The Crystal Structure of La3Pb0.1Ga1.6S7 and Pr3Pb0.1Ga1.6S7 Compounds
The crystal structure of La3Pb0.1Ga1.6S7 and Pr3Pb0.1Ga1.6S7 compounds was determined by X-ray powder diffraction method. The samples were prepared by solid-solid reactions of the elements at 1370 K. The compounds crystallize in the La3CuSiS7 structure type, space group P63, with the lattice and computation parameters a = 1.01902(3) nm, c = 0.60661(2) nm, RI = 0.0673, RP = 0.1546 (for La3Pb0.1Ga1.6S7) and a = 1.00034(3) nm, c = 0.60587(3) nm, RI = 0.0847, RP = 0.1623 (for Pr3Pb0.1Ga1.6S7). The crystalline structure is described with hexagonal system, in which La(Pr) atoms center trigonal prisms with one additional atom. These prisms are formed by S atoms. The atoms of the statistical mixture M1(0.600 Ga + 0.090 Pb) and M2 (0.59(1) Ga + 0.095(7) Pb) are located practically in the centers of the octahedra of S atoms. Ga atoms are located in tetrahedra of S atoms. Asymmetric structure of the studied compounds points out their non-linear properties. Adding Pb atoms to the structure of La3Ga1.67S(Se)7 compounds increases the probability of formation of the ionic component of the inter-atomic bond, and this in turn improves thermoelectric properties.
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