Chem. Mater. 1993,5, 17-22
17
Art ic 1es Synthesis and Characterization of a Family of New Intermetallic Materials M5(InTe& (M = Cr, Mn, Fe, Co, Ni) Jian H. Zhang,la Biao Wu,lb and Charles J. O’Connor*Jb Departments of Chemistry, Xavier University, New Orleans, Louisiana 70125, and University of New Orleans, New Orleans, Louisiana 70148 Received May 8, 1992. Revised Manuscript Received October 15, 1992
A family of new amorphous intermetallic materials, Ms(InTe4)2, where M = Cr, Mn, Fe, Co, Ni, has been prepared by solution reaction of the Zintl polyanion InTe4“ with transition-metal cations M2+. The magnetic and conducting properties for these materials have been examined as a function of temperature. Mns(InTe4)n and Crs(InTe4)z exhibit strong antiferromagnetic coupling between localized magnetic moments and highly temperature dependent semiconductivity. The variable range hopping mechanism is found to dominate the conductivity in Crs(InTe4)z over a wide temperature range. Fes(InTe4)~undergoes double magnetic transitions, the para- to weak ferromagnetic transition a t -32 K and the second transition to a spin glass state a t -13 K. The magnetic properties of Cos(InTe4)~are dependent on the preparation conditions. Both Fes(InTe4)~and Cos(InTe4)z display conductivity consistent with a narrow energy gap semiconductor. Nis(InTe4)z shows a non-CurieWeiss paramagnetism and con., ductivity in the boundary of metals. It appears that the magnetic and conducting properties of these materials are roughly correlated with the ability of the precursor transition metal cations to be reduced by the Zintl polyanion. Introduction Recent reports from our lab~ratoryl-~ have described the synthesis of novel molecular solids from the oxidation of Zintl anions in solution. For example, the series of solids of the formula MzSnTe4 was obtained from the reaction of the ternary Zintl phase material K4SnTe4 with transition-metal halides in solution.1*2 Zintl materials are “valence” compounds; the metals and metalloids that comprise the material will transfer or share electrons in order to achieve an octet in the outermost electron This often results in the formation of covalently bonded polyatomic anionic units, referred to as Zintl anions. The octet valence shell of the Zintl phase arises when the more electropositive metals (i.e., alkali and alkaline earth metals) are allowed to react with the less electropositivepost transition main group metals and metalloids. Because of the octet rule, Zintl phases are formally semiconductors. In practice, however, the bandgap may be sufficiently narrow that the material behaves as a metallic conductor, or large enough that the material will effectively be an insulator. Zintl materials lie at the boundary between intermetallic alloys and ionic salts and (1) (a) Xavier University. (b) University of New Orleans. (2) (a) Haushalter, R. C.; O’Connor, C. J.; Haushalter, J. P.; Umarji,
A.M.; Shenoy, G. K. Angew. Chem. 1984,97,147. (b) Haushalter, R. C.; OConnor, C. J.; Umarji, A. M.; Shenoy, G. K.; Saw, C. K. Solid State Commun. 1984,49,929. (3) Zhang, J. H.; v d u y n e v e l d t , A. J.; Mydosh, J. A,; O’Connor, C. J. Chem. Muter. 1989,1, 404-406. (4) Schiifer, H. Annu. Reu. Muter. Sci. 1985, 15, 1. ( 5 ) SchSer, H.; Eisenmann, B.; Muller, W. Angew. Chem. Znt. Ed. Engl. 1973,12, 694 and references therein. (6) von Schnering, H. G. Bol. SOC. Quim. 1988, 33, 41. (7) Corbett, J. D. Chem. Reu. 1986, 85, 383 and references therein.
