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 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 were synthesized and structurally characterized by a combination of single-crystal X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. Li 2 Zn 1– x Cd x SnS 4 undergoes structural transitions from space group Pn to Pna 2 1 to Pmn 2 1 as Cd substitutes for Zn, associated with different arrangements of the metal atoms. Li 2 CdSn(S 1– y Se y ) 4 undergoes a structural transition from Pmn 2 1 to Pna 2 1 as Se substitutes for S. Li 2 ZnSn(S 1– y Se y ) 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 Li 2 Zn 1– x Cd x SnS 4 upon Cd substitution and decreasing to no lower than 2.0 eV in Li 2 CdSn(S 1– y Se y ) 4, and Li 2 ZnSn(S 1– y Se y ) 4 upon Se substitution. The majority of compounds evaluated show modest SHG responses (reaching a maximum of 0.4× AgGaS 2 ) and type-1 phase-matchable behavior at a laser wavelength of 2090 nm, but the trends are irregular. They melt congruently above 800 °C.