Band Gap Tuning in New Strontium Seleno-Stannates

The new ternary seleno-stannates Sr 2 SnSe 5 and SrSn 2 Se 4 were both prepared from directly reacting the elements in stoichiometric ratios at 850 °C, followed by slow cooling. Sr 2 SnSe 5 crystallizes in the Eu 2 SnSe 5 type with Sn IV, Se - II, and nonlinear Se 3 2 - units, space group P 2 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). SrSn 2 Se 4 forms a distorted variant of the SrIn 2 Se 4 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 Fdd 2, occurring with differently sized and distorted Sn II Se 4 and Sn IV Se 4 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 Sn II atoms may be replaced by Mg, producing the new isostructural compound SrMgSnSe 4 with a = 2176.0(1) pm, b = 2185.9(1) pm, c = 1340.60(9) pm, V = 6.3765(7) nm 3 . SrMgSnSe 4 is a red material (indicative of a band gap of 2.0 eV), while the two other seleno-stannates are black, pointing toward gaps <1.7 eV. The computed band gaps are 1.8 eV for SrMgSnSe 4, 0.9 eV for Sr 2 SnSe 5, and 0.2 eV for SrSn 2 Se 4 .