Mol. Cryst. Liq. Cryst. 1994, Vol. 256, pp. 855-860 Reprints available directly from the publisher Photocopying permitted by license only 0 1994 OPA (Overseas Publishers Association) Amsterdam B.V. Published under license by Gordon and Breach Science Publishers S.A. Printed in the United States of America PRESSURE AND TEMPERATURE INDUCED FREQUENCY SHIFTS OF RAMAN-ACTIVE PHONONS IN THE MX SOLID pt~Br6(NH3)~ G.S. KANNER, S.R. JOHNSON, B. SCOTT, S.P. LOVE, AND B.I. SWANSON Los Alamos National Laboratory, CST 14, MS C345, Los Alamos, NM, 87545 Abstract We report the dependencies on hydrostatic pressure up to 100 kbar, and on temperature from 10 K to 300 K of the frequencies of Raman-active stretching modes in the single crystal MX chain solid Pt2Br6(NH3)4 (PtBrn). The data show that a predicted pressure- induced metallization does not occur, and suggests instead that either a 3-D structural distortion or a transition to a broken-symmetry ground state different from the charge-density wave (CDW) takes place. INTRODUCTION The halogen-bridged (X=Cl,Br,I) transition metal (M=Pt,Pd,Ni) compounds (MX solids) are attracting a great deal of attention as paradigms of 1-D systems.1 One recently discovered phenomenon in these materials, known as the template effect, is the influence of ligands and counterions on the distance d between adjacent M atoms along the chain.2,3 We have observed3 that both the frequency of the Raman-active chain mode v1, and the energy gap E,, increase asdincreases. This tunability of E, is clearly a desirable feature in materials used in optical devices, such as light-emitting diodes.4 Alouani et a1.5 predicted that applying uniaxial stress along the chain axis in PtzBr6(NH3)4 (PtBrn), an MX solid with the smallest number of atoms per unit cell, would lead to metallization (equal distances between all axial Br and Pt atoms) at 89 kbar. v1 would then vanish as the stretching mode became asymmetric (and, therefore, Raman inactive) with respect to the new (two atom) unit 855 Downloaded By: [LANL Research Library] At: 00:42 18 March 2010