-1965. (2) (a) S. M. Ariya, A. I. Zaslavskii, and I. T. Matveeva, Zh. Obshch. Khim., 26, 2373 (1956); (b) L. H. Brixner, J.Imrg. Nucl. Chem., 24, 257 (1962). (3) L. A. Aslanov, Y . M. Ukrainskii,and Y .P. Simanov, Zh. Neorgan. Khim., 8,180 (1963). (4) F. Kadijk, R. Huisman, and F. Jellinek, Rec. trav. chim., 83, 768 (1964).
MAGNETIC SUSCEPTIBILITY OF TANTALUM DISELENIDE
fibrous appearing material were observed in the reaction tube. After reaction, these samples were ground in an inert atmosphere. Both these and the lower selenides were annealed at 600' for at least 24 hr. before any measurements were made. To check the compositions, the selenium content of the samples wm determined by oxidizing the selenides and vaporizing selenium as Se02 at 4ooo, the tantalum content by cupferron. The agreement between preparation and analysis indicated an indetermination in the atomic ratio of selenium to tantalum of less than seven parts per thousand or k0.02 in the compositions close to TaSez. Magnetic Susceptz%ilities. As previously described, susceptibilities were determined by the Faraday method with a Sartorius vacuum microbalance to measure the forces and a Varian 4-in. electromagnet to generate the field.5 Measurements were made at field strengths of 1500,2400, and 2900 oersteds. When field dependence was observed, the values given are those extrapolated to infinite field strength. The susceptibility apparatus was calibrated using both (NH4)2Fe(S04)2.6H20* and H ~ [ C O ( C N S ) ~ ] . ~ X-Ray Diffraction Patterns. Room temperature powder X-ray diffraction patterns were made with a Norelco diffractometer using copper radiation and a nickel filter. The temperature was within 1' of 27' for all patterns. The 900' patterns were made with a Rigaku high-temperature camera 90 mm. in diameter, also with copper radiation. Since this camera permits multiple exposures on the same film,the first and last exposures on each film were those of a platinum wire standard.
Results and Discussion The first measurements made were on the selenide with composition TaSez.oz. These indicated antiferromagnetic behavior with a NBel peak at about 130OK. A rough cooling curve indicated that a maximum in the heat capacity also occurred at 130OK. Duplicate samples gave results which agreed within 3% over the entire range from liquid nitrogen temperatures to 1OOOO. Since extensive ability to accommodate excess tantalum had been reported, other compositions close to TaSez were synthesized. The low-temperature results obtained on samples ranging from TaSe1.g5to TaSe2.07 in composition are shown in Figure 1. In addition, data were taken on a selenide with composition TaSe2.22. The susceptibility was determined at only a few temperatures for this sample and therefore it is not shown in Figure 1. All samples were field dependent in the low-temperature region with the field dependence diminishing as the temperature increased
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and disappearing completely at about 250'. In the region of the NBel temperature, where the field dependence was greatest, the susceptibilities at 2900 oersteds were about 5% lower than those at 1500 oersteds. Furthermore, slight thermal hysteresis, of about 5%, was observed in the vicinity of the NQel temperature. The results shown are the average ones. The behavior in all cases is antiferromagnetic in nature with NBel temperatures in the vicinity of 130OK. A plot of two susceptibility us. composition isotherms is shown in Figure 2. Ordinarily, at any temperature the susceptibility of a mixture of two phases varies linearly with composition while the susceptibility vs. composition behavior of solid solution regions is often nonlinear. Discontinuous changes of slope in the isotherms occur at phase boundaries. The results in Figure 2 therefore suggest that the selenium-rich limit of the diselenide phase may not extend to TaSez.oo but only to about TaSel.g2. The susceptibility of TaSes was not determined in this work. However, Bjerkelund and Kjekshuss have reported a value of -0.13 X ~ m . ~ / gwith . little temperature dependence. It is interesting to note that if the linear portions of the isotherms shown in Figure 2 are extrapolated to the composition of TaSe3, values in good agreement with that of Bjerkelund and Kjekshus are obtained. The diffraction pattern of the samples showed them to be primarily the 2s form of TaSe2 as reported by Kadijt, Huisman, and Jellinek4 with a = 3.430 0.002 A. and c = 12.71 i 0.01 A, Faint but definite reflections of TaSeo as reported by Bjerkelund and Kjekshus were detected in TaSel.e, and other compositions richer in selenium. Therefore, the diffraction results support the suggestion that the limit of the diselenide phase does not reach TaSe2.00. I n TaSel.go and TaSel.gs,faint 3s reflections were also noted. In the region above room temperatures, the magnetic susceptibilities were determined for compositions corresponding to TaSel.H, TaSel .w, and TaSe2.02 (Figure 3). An apparent Weiss-Curie region exists from the NBel peak to about 1000OK. Above 1000°K., the abrupt changes in susceptibility signal either magnetic anomalies or phase changes. In practice, continuous recordings of susceptibility us. temperature were made
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(5) J. J. Banewica, R. F. Heidelberg, and A. H. Luxem, J . Phys. Chem., 65, 615 (1961). (6) P. W. Selwood, "Magnetoohemistry," Interscience Publishers, Inc., New York, N. Y., 1956. (7) B. N. Fig& and R. S. Nyholm, J . Chem. SOC.,4190 (1958). (8) E. Bjerkelund and A. Kjekshus, 2. anorg. allgem. Chem., 328, 236 (1964).
