  BaFe2Se3 (Pnma, CsAg2I3-type structure), recently assumed to show
superconductivity at ~ 11 K, exhibits a pressure-dependent structural
transition to the CsCu2Cl3-type structure (Cmcm space group) around 60 kbar, as
evidenced from pressure-dependent synchrotron powder diffraction data.
Temperature-dependent synchrotron powder diffraction data indicate an evolution
of the room-temperature BaFe2Se3 structure towards a high symmetry CsCu2Cl3
form upon heating. Around 425 K BaFe2Se3 undergoes a reversible, first order
isostructural transition, that is supported by the differential scanning
calorimetry data. The temperature-dependent structural changes occur in two
stages, as determined by the alignment of the FeSe4 tetrahedra and
corresponding adjustments of the positions of Ba atoms. On further heating, a
second order phase transformation into the Cmcm structure is observed at 660 K.
A rather unusual combination of isostructural and second-order phase
transformations is parameterized within phenomenological theory assuming
high-order expansion of Landau potential. A generic phase diagram mapping
observed structures is proposed on the basis of the parameterization.
