AAg₃Ga₈Se₁₄ (A = Rb, Cs): second-harmonic generation responses realized through the parallel arrangement of AgSe₄ and GaSe₄ tetrahedrons

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via solid-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group Cm (no. 8) and their three-dimensional [Ag3Ga8Se14]- anionic frameworks were comprised of AgSe4 and GaSe4 tetrahedrons. Their UV-Vis-near infrared diffuse reflectance spectra showed that 1 and 2 possessed wide band gaps of 2.17 and 2.10 eV, respectively. Notably, under incident laser irradiation at 1910 nm, compounds 1 and 2 presented moderate second-harmonic generation responses of 0.6 and 0.7 × AgGaS2, respectively, with phase-matching behaviours due to the parallel arrangement of nonlinear optical (NLO) functional tetrahedral AgSe4 and GaSe4 units. The laser-induced damage thresholds of 1 and 2 were estimated to be 25.4 and 18.0 MW cm-2, respectively, which were 2.1 and 1.5 times the threshold of AgGaS2. This study revealed that the title selenides, which were constructed from tetrahedral units arranged in a parallel array, are promising infrared NLO materials.