Ca2La(MS4)(BS3) (M = Ge/Si and Sn/Si): high-performance infrared nonlinear optical materials designed using an atomic site co-occupancy strategy

Abstract

Exploration of new material systems and optical performance enhancement are huge challenges for the study of infrared nonlinear optical (IR NLO) materials. In this work, the first thioborate-thiogermanate and thioborate-thiostannate compounds, Ca₂La(Ge_0.72Si_0.28S₄)(BS₃) and Ca₂La(Sn_0.75Si_0.25S₄)(BS₃), containing both co-occupied Ca²⁺/La³⁺ cation and [Ge/SiS₄]⁴⁻ or [Sn/SiS₄]⁴⁻ anion sites, respectively, were designed through an atomic site co-occupancy strategy. They inherited favourable 3D network structures in which the effectively aligned [MS₄]⁴⁻ and [BS₃]³⁻ functional anions were bridged by Ca²⁺/La³⁺ cations. Remarkably, the title compounds achieved excellent IR NLO properties, including good chemical and thermal stabilities, wide light transmission ranges (0.45–11 μm), strong second harmonic generation responses (1.5 and 2.0 times that of commercial AgGaS₂ at 2.05 μm) and high laser-induced damage thresholds (7 and 6 times that of AgGaS₂). Theoretical calculation and experimental results revealed that, on the basis of excellent structural framework, introducing more active functional groups through atomic site co-occupancy could simultaneously enhance the second harmonic generation effect and maintain a relatively high laser-induced damage threshold. This work not only offers an easier synthetic route for mixed anionic thioborates but also provides inspiration for the design of well-performed NLO materials.