may have a combination of properties related to both classes of materials. There have been relatively few reports in the literature that characterize the chemical reactivity of Zintl phases. Much of the chemistry that has been reported sofar results from the ionic character of the Zintl phase and the reaction of Zintl anions with other salts. Because of the difference in the electronegativityof the elements that comprise a Zintl phase material, there is often a great deal of ionic character in the Zintl phase material, and this ionic character may be sufficientto allow the solvation of salt like ions (cations and Zintl anions) in polar solvents (e.g., KSn dissolves in liquid NH3 and &SnTe4 dissolves in H20).Thus, the reaction of solutions of these Zintl phases with transition metal cations offers a new synthetic route to novel intermetallic solids. The reaction consists of electron transfer from Zintl polyanions with very high chemical reactivity to transition-metal cations, which results in the rapid precipitation of the neutral solid product. The intermetallic solids formed are often amorphous and metastable. By using this simple metathesis reaction with K4SnTe4, a seriesof ternary metal chalcogens of the formula MzSnTe4 (M = Cr, Mn, Fe, Co, Ni, and Cu) have been prepared and characterized. These materials exhibit some remarkable properties including specificresistivity range from IO4 to IOm43 cm, a spin glass transition at temperatures ranging from 5 to 18 K, a photomagnetic effect in FezSnTer, and amorphous structure down to the lo-A level.&12 (8)O’Connor, C. J.; Foise, J. W.; Haushalter, R. C. Proc. Natl. Acad. Sci., Znd. Chem. Sci. 1987, 58, 69. (9) O’Connor, C. J.; Foise, J. W.; Haushalter, R. C. Solid State Commun. 1987,53, 349.
0897-475619312805-0017$04.00/0 0 1993 American Chemical Society
Zhang et al.
18 Chem. Mater., Vol. 5, No. 1, 1993 Other examples of the chemistry of Zintl phases include reports by Kolis15J6and Dance17that describe complexes in which Zintl anions (X= Te, Se) act as chelating ligands in coordination compounds. Haushalter has also reported the preparation of metal polytslluride complexes18J9 as well as metals coordinated to arsenicz0and tinz1 clusters. As part of our effort in synthesis and characterization of novel solid materials, we have been investigating the magnetic and conducting properties of materials of the general formula MdInTep)~,where M = Mn, Cr, Fe, Co, Ni. They are prepared by the metathesis reaction between the transition metal halides and the ternary Zintl phase KsInTe4 in aqueous solution. The preliminary result of magnetic characterization of Fes(InTe4)z has been previously reported3as a spin glass with a freezing temperature of -15 K. In this paper we report a systematic examination of the magnetic and conducting properties as functions of temperature for these new intermetallic materials.
s2-
probe van der Pauw technique.Z2 The pressed pellets were made under a pressure of 16 OOO psi. The current was supplied by a KeithleyModel 224programmable currentsource and the voltage drop across sample measured with a Keithley Model 181digital nanovoltmeter. Electrical contacts to the samples were made using gold wires attached with silver paint. Resistivity of Mm(1nTed)z is quite large and approaches the upper limit of our measurement capabilitiesusing the four probe technique. Thus, the resistivity measurement on the Mnd1nTer)z sample was performed using a two probe technique for highly resistive materials.23 Magnetic Measurements. The dc magnetic Susceptibility and magnetization measurementswere conducted on a SHECorp. VTS-50 superconductingsusceptometer that is interfaced to an IBM XT computer system. Measurements and calibration techniques me reported el~ewhere.~‘In the susceptibilitymeasurement, two different procedures were used (i) zero-field cooling, where the sample waa slowly cooled in zero field to the lowest measured temperature at which the field was switchedon and the M(T) waa measured as temperaturewas raised; (ii)field cooling,where the field was turned on at a high temperature and the sample was cooled in the field.
