Synthesis,Crystal Structures,and Optical Properties of New Quaternary Metal Chalcogenides of Group 5: Cs2AgVS4, K2AgVSe4, Rb2AgVSe4, Rb2AgNbS4, and Cs2AgNbSe4

The new quaternary compounds Cs₂AgVS₄, K₂AgVSe₄, Rb₂AgVSe₄, Rb₂AgNbS₄, and Cs₂AgNbSe₄ were prepared using the reactive flux method. In this structure type infinite chains of edge-sharing AgQ₄- and M^VQ₄-tetrahedra are running parallel to the crystallographic a-axis. The chains are separated by alkali cations. A linear relationship between the size of the alkali cation A⁺ and the Q–Q interchain distances was found. These compounds are isostructural with the analogous quaternary copper chalcogenides. The optical properties were studied by collecting UV/Vis transmission and reflectance spectra which allowed to derive the optical band gaps. The colours of the vanadium compounds range from black to dark violet with optical band gaps between 1.7 and 1.8 eV. In addition, the behaviour of the samples was studied using polarized light. Under these experimental conditions the niobium compound Rb₂AgNbS₄ changes its colour from green to red when the direction of the polarization plane is changed by 90°.