Raman and X-ray diffraction studies of ternary chalcogenides at ambient and high pressure

Full text : Ternary chalcogenides as well as rare earth chalcogenides are studied at ambient and high pressure using various techniques to check claims of high pressure phase transitions in these materials. Most interesting is the result of a study of the nature of the phase transition of TlGaSe2 measuring the real part of the dielectric constant, ε, the heat capacity, cp, infrared and X-ray diffraction. The results prove unambiguously that we have found the first example of lone pair ferroelectricity predicted earlier to exist by Orgel (L.E. Orgel, J. Chem. Soc. 4 (1958) 3815). In the case of rare earth chalcogenides our high pressure optical and X-ray diffraction studies show that the application of high pressure leads to the formation of Se-Se bonds in the case of KTbP2Se6 whereas It does not occur in KPrP2Se6. There is a suggestion of a model for the formation of such bonds and show why it can happen in KTbP2Se6 but not in KPrP2Se6 based on structural arguments