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

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 [M4Se8](-) layers separated by Na(+) cations, while the structures of K3Ga3Ge7Q20 (Q = S, Se) are constructed by incompletely isolated quasi-2D [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.