Results and Discussion
Chemistry and Chemical Composition. Potassium indium telluride is synthesized either by solid reaction of Preparation. The Zintl phase material, KsInTe4, was prethe binary Zintl phase K 5 h 8 with Te or by reaction of a pared and purifiedin an argon-fiiedgloveboxas described before? mixture of three elements in stoichiometric ratio. The The transition-metalindium tellurides Ms(InTed2(M = Cr, Mn, chemical composition of this ternary Zintl phase has been Fe, Co, Ni) were prepared by the reaction of KsInTer with well established as KsInTe4 by AA analysis in our lab and transition-metal halides in degassed water. In a typical prethe commercial analytical lab.3 The dry KsInTe4 is yellow paration, while stirring the MX2 solution (25 mL, 0.1 M), a and solvated InTed” is orange. This compound is very stoichiometricquantity of the KsInTer solution (20mL, 0.05 M) was slowly added. A fine black precipitate was immediately soluble in water and formamide but insoluble in ethylformed, separated by suction filtration, washed with degassed enediamine, dimethlformamide, and dimethyl sulfoxide. water and acetone, and dried overnight under vacuum. All The ionic character of KsInTer permits it to dissolve in samples were treated as air-sensitive in the course of physical a polar solvent such as water to form very reactive measurementa to prevent from possible decomposition. Since polyanion InTe4”. In the presence of a transition-metal magnetic properties of the sample may be dependent on the cation of sufficient electron affinity, a rapid precipitation preparation conditions,some samples were prepared by adding reaction results in insoluble intermetallic products of a 2-fold excess of a aqueous solution of metal halides. formula Ms(InTe4)~that are very fine and black powders Elemental Analysis and X-ray Diffraction Power Analexcept Mn2+. When mixing InTed“ with Mn2+,an orange ysis. The presence of only three elements in each sample and the homogeneity of each sample were confirmed with an EDSprecipitate forms that turns black after it is filtered and equipped AMRAY Model 1820 scanning electron microscope. dried. The quantitative elemental analysis for these materials was It has been observed that there is a trend that correlates carried out on a IL-S-12AA Spectrometer, Thermo Jarrell Ash the solubility properties of ternary Zintl phases with their Corp. The atomic absorption standards were purchased from crystal structures. The Zintl-phase materials which conJohnson-Matthey. tain isolated Zintl anions in their solid are soluble in some The powder X-ray diffraction patterns were obtained with a polar solvents, for instance, K4SnTer soluble in methanol XDS 2000 Scintag diffractometer equipped with a micro VAX and water, KsSbTe3 soluble in DMSO and DMF.25 On computer for data acquisition and analysis. the other hand, the Zintl-phase materials which have Resistivity Measurements. Resistivity measurements for polymeric structures such as K~Ga3As4~~ and K4In4& (X all materials except Mns(InTe4)n were performed on pressed = As, Sb)27are insoluble in any solvents. The growth of pellets over the temperature range 20-320 K using the foursingle crystals of KsInTe4 from an aqueous solution and mixtures of Hz0-DMF and Hz0-EN has not been (10) O’Connor, C. J.; Noonan, J. F. J. Chem. Phys. Solid. 1987,48,69. successful. However, on the basis of solubility behavior (11) Haushalter, R. C.; Goshorn, D. P.; Sewchok, M. G.;Roxlo, C. B. Mater. Res. Bull. 1987, 22, 761. of KsInTe4, its structure likely consists of an ionic lattice (12) Foise, J. W.; O’Connor, C. J. Znorg. Chim. Acta 1990. of K+ and discrete InTe& Zintl polyanions. (13) Haushalter, R. C.; Krauss, L.J. Thin Solid Films 1983,202,2312. The elemental analysis for the materials that were (14) Haushalter, R. C. Angew. Chem. 1983, 95,560. (15) Flomer,W.A.;ONeal,S.C.;Pennington,W.T.;Jeter,D.;Cordes,prepared from the reaction of KsInTe4 and MC12 or MBrz Experimental Section
A.; Kolis, J. W. Angew. Chem., Znt. Ed. Engl. 1988, 27, 1702. (16) Zhang, J. H.; Flomer, W. A.; Kolis, J. W.; O’Connor, C. J. Inorg. Chem. 1990,29, 1108. (17) Cusick, J.; Scudder, M. L.; Craig, D. C.; Dance, I. C. J. Aust. Chem. 1990,43, 209. (18) Adams, R. D.; Wolf, T. A.; Eichhorn, B. W.; Haushalter, R. C. Polyhedron 1989,8, 701. (19) Eichhorn, B. W.; Haushalter, R. C.; Mercola, J. S.Znorg. Chem. 1990,29,728. (20) Eichhorn, B. W.; Haushalter, R. C.;Huffman, J. C. Angew. Chem., Znt. Ed. Engl. 1989,28, 1032. (21) Eichhorn, B. W.; Haushalter, R. C.; Pennigton, W. T. J . Am. Chem. SOC.1988, 210,8704.
