Synthesis, polymorphism, and electronic structures of Sr3Sn2As4†
Two polymorphs of Sr3Sn2As4 were synthesized from the Sn-flux reactions, and their structures were determined by the single-crystal X-ray diffraction technique. α-Sr3Sn2As4 adopts the Sr3Sn2P4 structure type with the orthorhombic space group Cmca (a = 25.798(2) Å, b = 12.8883(11) Å, c = 19.1244(16) Å, V = 6358.8(9) Å3, Z = 24), whereas β-Sr3Sn2As4 belongs to Ca3Si2As4 structure type and crystallizes in the monoclinic crystal system P21/c (a = 7.7049(13), b = 19.118(3), c = 7.6877(13), β = 112.003(2)°, V = 1049.9(3), Z = 4). Despite the obvious structural differences, the polyanion units of both compounds feature similar [Sn2As6] octahedra. Differential thermal analysis and thermogravimetry measurements indicate that α-Sr3Sn2As4 has good thermal stability and melts at 1185 K, while β-Sr3Sn2As4 starts to decompose above 800 K. Diffuse reflectance spectral measurements proved that both compounds had a band gap of about 0.9 eV, supported by the density functional calculations.