High pressure optical and X-ray diffraction studies of two polymorphs of K(RE)P2Se6 (RE = Pr and Tb)

The results of a high pressure absorption, X-ray diffraction and Raman scat tering study indicate a pressure induced phase transition in KTbP2Se6 at ab out 9.2 GPa which is accompanied by a discontinuous jump of the absorption edge of about 0.5 eV into the red. The phase transition is also reflected i n the Raman spectrum at this pressure. The result of the energy dispersive X-ray diffraction study shows that the high pressure phase above 9.2 GPa is also orthorhombic but the b-axis is doubled with respect to the low pressu re phase. We propose that the high pressure phase is due to charge transfer from the selenium to the phosphorus accompanied by the formation of a Se-S e bond. We also discuss a model which involves a 4f-5d charge transfer in T b, leading to the intervalence charge transfer from: Tb3+P4+ --> Tb4+P3+. S tudies on KPrP2Se6 did not show a phase transition in this pressure range. (C) 1999 Elsevier Science Ltd. All rights reserved.