~~
(22) Standard F374-84, in ASTM Annual Book of Standards 1986, vol. 10.05. (23) Schroder, D. K. Semiconductor Material and Deuice Characterization; John Wiley & Sons: New York, 1990; p 23. (24) OConnor, C. J. B o g . Znorg. Chem. 1982,29,203. (25) Jung, J. S.;Wu, B.; Stevens, E. D.; O’Connor, C. J. J.Solid State Chem. 1991,94,362. (26) Birdwhistel1,T.L. T.; Stevens, E. D.;OConnor,C. J.Znorg. Chem. 1990,29, 3892. (27) Birdwhistell, T. L.T.; Klein, C. L.;Jeffries, T.; Stevens, E. D.; O’Connor, C. J. J. Mater. Chem. 1991,4,555.
Family of New Intermetallic Materials 1
I
I
Chem. Mater., Vol. 5, No. 1, 1993 19 I
0.5
--
0.4
a
E
\
3
s
Y
x
0.3
0.2 I
0
100
200
T
0
300
I
10
20
30
T
(K)
I
I
I
40
I
50
(K)
Figure 1. Temperature dependence of inverse molar susceptibility for Ms(InTe4)~ (M = Cr, Mn, Co, and Fe). The solid lines represent the Curie-Weiss fits.
Figure 2. Temperature dependence of the zero field susceptibility measured at 10 G for the fresh and aged samples of Feb-
in stoichiometric ratio gave the following chemical compositions: Cr2.7InTe3.64;Mn2.6sInTe4.03;Fe2.ssInTe4.07; Co2.3JnTe3.94 (sample A); Niz.mInTe3.99. Since it has been observed that the magnetic properties of Cos(InTe4)n are dependent upon preparation conditions (vide infra), two samples of Co~j(InTe4)e were prepared with CoClz in 2-fold excess. Sample B prepared from KsInTe4 (0.05M, 20mL) and CoC12 (0.2 M, 25 mL) has a composition of C02.32InTe4.w. Sample C prepared from KaInTe4 (0.1 M, 10 mL) and CoCl2 solution (0.2M, 25 mL) has a composition of Co2.37InTe3.w. Within the error of analysis, the materials can be represented by the formula corresponding to M5(InTe4)z. Some of the intermetallics also contained some inert material, either trapped solvent or residual potassium halide from the metathesis. The materials resulting from rapid precipitation from solutions are amorphous. The X-ray powder diffraction patterns exhibit a lack of diffraction peaks and are consistent with a minimal amount of long-range crystalline order for the freshly prepared materials. The measured physical properties, for instance, magnetic spin glass state and amorphous semiconductivity, are also consistent with those expected for an amorphous solid. Magnetism. The magnetic susceptibilities of all metal indium tellurides under investigation can be interpreted on the basis of localized magnetic moments except Ni5(InTe4)z. Figure 1 shows the inverse magnetic susceptibility as a function of temperature with Curie-Weiss fits for Ms(InTe&, (M = Cr, Mn, Co, and Fe). The leastsquare fit of llx to Curie-Weiss expression, llx = (2' 8)/C,in the temperature range 100-300 K gives 8 = -240 K for Mns(InTe&, and 8 = -46.6 K for Crb(InTe412, respectively. The magnetic moment of Mns(InTe4)z a t room temperature is 3.60 pg per Mn2+ion, and steadily decreases to 1.0 fig a t 2.2 K. The magnetic moment of Crs(InTe4)zat room temperature is 3.2 pg per Cr2+ion and decreases rapidly below 60 K to 1.2 pg at 2.5 K. From the moment variation with temperature and the large negative 8 values, it is apparent that the dominant magnetic
