Structural modification and heat capacity of ZrTe5

Synthetic Metals, 10 (1985) 161 - 168

161

S T R U C T U R A L MODIFICATION AND H E AT CAPACITY OF ZrTe s T. SAMBONGI Department of Physics, Hokkaido University, Sapporo 060 (Japan)

K. BILJAKOVIC and A. SMONTARA Institute of Physics of the University, Zagreb 41001 (Yugoslavia)

L. GUEMAS Laboratoire de Chimie des Solides, Universitd de Nantes, Nantes 44072 (France)

{Received July 23, 1984; accepted August 6, 1984)

Abstract O r t h o r h o m b i c and monoclinic p o l y t y p e s of ZrTe s are examined by X-ray camera techniques at r o o m temperature. Long period m odul at i on of t h e f o r m e r was determined. Lattice parameters of t he monoclinic p o l y t y p e are given. Two peaks are observed in the heat capacity of the o r t h o r h o m b i c crystal. One is assigned t o a phase transition in the monoclinic inclusion. A n o t h e r peak at 135K appears as the result of thermal excitation of electrons in a semimetallic energy band.

1. Introduction We r ep o r t on X-ray diffraction at r o o m temperature, heat capacity and electrical resistivity o f ZrTes. Though several resistivity studies [1 - 4] have been published, a supplementary measurement has become necessary by the recent discovery o f a new elastic anomaly. Transition-metal pentatellurides, ZrTes and HfTes, show giant resistivity anomalies at ~ 150K and ~ 80K respectively [1, 2]. Associated changes have been observed in the Hall coefficient [5], the thermoelectric pow er [6] and various o t h e r properties. As their origin, a phase transition, e.g., onset o f the charge density wave (CDW), has been proposed because the t em perat ure dependences o f these properties are quite similar t o those observed in NbSe3, the typical CDW c onduc t or . On t he o t h e r hand, t he absence of any new structural modification at low t e m p e r a t u r e has led to the proposal t hat a semi-metallic b a n d s t r u c t u r e is responsible for the transport anomalies. Band structure calculations [7, 8] support t he absence of the CDW state at low t e m p e r a t u r e ; th e calculated band structure is n o t likely to be one dimensional. At present, however, careful measurements on ot her properties in the temperature range o f interest are desirable to look into the mechanism of the 0379-6779/85/$3.30

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162

transport anomalies. Recent elastic measurements on ZrTe s [9] revealed a new anomaly, indicating a structural transition at ~ 85K, where other properties are not singular. The crystal structure of pentatellurides, determined by Furuseth et al. [ 10], is closely related to those of trichalcogenides; triangular MTe 3 columns run parallel to the orthorhombic a-axis but are linked together along the caxis by 'Te2' chains. Several kinds of structural modification have been found at room temperature. DiSalvo et al. [4] observed half-integer X-ray reflections within the a * - b * plane. Skelton et al. [11] found (0,0,2n + 1) reflections which are forbidden by the space group Cmcm determined by Furuseth et al. They observed that the intensities of low n extra reflections show maxima at the resistivity peak temperatures. The half-integer reflections were not detected in samples used by Skelton et al. The diversity of the above results is one of the motivations of the present work. Preparation of a single crystal has been described elsewhere [2 ].

2. X-ray examination at room temperature and electrical resistivity Several crystals were examined at room temperature by oscillation, Weisenberg and precession cameratechniques. The presence of two polytypes (orthorhombic [10] and monoclinic [2]) was confirmed. It is rather difficult to distinguish them by microscopic examination, but on the surface of the monoclinic crystal many striations run parallel to the growing axis. O r t h o r h o m b ic p o l y t y p e From strong reflections, lattice parameters are determined: ao = 3.98 A, bo = 14.50 A, Co = 13.73 A, and ~ = ~ = 7 = 90 + 0.1 °, where the subscript o denotes the orthorhombic lattice. The observed reflections are summarized in Table 1. The space group is Cmcm (no. 63). The above parameters and the space group are identical to those determined by Furuseth et al. [10]. In the orthorhombic polytype, another set of reflections, mostly weak, was observed at room temperature. In the oscillation photographs around the ao axis, five weak layers were observed. The new period al is equal to 26.5 A and the a~axis is parallel to the ao. From the (0, k, l) Weissenberg photographs the TABLE 1 F u n d a m e n t a l reflections observed in the o r t h o r h o m b i c ZrTes Observed reflections (0, (1, (~, (~,

~, ~)o r/, ~)o 0, ~)o r/, 0)o

(0, even, l) and (0, 0, even) (1, odd, l) (even, 0, even) (h, k, 0) with h + k = even

