Lattice modes in the linear chain compound ZrTe5
Résumé
We report Raman scattering experiments on the linear chain compound ZrTe 5. Room and low temperature measurements rule out the existence of a structural phase transition, suggested by the resisitlve anomaly near 150 K. The eight ~ = ~ vibrationnal modes allowed by the scattering geometry are observed and their symmetries determined from polarized spectra. Some of the modes are identified on the basis of synmetry properties and (or) in comparison with the Raman spectra of the related compound ZrTe 3. Moreover, this comparison indicates surprisingly similar strengths for atomic interactions in ZrTe 3 and ZrTe 5. Introd1~ction ZrTe 5 crystallizes in a chain structure l closely related to that of the transition-metal trichalcogenldes. In some of these low dimensional compounds, such as NbSe 3, ~ particular form of the Fermi surface produces electronic insta-bi]ities which drive structural phase transitions 2,3. This explains the recent attention devoted to the pentatellurides ZrTe 5 and HfTe 5 4 and the subsequent investigation of" their structural and transport properties by means of various techniques. In spite of a resistive anomaly z observed near 150 K, the other measurements ~ ruled out the possibility of an electronically-driven phase transition s~nilar to that reported in h%Se 3. In the present paper, we report the results of room and low temperat~:e Raman scattering experiments on crystalline ZrTe 5. The crystal structure is described in Sec. 2. The experimental details are given in Sec. 3. Following the factor group analysis of the crystal and chain ~ = ~ vi-brationnal modes, the experimentally observed spectra are presented and discussed in Sec. 4. 2. Crystal Structure ZrTe 5 was shown to be isostructural with HfTe 5, th@ crystal structure of which has been detegmined by the X-ray work of Furuseth et al. z ZrTe 5 crystallizes in the Cmcm (D~) space group. The conventional non-primitive un1~ cell contains four foEmula units an~ has the dimensions a = 3.9876 A: b = 14.502 A and c = 13.727 A. ZrTe 5 possesses an interesting structure with both layer and chain features that are responsible for the easy (010) cleavage and the fibrous looking of the crystals. Fig. 1(a) shows projections of the structure into different crystallographic planes, and displays the 12-atom primitive unit cell, with two chains passing ttlrougn. One can build up the infinite atc~ic chains parallel to the a axis by stacking together distorted bicapped trigonal prisms of tel-lurium with the zirconium atoms located in the centre. The chains are linked by Te-Te bonds so as to form layers approximately parallel to the (010) plane. ~%e layers are held together by weak Van der Waals-type forces. This structure is closely related to that of ZrTe 3 6 and other IV b transition-metal tri-chalcogenides. The main difference, which accounts for the difference in composition, lies in the linkage between the basic coordination units [see fig. 1(b)]. 3. Experiment The samples studied in this work were single crystals of ZrTe~, with typical dimensions 6 x 0,2 × 0,h mm3. T6e Raman measurements were taken on freshly cleaved (010) surfaces in order to avoid scattering from Te, left behind by surface oxidation. The prepared samples were immediately immersed in the exchange gaz of an Ozfo~ CF 204 eryostat for room and low temperature experiments. • "~e Raman spectra of ZrTe=, excited with the 511~5 ~ and 5309 ~ lines of Spe~$ra Physics argon and krypton ion lasers, were measured in the back-scattering gec~etry. The laser beam was focused onto the surface sample at nearly normal incidence. The scattered light was collected along the crystal h axis direction and analyzed in a T800 Code~ triple monochromator, in conjunction with standard photoneountlng electronics. 89
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