exchange interaction is antiferromagnetic in these two materials. However, the maximum in the susceptibility variation with temperature characteristic of either long range antiferromagnetic ordering or spin glass transition has not been observed down to 2.2 K for both materials. On the other hand, the CurieWeiss fit of the hightemperature magnetic data (>2W K) for Fea(InTe4)zgives 8 = +88K, indicating dominant ferromagnetic coupling between the Fe moments. The magnetic susceptibility significantly deviates the Curie-Weiss behavior below 200 K, which is attributed to the formation of clusters with large moments. The striking feature of magnetism for thismaterial is that ac susceptibility measured as a function of temperature a t zero external field shows two peaks: one sharp peak occurring at -15 K and one broad peak occurring at -32 K.3 The peak at -15 K arises from the transition to a spin glass state, which was confirmed by the analysis of the field dependence of the isothermal remanent magnetization and thermal remanent magnet i z a t i ~ n .The ~ temperature dependence of the dc magnetic susceptibility at different fields has been measured on a zero-field-cooledsample to understand the natures of these transitions. The temperature dependence of dc susceptibility in the lowest measured field of 10 G is very similar to the ac susceptibility result as shown in Figure 2. The weak maximum of susceptibility at -32 K is sensitive to small external fields and completelyremoved in fields >500 G.3 Therefore it is likely that the broad peak a t -32 K arises from the weak ferromagnetic statewhich is saturated at relatively small fields. It is apparent that Fes(InTe4)z can be classified as a reentrant spin glass in which there exists a competition between ferromagnetic and antiferromagnetic interactions with a dominant ferromagn e t i ~ m . ~Consequently, J~ as temperature is lowered, this material becomes first dominated by the weak ferromag-
(InTe4h.
(28) Herlach, D. M.; Kastner, J.; Heller, A.; Wassermen, E. F. J. Appl. Phys. 1984,55 (61, 1706. (29) Mirebeau,I.; Hennion, M.;Lequien, S.;Hippert,F. J. Appl.Phy8. 1988, 63 (8),4077.
20 Chem. Mater., Vol. 5, No. 1, 1993
netism at -32 K and then undergoes a second transition into a spin-glass-like state at -15 K. Spin glasses are magnetic systems in which the interaction between the magnetic moments are "in conflict" with each other, due to some frozen-in disorder in the magnetic exchange fields. A spin glass state in an amorphous solid may be gradually lost due to the slow crystallization. The susceptibility on a sample of Fe5(InTe4)z sat for 6 months in a drybox was measured as a function of temperature at a field of 10 G. The result is shown in Figure 2 along with the susceptibility collected on the fresh sample for comparison. The broad peak at -32 K characteristic of the para- to ferromagnetism transition remains in the aged sample and has grown stronger. However, the peak characteristic of the transition to a spin glass state is shifted to 6 K as indicated by rapid decrease in the susceptibility. The magnetic measurement on the aged sample under field cooling condition reveals that it exhibits much larger susceptibility than that for the zero-field-cooled sample due to a large moment frozen in the spin glass state. This rules out the possibility of that the rapid decrease in the susceptibility in the aged sample arises from antiferromagnetic ordering. I t is clear that a weaker spin glass state remains in the sample, and however, the freezing temperature is shifted to a lower temperature due to the growth of ferromagnetic clusters. It is observed that the magnetic properties of Co5(InTe4)2 are dependent upon preparation conditions. Three samples of Cos(InTe4)z prepared under different conditions have been magnetically studied. The sample A