Polymorphic forms of Rb₂Te: in-situ high-pressure DAC X-ray diffraction investigations and structure modelling with CASTEP

We report on high pressure X-ray diffraction investigations of Rb 2 Te in diamond anvil cells DAC (Mo K α 1 radiation, Rietveld refinement of the diffraction diagrams), which resulted in a polymorphic transformation series for the compound Rb 2 Te from an anti-fluorite type into an anti-cotunnite type (transition pressure 0.5 GPa) and further on into a Ni 2 In type structure (transition pressure 1.5 GPa). Among the compounds of the composition A 2 B Rb 2 Te has up to now an exceptional position in so far, as this sequence of polymorphic forms is also observed as a high-temperature transformation series. Geometry optimizations on the basis of DFT calculations with the program CASTEP confirmed the phase transitions observed experimentally. Since the anti-cotunnite and the Ni 2 In type are structurally related — between both structure types there is a group-subgroup relation with the Ni 2 In type as aristo -type (1st Landau criterion) — the transition between the two types can be characterized as continuous in real space coordinates. According to experimental data as well as results of the CASTEP calculations the phase transition takes place with a discontinuity in the volume versus pressure curve. The geometry optimizations give direct insight in the continuous mechanism of the transition and offer the possibility to compare intermediate structural arrangements with other structures to be discussed in this context, as e.g. the post-cotunnite or the Co 2 Si structure-type.