Структурний тип Y1,32Pb1,68Ge1,67Se7

Abstract

Structure type Y1,32Pb1,68Ge1,67Se7 is realized for Sm1,32Pb1,68Ge1,67Se7. The crystal structure of the Sm1,32Pb1,68Ge1,67Se7 compound is built by [M(Sm + Pb)Se7] monocapped trigonal prisms, [Ge1Se4] tetrahedra, and [Ge2Se3] triangles which are connected by corners and faces. Similar structural motifs are observed in the crystal structure of Sm3Ge1,48Se7. The differences are only for the occupations of octahedra located along the c axes. The structure of Sm1,32Pb1,68Ge1,67Se7 can be obtained from the Sm3Ge1,48Se7 structure by random substitution of Sm atoms which occupy a single independent crystallographic position by Pb atoms. The substitution of Sm3+ in Sm3Ge1,48Se7 by Pb2+ with formation of Sm1,32Pb1,68Ge1,67Se7 is supported by increasing of the Ge content in octahedra (position Ge2) to satisfy charge balance requirement. The unit cell volume of the Sm1,32Pb1,68Ge1,67Se7 compound equals 0,6257 nm3 and is larger than that for Sm3Ge1,48Se7, 0,5692 nm3. The lattice parameter a for both compounds has approximately the same values, whereas the parameter c for Sm1,32Pb1,68Ge1,67Se7 is significantly larger than the same for Sm3Ge1,48Se7. Similar situation exist for both series of ternary chalcogenides R3Ge1+xSe7 (R – Lanthanide; x = 0,43 ¸ 0,49) and quaternary compounds R1,32Pb1,68Ge1,67Se7. The differences of the lattice parameter a between ternary R3Ge1+xSe7 and quaternary R1,32Pb1,68Ge1,67Se7 compounds is not significant in comparison with differences of lattice parameter c . The values of parameters c for quaternary R1,32Pb1.68Ge1,67Se7 compounds are significantly larger than those for the ternary R3Ge1+xSe7 compounds. Such increasing of the c / a ratio for quaternary R1,32Pb1,68Ge1,67Se7 compounds when compared with ternary R3Ge1+xSe7 compounds is related to the existence of large Pb2+ ions in the quaternary chalcogenides. As a result, elongation of the [Ge2Se6] octahedron along the c axis in the structure of R1,32Pb1,68Ge1,67Se7 compounds is observed. Besides, an increase of the Se – Se distances in the c direction and a decrease of the Se – Se distances in the ab plane are observed for Sm1,32Pb1,68Ge1,67Se7 when compared to Sm3Ge1,48Se7. Keywords: crystal structure; X-ray powder method; rare-earth chalcogenides.