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 C, followed by slow cooling. Sr2SnSe5 crystallizes in the Eu2SnSe5 type with SnIV, Se-II, and nonlinear Se32- units, space group P21212, with lattice dimensions of a = 1205.7(2) pm, b = 1658.3(3) pm, c = 863.9(2) pm, V = 1.7272(6) nm3 (Z = 8). SrSn2Se4 forms a distorted variant of the SrIn2Se4 type with mixed valences, i.e., SnII and SnIV instead of two InIII. The distortion is reflected in lowering the symmetry from Fddd to Fdd2, occurring with differently sized and distorted SnIISe4 and SnIVSe4 tetrahedra. The lattice dimensions are a = 2189.2(1) pm, b = 2189.3(1) pm, c = 1354.70(7) pm, V = 6.4928(6) nm3 (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) nm3. SrMgSnSe4 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 SrMgSnSe4, 0.9 eV for Sr2SnSe5, and 0.2 eV for SrSn2Se4.