Band gap tuning in new strontium seleno-stannates

The new ternary seleno-stannates Sr2SnSe5 and SrSn2Se4 were both prepared from directly reacting the elements in stoichiometric ratios at 850 degreesC, followed by slow cooling. Sr2SnSe5 crystallizes in the Eu2SnSe5 type with Sn-IV, Se-II, and nonlinear Se-3(2-) units, space group P2(1)2(1)2, with lattice dimensions of a = 1205.7(2) pm, b = 1658.3(3) pm, c = 863.9(2) pm, V = 1.7272(6) nm(3) (Z = 8). SrSn2Se4 forms a distorted variant of the SrIn2Se4 type with mixed valences, i.e., Sn-II and Sn-IV instead of two In-III. The distortion is reflected in lowering the symmetry from Fddd to Fdd2, occurring with differently sized and distorted (SnSe4)-Se-II and (SnSe4)-Se-Iv tetrahedra. The lattice dimensions are a = 2189.2(1) pm, b = 2189.3(1) pm, c = 1354.70(7) pm, V = 6.4928(6) nm(3) (Z = 32). The SnII atoms may be replaced by Mg, producing the new isostructural compound SrMgSnSe4 with a = 2176.0(1) pm, b = 2185.9(1) pm, c = 1340.60(9) pm, V = 6.3765(7) nm(3). SrMgSnSe4 is a red material (indicative of a band gap of 2.0 eV), while the two other selenorstannates are black, pointing toward gaps < 1.7 eV. The computed band gaps are 1.8 eV for SrMgSnSe4, 0.9 eV for Sr2SnSe5, and 0.2 eV for SrSn2Se4.