Crystal structures of the La3AgSnSe7 and R3Ag1−δSnS7 (R=La, Ce; δ=0.18−0.19) compounds

The crystal structures of new quaternary compounds La₃AgSnSe₇ (space group P6₃, Pearson symbol hP24, a=1.0805(4) nm, c=0.6245(1) nm, R1=0.0315), La₃Ag_0.82SnS₇ (space group P6₃, Pearson symbol hP23.64, a=1.0399(1) nm, c=0.6016(1) nm, R1=0.0149) and Ce₃Ag_0.81SnS₇ (space group P6₃, Pearson symbol hP23.62, a=1.0300(1) nm, c=0.6002(1) nm, R1=0.0151) were determined by means of X-ray single crystal diffraction. Structural investigations of the R₃Ag_1−δSnS₇ (R=La, Ce; δ=0.18-0.19(1)) compounds at 450 and 530 K were performed. Low temperature data (12 K) for Ce₃Ag_0.81SnS₇ were also collected. The nearest neighbours of the La(Ce), Ag and Sn atoms are exclusively Se(S) atoms. The latter form distorted trigonal prisms around the La(Ce) atoms, and distorted tetrahedrons around the Sn atoms. The Ag (Ag1) atoms have triangular surroundings: they are located very close to the planes built of three Se(S) atoms. The Ag2 atoms in the structures of the La₃Ag_0.82SnS₇, Ce₃Ag_0.81SnS₇ compounds are located practically in the centres of trigonal antiprisms. The pseudo-potentials determined through the Ag atoms show relatively low barrier between two nearest positions which decreases when temperature rises.