Structural Evolution and Optical Tunability of Diamond-Like Semiconductors Li 2 Zn 1– x Cd x SnS 4 , Li 2 CdSn(S 1– y Se y ) 4 , and Li 2 ZnSn(S 1– y Se y ) 4

Lithium-containing chalcogenides with diamond-like structures are potential candidates for infrared nonlinear optical (NLO) materials because they provide high second harmonic generation (SHG) responses while maintaining large band gaps required for high laser-induced damage thresholds. To evaluate the effects of mixed cations and anions on these optical properties, the complete solid solutions Li2Zn1–xCdxSnS4, Li2CdSn(S1–ySey)4, and Li2ZnSn(S1–ySey)4 were synthesized and structurally characterized by a combination of single-crystal X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. Li2Zn1–xCdxSnS4 undergoes structural transitions from space group Pn to Pna21 to Pmn21 as Cd substitutes for Zn, associated with different arrangements of the metal atoms. Li2CdSn(S1–ySey)4 undergoes a structural transition from Pmn21 to Pna21 as Se substitutes for S. Li2ZnSn(S1–ySey)4 retains the Pn structure within its entire range. The band gaps are large and direct, remaining relatively constant at 3.0–3.3 eV in Li2Zn1–xCdxSnS4 upon Cd substitution and decreasing to no lower than 2.0 eV in Li2CdSn(S1–ySey)4, and Li2ZnSn(S1–ySey)4 upon Se substitution. The majority of compounds evaluated show modest SHG responses (reaching a maximum of 0.4× AgGaS2) and type-1 phase-matchable behavior at a laser wavelength of 2090 nm, but the trends are irregular. They melt congruently above 800 °C.