Volume 70, Number 1
January 1966
232
R. K.
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QUINN,
R. SIMMONS, AND J. J. BANEWICZ
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Temp., OK.
Figure 3. Susceptibilities of tantalum selenides with compositions close to TaSes as a function of temperature.
Figure 1. Low-temperature susceptibilities of tantalum selenides.
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2 0.3 a
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Atomic ratio of selenium t o tantalum.
Figure 2. Susceptibility us. composition isotherms of tantalum selenides.
in the high-temperature region. In TaSel.w and TaSe2,0z,the recorder plot showed clearly that the decrease in susceptibility took place in two stages, beginning at below 1100"K, leveling off, and then continuing again at a temperature between 1120 and 1140OK. In the case of TaSel.92,the leveling-off portion was not observed. Ariya, Zaslavskii, and Matveeva2a have reported that below 850 and 900" a rapid decomposition of TaSe2takes place with loss of selenium. We have treated a sample of TaSez.ooin vacuo at different temperatures. Loss of selenium occurs at The J o u d of Ph,ysicaE Chemistrv
600°K. but only to a sufhient extent to yield TaSel.92, which is stable even after 1 hr. at 1073°K. Further heating for 1-hr. periods in vacuo at 1123°K. gives TaSel.8, while at 1273'K. TaSel.n is obtained. The final drop in susceptibility which occurs at above 1100°K. therefore seems to correspond to the decomposition of the diselenide by the loss of selenium and the formation of some lower selenide. The resulting lower selenide has an extremely low susceptibility which, at least over the small range of temperatures observed, does not vary appreciably with temperature. In an attempt to elucidate the nature of the high-temperature selenide, powder X-ray diffraction patterns of TaSel.g2were made at 1200-1250°K. in a Rigaku high-temperature X-ray camera. The reflections observed, as shown in Table I, correspond to the 2s form of Kadijk, Huisman, and Jellinek,4 with a equal to 3.458 A. and c equal to 12.82 A. Therefore, the lower selenide formed on decomposition still retains the diselenide structure even though its magnetic susceptibility suffers a drastic decrease. The change in lattice parameters with temperature shows a linear coefficient of expansion of 9 X 10" for both the a and c axes. Aslanov, Ukrainskii, and Simanov8 have reported that TaSe3 decomposes to TaSe2 and selenium under vacuum at 1053 i 5'K. Even though smdl amounts of TaSea appear to be present, at least in the TaSe2.0z, the effect of the decomposition of the triselenide would not be large enough to be observed in the data shown in Figure 3. The first part of the two-stage drop in susceptibility may be due to the formation of the 6s form, or the more elusive 4s form, from the starting 2s.
MAGNETIC SUSCEPTIBILITY OF TANTALUM DISELENIDE
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Table I: 1200°K. Powder X-Ray Diffraction Data for TaSel.oz
2 X
a, 4. Obsd.
Celod.
hkl
Intensity
6.34 2.140 1.729 I.671 1.604 1.344 I.280 1.175 1.114 1.029 0.907 0.856 0.849
6.42 2.137 1.?29 1.670 1.603 1.344 1.282 I.176 1.115 I.030 0.909 0.857 0.849
002 006 110 112 008 116 0010 118 212 1110 1112 222 2110
vst st W W
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a a
B
vw st
vw vw W
vw m
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I 400
I
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I
600
I
I
800
I 1000
Temp., OK.
Figure 4. Curie-Weiss plot for tantalum selenides.
A plot of the reciprocal molar susceptibility vs. the absolute temperature is shown in Figure 4. The molar susceptibility was corrected for the diamagnetic contributions of the constituent ions using 20 X cm.a/ mole for Ta4+and 48 X for Se-2. Apparently, the Weiss-Curie relation does hold for all three compositions with values of the Curie constant of 0.717, 0.670, and 0.735 for TaSel.gz,TaSe1.9,, and TaSez.oz,respectively. The Weiss constants lie between 2400 and 2800°, considerably higher than expected for an antiferromagnetic substance with a NBel temperature of 130'K. I n
other chalkogenides of transition metals, temperatureindependent paramagnetism commonly occurs, and it seems likely that even in the antiferromagnetic diselenides the observed susceptibility includes the effect of a temperature-independent term which cannot be neglected considering the low magnitude of the total susceptibility. I n view of this, any calculations of magnetic moments from the Curie constants would have little significance.
Volume 70,Number 1 January 1966