of Co5(InTe4)zprepared with I n T e P and Co2+in stoichiometric ratio follows the Curie-Weiss law above 30 K as shown in Figure 1. The CurieWeiss fit of magnetic data gives B = -4.3 K. The magnetic moment is 3.30 pg per Co2+ion at room temperature and nearly temperature independent down to 30 K. The low-temperature magnetic data is shown in Figure 3. There is a rise at 20 K in magnetic moment measured a t low fields. This pronounced fielddependent behavior (susceptibility inverse to external field) is characteristic of the generation of a magnetized ferromagnetic ground state. The material is magnetized to saturation at the high fields (>lo kG). Sample B and C of Co5(InTe& prepared with Co2+in 2-fold excessexhibit similar magnetic behavior. At high temperatures the susceptibility obeys the Curie-Weiss behavior with B = -8.5 K and the magnetic moment steadily decreases from 3.25 pg at 300 K to 2.40 pg a t 10 K. Both samples exhibit a spin glass state with Tf= -3.5 K for sample B and -3.7 K for sample C as indicated by the maximum in the variation of zero-field susceptibility with temperature. The low-temperature magnetic data for sample B is shown in Figure 4 to illustrate the spin glass transition. Since all three samples have nearly the same composition within experimental error, a possible explanation for the variance in the magnetic properties is the difference in the size of the particles that are formed in the rapid precipitation reaction. However, the cobalt analog is the only one that exhibited this phenomenon. The high-temperature susceptibility of Nis(InTe4)~ displays non-Curie-Weiss paramagnetism and is nearly temperature independent with a susceptibility of 5 X emdmol. A t low temperatures, thematerial exhibits weak ferromagnetism as indicated by a small positive B value (0 = 10 K). No spin glass transition for this material has
-
0 l00G
V 500 G A 1 kG 0 10 kG
0 10
0
20
30
50
40
T(K)
Figure 3. Field dependence of the susceptibility (a), and the effective magnetic moment (b) versus temperaturefor the sample A of Cos(1nTer)zprepared with InTer" and Coz+in stoichiometric ratio.
c @i F
1.o
I
I
I
I
I
-
t
0 field cooling 0
zero field cooling
i I
I
0
0
I
I
10
I
20
I
1
30
T(K)
Figure 4. Temperature dependence of the zero-fieldand fieldcooled susceptibility for the sample B of Co6(InTer)nprepared with eo2+ in 2-fold excess.
been observed down to 2.2 K. The isothermal magnetization measurement a t a temperature of 2.2 K indicates that this material approaches saturation at 30 kG. However the saturated magnetization is only 0.004 p~ per Ni2+ ion, suggesting that the Ni ions carry little magnetic moment, that is, the electrons are essentially delocalized. This is consistent with high electrical conductivity in this material as shown below. At low temperatures this material can be described as an itinerant ferromagnet. Conductivity. The room-temperature specific resistivity for Ms(InTe4)z varies considerably. It has a value of 3.0 x lo7 0 cm for Mn5(InTe4)2,9.2 X 10 0 cm for Cr5in Ted)^, 7.0 X lo-' 0 cm for Feb(InTe4)2,3.5X 52 cm
Family of New Intermetallic Materials
lo
Chem. Mater., Vol. 5, No. 1, 1993 21 -7
d
Mns(lnTe,)z E, = 0.51ev (328-26810
-8 h
E, I 0.20ev (268-200K)
7
T-
E,
'E s
0
v
0
-9
CS,
0 -
I
0: Fe-In-Te
a: Co-In-Te
-I
-5
-10
0: Ni-In-Te I -1 .o
I
-1.5
I -0.5
1
4
0
(M = Mn, Cr, Fe, Co, and Ni) against the standard reduction potentialmof MIM2+showing the correlation (the solid line as a guide to the eye).
