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 Sn^IV, Se^-II, and nonlinear Se₃^2- 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., 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 Sn^IISe₄ and Sn^IVSe₄ 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 Sn^II 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.