Negative Second Harmonic Response of Sn⁴⁺in the Fresnoite Oxysulfide Ba₂SnSSi₂O₇

Oxychalcogenides are promising candidates for the design of IR nonlinear optical materials. Here, we prepared the first oxysulfide in the polar Fresnoite mineral type, Ba2SnSSi2O7, and show that it has rare tin square pyramids, SnO4S, with apical Sn–S bond. These units and the Si2O7 groups are corner-shared to form the SnSSi2O7 layer with their Sn–S and apical Si–O bonds pointed along the polar c axis. Second harmonic generation measurements reveal that Ba2SnSSi2O7 is an IR nonlinear optical (NLO) compound. Formally, Ba2SnSSi2O7 results from the mineral Fresnoite, Ba2TiOSi2O7, by replacing the TiO5 square pyramid with the SnO4S square pyramid. This substitution increases the apical/the equatorial bond ratio of the square pyramid, namely, Ti–Oap/Ti–Oeq = 1.66/2.00 = 0.83 in TiO5, and Sn–S/Sn–O = 2.03/2.31 = 0.88 in SnO4S. This change has a major impact on the cleavage along the stacking polar c axis and on the second harmonic generation response, which decreases from Ba2TiOSi2O7 to Ba2SnSSi2O7 by a factor greater than 2. The atom response theory analyses based on the density functional theory calculations reveal a remarkable difference between Ba2TiOSi2O7 and Ba2SnSSi2O7; the second harmonic generation is positive for the Ti4+ ion of Ba2TiOSi2O7 but negative for the Sn4+ ion of Ba2SnSSi2O7. A Tauc plot analysis for Ba2SnSSi2O7 assuming indirect and direct transitions led to the optical band gaps of 2.4 and 2.7 eV, respectively.