0 Fe-In-Te v Co-In-Te 0 Ni-In-Te 4
"
0
0
O
I "
~
v
v
v
ocmwooooo
100
0
v 0
v 0
0
v 0
0
v 0
v 0
200
0
0
0
0
v 0
v
v
000
O l
1 v
v
0 1
300
T (K) Figure 6. Temperature dependence of the specific resistivity for &(InTe,)z (M = Fe, Co, and Ni).
for Cos(InTed)z, and 1.3 X s2 cm for Nis(InTe4)a. We have observed that the trend of room-temperature specific resistivity appears to parallel to that of the standard reduction potential of the divalent transition-metal ions that were used to prepare the amorphous solids. Figure 5 illustrates the plot of the logarithm of the resistivity of Ms(InTe4)~ versus the standard reduction potential of the precursor divalent transition metal An excellent linear correlation is observed for all five transition-metal cations under investigation. The temperature dependence of the resistivity for M5(InTed2 (M = Fe, Co, and Ni) is shown in Figure 6. The Fe and Co analogues exhibit a negative temperature coefficient (dpldT) throughout the temperature range ~~
(30) Robert,C. W.;Melvin, J. A. CRC Handbook of Chemistry and Physics, 60th ed.; CRC Presa: Cleveland, 1981; p D-155. (31) Mott, N.F.;Davis,E.A. Electronic Processes in non-Crystalline Materials, 2nd ed.; Oxford New York, 1979.
8
6
1OOO/T
Figure 5. Plot of the room-temperature resistivity of Ms(InTe4)z
~
oooooo
0
0
I
(V)
EO
z
\
10
(K-')
Figure 7. Temperaturedependence of the specificconductivity plotted as long u versus 1/T for Mns(1nTer)z. The solid lines represent the Arrhenius fits.
investigated, indicating semiconductivity. The Ni analogue displays nearly temperature independent resistivity which is at the boundary expected for metallic behavior. The specific resistivity of Mns(InTe4)~is significantly larger than that of other materials in this series, for example, 6 orders larger than that of Crs(InTe4)~. The specific conductivity of this material is highly temperature dependent as shown in the plot of log u versus 1/Tin Figure 7. The plot shows three linear regions. A linear least-squares fit of conductivity data to the relationship, u = C exp(-EdkBT) gives the activation energies for conduction, E a = 0.51 eV in the temperature region 328268 K and E a = 0.20 eV in the region 268-210 K. The change in the activation energy may be due to a structural phase change. Below 210 K, the specific conductivity is essentially temperature independent. This electronic behavior is consistent with that expected for a semiconductor with p- or n-type impurity centers. The onset of temperature-independent behavior may then be attributed to a transition from intrinsic to extrinsic semiconductor behavior. The specific conductivity of Crs(InTe4)z varies with a range of over 4 orders of magnitudes in the temperature range measured, indicating a semiconductor of a small energy gap. The logarithm of the conductivity plotted against 1/T exhibits linear region at temperatures above 200 K. A least-squares fit of the linear portion to the relation, u = C exp(-EdkT), gives E a = 0.08 eV and C = 0.23 s2-I cm-'. I t was30 suggested that the conduction mechanism for a non-crystalline material having a value of C of order 10 Q-l cm-' or less is due either to carrier hopping between the localized states at a band edge or to phonon-assisted tunneling among localized states at the fermi level (i.e., variable range hopping). In the latter case, a straight line in the plot of log Q versus 1/T is expected near room temperature since hopping occurs between nearest neighbors and at lower temperatures it becomes favorable for the centers to tunnel to more distant sites and thus, the specific conductivity is expected to follow
Zhang et al.
