Syntheses, crystal structures and physical properties of three new chalcogenides: NaGaGe3Se8, K3Ga3Ge7S20, and K3Ga3Ge7Se20

Abstract

Three new chalcogenides, namely NaGaGe₃Se₈, K₃Ga₃Ge₇S₂₀, and K₃Ga₃Ge₇Se₂₀, of an A–Ga–Ge–Q (A = Na, K; Q = S, Se) system were obtained for the first time. They crystallize into two different new structures, albeit both in the monoclinic space group P2₁/c. NaGaGe₃Se₈ has a layered structure consisting of two dimensional ^∞₂[M₄Se₈]⁻ layers separated by Na⁺ cations, while the structures of K₃Ga₃Ge₇Q₂₀ (Q = S, Se) are constructed by incompletely isolated quasi-2D ^∞₂[M₁₀Q₂₁]⁵⁻ layers, leading to large channels loosely occupied by K⁺ cations. Interestingly, thermal analysis indicates that the three title compounds are all congruent-melting compounds, which is uncommon for quaternary compounds, and makes bulk crystal growth using the Bridgman technique possible. UV-vis-NIR spectroscopy measurements reveal that the optical band gaps of the three compounds are 2.35, 3.25, and 2.23 eV. In addition, the electronic structure calculations on NaGaGe₃Se₈ show that the band gap is mainly determined by the GaSe₄ and GeSe₄ groups.