Negative Second Harmonic Response of Sn4+ in the Fresnoite Oxysulfide Ba2SnSSi2O7
Résumé
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 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 two. 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.
Domaines
Chimie
Origine : Fichiers produits par l'(les) auteur(s)