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

The results of a high pressure absorption, X-ray diffraction and Raman scattering study indicate a pressure induced phase transition in KTbP2Se6 at about 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 in 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 pressure 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–Se bond. We also discuss a model which involves a 4f–5d charge transfer in Tb, leading to the intervalence charge transfer from: Tb3+P4+→Tb4+P3+. Studies on KPrP2Se6 did not show a phase transition in this pressure range.