Pr4- m-n Tb m Er n Ge3S12: from structure to properties

Abstract

Sulfides composition Pr4-m-nTbmErnGe3S12 were obtained by sintering of elemental components in evacuated quartz ampoules at 1100 °C for 4 hours. To achieve structural homogeneity, the alloys were subjected to homogenizing annealing at 500°C for one month. The studied sulfides are formed by isomorphic substitution of praseodymium atoms in the 6a and 18b sites of the compound Pr4Ge3S12 (La4Ge3S12 structure type; S.G. [[EQUATION]]) with Tb or Er atoms with smaller ionic radii. Such substitution ensures the stability of the crystal lattice and significantly affects the electronic structure, and therefore the functional properties of the synthesized sulfides. Our structure studies established the unit cell parameters of the synthesized sulfides. The determined parameters have the following values:a = 19.231(2) – 19.2560(9) Å and c = 7.925(1) – 7.9477(7) Å. Analysis of the crystal structure showed that the rare earth atoms in the sulfides do not occupy strictly ordered positions, but are statistically distributed between two sites, 6a and 18b. Atoms of statistical mixture M1 (Pr+ErTb) occupy the 6a sites, centering trigonal prisms with three additional atoms. Atoms of the statistical mixture M2 (Pr+TbEr) occupy the 18b sites, centering trigonal prisms with two additional atoms. Ge atoms form tetrahedra with sulfur atoms. The band gap values ​​for these sulfides were determined by the Tauc method to be 2.45-2.83 eV. Isomorphic substitution by cations with smaller ionic radii significantly affects the electronic structure and functional properties. The stable non-centrosymmetric lattice and tunable structural parameters position these sulfides as promising materials for applications in nonlinear optics and photonics.

Keywords: rare earth metals, sulfides, crystal structure, X-ray powder diffraction, energy dispersive X-ray spectroscopy