Reversible Reconstructive Phase Transition of Ba₂SnSe₅: A New High Temperature Modification with Completely Different Structural Motifs

A new modification of Ba2SnSe5 was prepared by high temperature synthesis. In contrast to its low temperature modification that adopts the orthorhombic space group P212121, the new β-Ba2SnSe5 crystallizes in the monoclinic system, space group P21/c, with the lattice parameters a = 9.3949(6) Å, b = 8.8656(6) Å, c = 12.5745(7) Å, β = 113.299(4)°, V = 961.9(1) Å3, Z = 4. α-Ba2SnSe5 is comprised of Sn3Se108− units, SnSe44− tetrahedra, and isolated Se32− units, while β-Ba2SnSe5 contains only SnSe54− units, wherein Sn is tetrahedrally coordinated by four Se atoms. The fifth Se atom is connected to one Se atom of the SnSe44− tetrahedron, thereby forming a Se22− dumbbell. Different band gaps are a result of the different structure motifs, which are reflected in different colors of the two Ba2SnSe5 modifications, the α-form being dark brown and the β-form being red.