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 P2(1)2(1)2(1), the new beta-Ba2SnSe5 crystallizes in the monoclinic system, space group P2(1)/c, with the lattice parameters a = 9.3949(6) angstrom, b = 8.8656(6) angstrom, c = 12.5745(7) angstrom, beta = 113.299(4)degrees, V=961,9(1) angstrom(3), Z=4. alpha-Ba2SnSe5 is comprised of Sn3Se108- units, SnSe44- tetrahedra, and isolated Se-3(2-) units, while beta-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 Se-2(2-) dumbbell. Different band gaps are a result of the different structure motifs, which are reflected in different colors of the two Ba2SnSe5 modifications, the alpha-form being dark brown and the beta-form being red.