Negative Second Harmonic Response of Sn(4+)in the Fresnoite Oxysulfide Ba2SnSSi2O7

摘要

Oxychalcogenides are promising candidates for the design of IRnonlinear optical materials. Here, we prepared thefirst oxysulfide in the polarFresnoite mineral type, Ba2SnSSi2O7, and show that it has rare tin square pyramids,SnO4S, with apical Sn-S bond. These units and the Si2O7groups are corner-sharedto form the SnSSi2O7layer with their Sn-S and apical Si-O bonds pointed alongthe polarcaxis. Second harmonic generation measurements reveal that Ba2SnSSi2O7is an IR nonlinear optical (NLO) compound. Formally, Ba2SnSSi2O7results from themineral Fresnoite, Ba2TiOSi2O7, by replacing the TiO5square pyramid with theSnO4S square pyramid. This substitution increases the apical/the equatorial bondratio 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 onthe cleavage along the stacking polarcaxis and on the second harmonic generationresponse, which decreases from Ba2TiOSi2O7to Ba2SnSSi2O7by a factor greater than2. The atom response theory analyses based on the density functional theory calculations reveal a remarkable difference betweenBa2TiOSi2O7and Ba2SnSSi2O7; the second harmonic generation is positive for the Ti4+ion of Ba2TiOSi2O7but negative for the Sn4+ion of Ba2SnSSi2O7. A Tauc plot analysis for Ba2SnSSi2O7assuming indirect and direct transitions led to the optical band gaps of 2.4and 2.7 eV, respectively.