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

Abstract

We report on high pressure X-ray diffraction investigations of Rb₂Te in diamond anvil cells DAC (Mo K_α1 radiation, Rietveld refinement of the diffraction diagrams), which resulted in a polymorphic transformation series for the compound Rb₂Te from an anti-fluorite type into an anti-cotunnite type (transition pressure 0.5 GPa) and further on into a Ni₂In type structure (transition pressure 1.5 GPa). Among the compounds of the composition A₂B Rb₂Te 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 Ni₂In type are structurally related — between both structure types there is a group-subgroup relation with the Ni₂In 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 Co₂Si structure-type.