RAMAN SCATTERING STUDIES OF KNBO, TCNQ AND CUTCNQ

Abstract

The high pressure and low temperature variations of KNbO3, TCNQ and CuTCNQ have been studied by Raman spectroscopy. The effect of pressure on KN bO3 is great and a lot of phase transitions have occurred. Under high pressure up to 200 kbar, three new crystalline phases and an amorphous phase have been observed. Two of the phase transitions are thought to be displacive, they appear respectively at 20 and 90 kbar, which are mainly characterized by the soft modes with respective ambient frequencies at 50 and 266 cm-1. The third phase transition occurs at 60 kbar and it is indicated by the new broad band at 150 cm-1. As the pressure exceeds 150 kbar, all Raman bands become broad and the phase is suggested to be amorphous. Low temperature Raman studies of single crystals KNbO3 from room temperature down to 10 K show that there exists a phase transition from the orthorhombic to the rhombohedral phase at 224.3 K on cooling and it returns to the orthorhombic phase at 244.4 K on heating. A study of TCNQ single crystals under high pressure up to 80 kbar is presented. A phase transition at 22 kbar is found, but not at 12 kbar as earlier reported. High pressure Raman studies on polycrystalline CuTCNQ show that CuTCNQ undergoes a first-order phase transition at 30 kbar, which is characterized by the abrupt disappearance of all the Raman bands. Low temperature Raman studies on crystalline CuTCNQ show that no phase transition appears in the temperature range (room temperature down to 7.5 K) and the room temperature phase is found to be stable down to 7. 5 K.