163 following results were obtained: bl (parallel to bo) = 31.6 A and c] (parallel to Co) = 30.1 A. Comparison of (0, k, l)l, (6, k, l)] and (3, k, I)1 photographs seems to indicate t h a t the space group of the extra set is Cmcm, but this assignment is not conclusive because these reflections are weak. The new set of reflections may correspond to a kind of polytype. However, we temporarily propose that it is indicative of a modulation of the basic lattice because it is observed repeatedly and the relation with the basic one is quite reproducible. DiSalvo e t al. [4] observed five half-integer reflections in the (~, ~7, 0)o plane. They proposed that the real unit cell is 2a × 2b × c. However, some of these lines can be indexed by t h e newly found set, if slight deviations from half-integers are allowed. For example, (2.55, 0.5 + 0.04, 0)o can be indexed as (17,11,0)1. Skelton e t al. [11] observed (0,0,odd)o reflections, which are not allowed by the space group Cmcm. These extra reflections cannot be explained by the above transformation w i t h o u t large uncertainty in the assignment given by Skelton e t al. For example, an error of 10% must be allowed if (0,0,1)o is to be indexed as (0,0,2 + 0.2)1. Similarly the lattice spacing d(0,0,3)o = 4.58 A is much smaller than the possible one in the new set: d(0,0,6)~ = 5.02 A. The peculiar feature of the results of Skelton e t al. is that these extra lines can be observed only by the counter m e t h o d but not by camera techniques. It has not been solved why various modifications of the structure have been reported. One of the possible explanations is that the structure determined by Furuseth et al. is the basic one but the real (modulated) structure is sensitive to, for example, the degree of stoichiometry. Another problem is that incorporation of iodine, used as the transport agent, has not been ruled out. Monoclinic phase Existence of the monoclinic polytype, reported previously [2], was verified. The lattice parameters are am = 14.59 A, b m = 3.98 A, Cm = 31.29 A and j3m = 96.7 °, where the subscript m denotes the monoclinic phase. The growing axis is parallel to the bm -axis. In the Weissenberg photographs, the following reflections were observed: (even, 0, l) and (0, 0, even) in the (~, 0, ~) plane and (odd, 1, l) in the (~, 1, ~'). The possible space group is C2/c (no. 15). The following relation can be found between parameters of the two polytypes: bm = ao (both are parallel to the growing axes} and am = bo. In some cases crystals were found in which orthorhombic and monoclinic ones are stacked together, with bm parallel to ao and am parallel to bo, consistent with the above relation (Fig. 1). The electrical resistivity was measured using samples cleaved from a crystal which was assigned as monoclinic, along the growing axis by the four probe technique. The results are shown in Fig. 2. A large difference can be found; a peak at ~ 150 K is found in Fig. 2(a), while it is much smaller in Fig. 2(b). The broad peak is probably due to an inclusion of the orthorhombic polytype, which could not be detected by X-ray examination. Presumably the resistivity of the monoclinic polytype increases monotonically

164 %

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with increasing temperature, as inferred from Fig. 2(b). In Fig. 2(b) a small hump can be found near 85 K. In this temperature range anomalies have been found in the Young's modulus [9] and in the heat capacity {next section). They are indicative of a second-order phase transition [9]. Though samples had been confirmed as orthorhombic by X-ray before measurement, both the elastic and heat capacity results are indicative of the presence of the monoclinic polytype. On the other hand, the resistivity anomaly at ~ 85 K has not been observed in 'orthorhombic' samples [2, 9]. Because both the constituent elements, Zr and Te, are heavy with large X-ray absorption coefficients, it is probable that a second phase was missed accidentally, for example, because of the diffracted X-ray absorbed by the parent phase. More systematic examination is necessary to clarify the nature of the anomalies at 85 K, but unfortunately the monoclinic crystals have not been separated.

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165 3. H e a t c a p a c i t y

In order to clarify the problem of existence of the phase transition, we p e r f o r m e d specific heat measurements in t he t e m p e r a t u r e region between 60 K and 190 K. The basic principles o f the m e t h o d used, the improved d.c. relaxation technique, have been described previously [ 12 ]. 2

