High-pressure polymorphism of BaFeSe.

BaFeSe is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFeSe phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is most likely intrinsic to the BaFeSe phase and not to FeSe with T   >  20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder x-ray diffraction measurements revealed two structural transformations in BaFeSe: a second order transition above 3.5 GPa from Pnma (CsAgI-type structure) to Cmcm (CsCuCl-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFeSe transforms into δ-BaFeSe (Cmcm, CsCuCl-type average structure) via a first order phase transition mechanism. This transition is characterized by a significant shortening of the b lattice parameter of γ-BaFeSe (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.