22 Chem. Mater., Vol. 5, No. 1, 1993
standard reduction potential than the potassium ion. The Mns(InTe4)zand Crs(InTed2 exhibit a highly temperaturedependent semiconductivity. The Cos(InTe4)z and Nib(InTed2 are formed from more easily reduced metal ions and show substantially increased conductivity within the boundary expected for metallic behavior. However, an inspection of the data plotted in Figure 6 reveals that the temperature-dependent slope (dpldT) begins to level for Coa(1nTe.d but nevertheless remains negative, indicating that the material is formally a semiconductor. Therefore, it appears that the magnitude of resistivity for the materials of MsInTe4 shows a correlation with the standard reduction potential of the parent divalent transition-metal ion. This correlation likely arises from the degree of the transfer of electrons from the Zintl polyanion to cation when redistribution of charge occurs in the formation of the intermetallic materials.
q h
r
'E r -4
q 0
v
-
-5
Summary
A family of new intermetallic materials, Ms(InTe& (M = Cr, Mn, Fe, Co, Ni), has been prepared by reaction of the Zintl polyanion I n T e P with transition metal cations M2+ in aqueous solution. The resultant intermetallic materials exhibit varieties of properties, such as reentrant spin glass in Fes(InTe&, variable range hopping conduction in Crs(InTe&, and itinerant magnetism for Ni5the u = Aexp(-B/Pl4). As shown in Figure 8, the specific (InTed2. conductivity of Crs(InTe4)aexhibits e ~ p ( - B l T " / behavior ~) It appears that the magnetic and conducting properties over the temperature range 180-50 K. A hopping transport of these intermetallic materials are roughly correlated with mechanism has been observed in many mixed valence the standard reduction potentials of M2+,that is, with the semiconductors such as the simple 3d oxides, glasses containing 3d ions132*33 and amorphous s e m i c ~ n d u c t o r . ~ ~ ability of the precursor M2+ ions to be reduced by the Zintl polyanion. Although the formation of these materials From the magnetic measurement each Cr ion has a is perhaps better viewed as a metathesis reaction rather magnetic moment of 3.2 PB or less as opposed to the normal than a formal redox reaction, there is some degree of values of moments: 3.8 PB for Cr3+ and 4.9 PB for Cr2+. electron transfer or redistribution from electron-rich Therefore, it is likely that the hopping conductivity in polyanion to the cation. For instance, Mn2+is the most Crs(InTe4)a results from the amorphous structure instead difficult to reduce ion in this series and thus there is little of electron transfer between the ions of different valence electron transfer from anion to cation in forming Mnsstates. Below 50 K, the logarithm of the conductivity (InTed2. The closed shell is localized on the InTed&and deviates from l/!W4dependence, which might result from the d electrons are localized on the individual Mn2+. The the effect of the antiferromagnetic correlation developed localization of the unpaired electrons on the Mn2+ is gradually between the momenta as observed from the responsible for the observed magnetic property and the susceptibility data. limited degree of electronic conduction in Ns(InTe4)z. On The Zintl phase, KeInTe4, is an ionic insulator ( p > 1O'O the other hand, Nia(InTe4)n displays itinerant magnetism cm). This is consistent with the relatively large and and metallic conduction due to the delocalization of negative standard reduction potential of K+ (Eo = -2.924 electrons over the entire lattice. V).30 The Mns(InTed2 and Crs(InTe4)n also contain difficult to reduce cations but with a more moderate Acknowledgment. C.J.O. wishes to acknowledge support from a grant from the donors of the Petroleum (32) West, A. R. Solid State Chemistry and Its Applications John Wiley & Sons: New York, 19% p 616. Research Fund, administered by the American Chemical (33) Ghosh, A. J. Phys.: Condens. Matter 1989,I , 7819. Society,and NSFILaSER grant administered by the board (34)Adler, D.Amorphous Semiconductor; CRC Press: Cleveland, of regents of the state of Louisiana. 1970. 0.25
0.35 0.40 0.45 0.50 -r-1/4(K-1/4) Figure8. Temperaturedependenceof the specificconductivity plotted as log u versus l/P/'for Crs(InTe4)~. 0.30