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Fig. 3. The sample holder arrangement. 1, ZrTes crystal; 2, copper-constantan thermocouple ; 3, silicone grease ; 4, heater ; 5, Pt-thermometer, 6, thermal sink. The sample holder used here is shown in Fig. 3. A single crystal of ZrTe s was m o u n t e d on a small plate of 20 /~m copper foil. For a good thermal c o n t a c t we used a thin layer o f silicon high vacuum grease. Heating pulses o f a b o u t 0.1 - 0.2 K were applied t o the sample using the small heater on the lower side o f the c o p p e r plate. The heat capacity measurements were carried o u t in t w o runs using 6.6 rag and 14.2 mg crystals respectively. The cooling rate was a b o u t 1 K/hour. Parasitic heat capacities were measured independently and subtracted from t h e total heat capacity. By using the pr ocedure described in ref. 13, it was f o u n d that no indication o f a latent heat is present, neither in the heating n o r in th e cooling run. In th e first e x p e r i m e n t (No. 1, Fig. 4) there was no clear specific heat anomaly. However, there were at least two marked changes in the slope, t he first at 90 K and the second one a r ound 140 K. The experimental conditions in the first measurement were a b o u t the same as for NbSe 3 [ 14] , b u t the molecular weight 2.2 thr~es larger led to a lower absolute resolution and more scattering in the data. Therefore t he m e a s u r e m e n t was repeated with a larger crystal (No. 2, Fig. 4). In the second run we observed t w o broad anomalies. The first is around 135 K, which is also t h e t e m p e r a t u r e of the m a x i m u m in t he resistivity measured on the same sample. There is a second anomaly at ~ 90 K. Both the observed anomalies are rather smeared. T he y e x t e n d over about 40 K and 15 K, respectively.

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By choosing the appropriate base line, the heights of the anomalies are about 2.5% and 2% (i.e., absolute values 0.5 R0 and 0.38 R0 where R0 is the gas constant), respectively. The low temperature peak is associated with the elastic and resistivity anomalies of the monoclinic polytype, which is described in the previous section. Although no clear diffraction spot was observed which would correspond to the monoclinic lattice in the samples used, the presence of the monoclinic p o l y t y p e cannot be ruled out in a large crystal. If the low temperature peak is to be assigned to the orthorhombic polytype, anomalies should be observed in properties more sensitive to a phase transition, e.g., the Hall effect and the thermoelectric power. The high temperature peak, at ~ 135 K, corresponds to the transport anomalies in the orthorhombic polytype. Since the elastic constant [9] varies smoothly in this temperature range, the peak is attributable to a change in the electronic system but not in phonons. Two different approaches have been proposed to explain the transport anomalies. One is that they are associated with a phase transition inherent in low dimensional conductors. Although this hypothesis can explain the peak of heat capacity qualitatively, the difficulties mentioned previously [2] cannot be removed by the present findings. For example, the broadness of the heat capacity peak, which would be a sign of strongly-low dimensionality of the system, cannot be reconciled with the small resistivity anisotropy and the strongly anisotropic diamagnetism [2]. Another approach is that there is no phase transition and that the anomalies are associated with a gradual change in the carrier distribution with temperature [2]. In this case the transport anomalies are indicative of a transition (not a phase transition) from a low carrier density state to a higher

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Acknowledgements The authors f r om the Institute o f the University, Zagreb wish to thank t h e members o f the group for chain c o n d u c t o r s for their support during the course o f this work. The X-ray work was done during the stay of T.S. in Nantes. He wants t o express his he a r t y thanks to J. Rouxel and his collegues fo r their hospitality and helpful discussions.

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168 5 M. Izumi, K. Uehinokura, S. Harada, R. Yoshizaki and E. Matsuura, Mol. Cryst. Liq. Cryst., 81 (1982) 141;Solid State Commun., 42 (1982) 773. 6 T. E. Jones, W. W. Fuller, T. J. Wieting and F. Levy, Solid State Commun., 42 (1982) 793. 7 D.W. Bullett, Solid State Commun., 42 (1982) 691. 8 M. H. Whangbo, F. J. DiSalvo and R. M. Fleming, Phys. Rev. B, 26 (1982) 687. 9 J. W. Brilt and T. Sambongi, J. Phys. Soc. Jpn., 53 (1984) 20. 10 S. Furuseth, L. Brattas and A. Kjekshus,.Acta. Chem. Scand., 27 (1973) 2367. 11 E. F. Skelton, T. J. Wieting, S. A. Wolf, W. W. Fuller, D. U. Gubser, T. L. Francavilla and F. Levy, Solid State Commun., 42 (1982) I. 12 D. Djurek, K. Franulovic, M. Prester, S. Tomic, L. Giral and J. M. Fabre, Phys. Rev. Lett., 38 (1977) 715. 13 D. Djurek and J. Baturic-Rubcic, J. Phys. E, 5 (1972) 424. 14 S. Tomic, K. Biljakovic, D. Djurek and J. R. Cooper, Solid State Commun., 38 (1981) 109. 15 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 5th edn., 1976 p. 453.