Physica 139 & 140B (1986) 458-460 North-Holland, Amsterdam PRESSURE DEPENDENCE OF TCNQ RAMAN SPECTRUM AT 25 K S. DESGRENIERS and C. CARLONE Centre de Recherche en Physique de Solide, Dgpartement de Physique, Universitd de Sherbrooke, Sherbrooke, Qudbec, Canada J I J 2RI H.D. HOCHHEIMER Max-Planck Institut fiir Festkorperforschung, 7000 Stuttgart 80, Fed. Rep. Germany The 514.5 nm laser excitation was used to excite the Raman spectrum of TCNQ crystals at 25 K. Using the diamond anvil cell and liquid argon as a pressure medium, the Raman spectrum was followed from 0 to 2.0 GPa. We find that the Gruneisen parameter is quite constant for the low frequency modes and that it drops by order of magnitude for the high frequency modes; TCNQ is a well behaved molecular crystal at 25 K. At 300 K, the Gruneisen parameter is a monotonically decreasing function of the frequency, indicating a hierarchy of forces. In going from 25 K to 300 K, the effect of phonon-phonon interaction offsets that of volume dilatation. The salts formed with tetracyanoquinodi- methane (TCNQ) as an electron acceptor are extensively studied because they have relatively high conductivity for organic materials [1]. Raman spectroscopy has already been used to understand the phonon structure in these com- pounds [2]; some high pressure results have ap- peared [3, 4], but these measurements were per- formed at room temperature. This work reports the first Raman and high pressure measurements of TCNQ crystals at 25 K. It is generally assumed that TCNQ is a molecu- lar crystal. Such crystals are characterized by two kinds of forces-the weak type between the molecules and the stronger type between atoms in the same molecule. The application of pressure reveals the force differential, as seen through the two values of the Gruneisen parameter F [5]. This behaviour has been observed in several crystals [5-8], that is to say, F is constant for the external modes, but drops by an order of magnitude for the internal modes. In other crystals [3, 5], F is a monotonically decreasing function of the frequen- cy, indicating a hierarchy of forces in the crystal. At room temperature, TCNQ falls in the latter category [3]. The present work was undertaken at 25 K with the purpose of elucidating the molecular dynamics at low temperatures. By changing the temperature, two parameters are affected. First, volume dilatation occurs: at high temperature, the intermolecular forces are weaker than they are at lower temperatures due to the increased interatomic distances, and the molecular approx- imation is expected to be better. At the same time, the phonon-phonon interaction increases at higher temperatures, thereby mixing the internal and external modes and making the molecular approximation worse. The two effects contribute in opposite fashion. The TCNQ crystals studied were obtained from two different crystal growing facilities. Crystals labelled A were grown by K.D. Truong by three subsequent slow recrystallizations from purified acetronitrile solvent of commercially available TCNQ powder. Crystals labelled B were pre- pared by M.L. Kaplan and P.H. Schmidt. Com- mercially available TCNQ powder was gradient sublimed in vacuum onto a teflon substrate in order to avoid contamination. A gasketed diamond anvil cell [9] with a sap- phire backing, a modified version of the Syassen- Holzapfel design [10] suitable for low tempera- ture measurements, was used for the Raman studies. High pressure data (0 < P < 20.0 GPa) could 0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)