The family of metal thiophosphates is an important but long-ignored compound system of the nonlinear optical (NLO) materials with desirable properties for the mid-infrared (mid-IR) coherent light generation. In the present work, the mid-IR NLO capabilities of metal thiophosphate crystals are systematically investigated based on their structure-property relationship. The linear and nonlinear optical properties of these crystals are predicted and analyzed using the first-principles calculations. In particular, several metal thiophosphate compounds are highlighted to exhibit good mid-IR NLO performances, as supported by the primary experimental results. These candidates would greatly promote the development of the mid-IR NLO functional materials.

A new mercury selenide BaHgSe2 was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe3](4-) anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.