Temperature dependence of the Raman spectra of 1T-TaS 2 T. Hirata a, * , F.S. Ohuchi b a National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibraki 305-0047, Japan b Department of Materials and Engineering, University of Washington, Box 352120, Seattle, WA 98195-2120, USA Received 18 August 2000; received in revised form 30 October 2000; accepted 30 October 2000 by F.J. DiSalvo Abstract The temperature dependence of the unpolarized Raman spectra from 1T-TaS 2 has been measured between 297 K and 48 K. The present work demonstrates that the three Raman modes observed above 220 cm 21 (high frequency region) at room temperature exhibit remarkable frequency changes toward low wavenumbers in the vicinity of 200 K, where 1T-TaS 2 under- goes a ®rst-order transition of the charge density wave (CDW) from nearly commensurate to a commensurate state. The monotonic line-broadening and intensity reduction with temperature has been identi®ed for these Raman modes. In contrast, the behavior observed in the low frequency region (less than 200 cm 21 ), suggested that the soft phonons are less likely to be involved in the temperature dependence of the high frequency Raman modes. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: D. Phase transitions; D. Phonons PACS: 63.20.2e; 64.60.2i; 78.30.2j A transition metal dichalcogenide compound, 1T-TaS 2 , exhibits a strong charge density wave (CDW) instability [1], which is characterized by two ®rst-order transitions of incommensurate to nearly commensurate and nearly commensurate to commensurate states that are observed at 350 K and 200 K, respectively. At these temperatures, discontinuous changes in the electrical resistivity, magnetic susceptibility and re¯ectivity were observed [2±4]. The CDW instabilities and the effects of cation doping in the transition metal chalcogenides have been studied exten- sively by electron, X-ray and neutron diffraction techniques, as well as infrared re¯ectivity and Raman scattering [4±16]. Several Raman studies on 1T-TaS 2 have shown that there are two Raman A g modes observed at 81 and 114 cm 21 . These low frequency modes behave like soft phonons, and exhibit their frequency shift toward the lower wavenumbers in the vicinity of T  200 K [11,12], as the temperature is raised from a low temperature. The temperature-dependent frequency (v ) of the 82 cm 21 Raman mode, for example, can be ®tted to a form of v  a  T 0 2 T  0:5 , where a is a constant and T 0 is approximately 1040 K is a transition temperature for the soft phonon mode [10]. There are additional Raman lines present in the 1T-TaS 2 Raman spectra when the commensurate CDW state is reached at 200 K. They are explained as the increase in the number of zone center (G -point) phonon modes due to folding of the original Brillouin zone by the formation of a commensurate superlattice [9,11]. These spectral changes are found in the low and/or high frequency region, but no particular attention has been paid to subtle changes in the high frequency region of the Raman spectra from 1T-TaS 2 , when 1T-TaS 2 undergoes the transition from the nearly commensurate to commensurate state at 200 K [11,12]. The present paper reports a new ®nding relating to the spectral changes in the high frequency region (above 2230 cm 21 ) when the transition from the nearly commensu- rate to commensurate state takes place in 1T-TaS 2 . Single crystals of 1T-TaS 2 were grown by an iodine vapor transport technique, details of which are given elsewhere [2]. Unpolarized Raman spectra of 1T-TaS 2 were collected from 1T-TaS 2 at different temperatures ranging from 297 K to 48 K using a Dilor XY spectrometer. Sample temperature was measured by a Au (Fe)-KP thermocouple, and controlled with an accuracy of ^1 K. A 514.5 nm line from the Ar 1 laser was used as the radiation source. Incident laser power was limited to 10 mW in order to minimize a possibility of sample heating. All the Raman spectra from 1T-TaS 2 were collected in a backscattering con®guration. A Solid State Communications 117 (2001) 361±364 0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-1098(00)00468-3 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: 181-298-59-2734; fax: 181-298- 59-2701. E-mail address: hirata@nrim.go.jp (T. Hirata).