Vidyanshu Mishra

The mixed-cation tellurides Eu(Al1–xGax)2Te4 (x = 0–1) were synthesized using two thermal profiles, resulting in subtly different structural features. Samples prepared by one-step annealing at 700 °C exhibit a composition-dependent transformation from a body-centered orthorhombic structure with a larger cell (space group Ibam, x = 0–0.50) to a side-centered orthorhombic structure with a smaller cell (space group Cccm, x = 0.75–1.0) as the Ga content increases. Samples prepared by two-step annealing at 700 and 400 °C retain the Ibam structure throughout the entire composition range, implying that the Ga-richest samples such as EuGa2Te4 undergo a temperature-dependent structural transition, as confirmed from variable-temperature powder X-ray diffraction experiments. The structures consist of a common framework of infinite linear chains of edge-sharing MTe4 (M = Al, Ga) tetrahedra but differ in terms of the site distribution of intervening Eu atoms. These compounds are semiconductors with narrow optical band gaps of 0.6–0.9 eV. Based on calculated transport properties, they may be suitable candidates for thermoelectric applications at high temperatures. They undergo electrocatalysis for water splitting, but with room for improvement in their performance.

The mixed chalcogenides La₃Ga_1.67(S_1-xSe_x)₇ form a complete solid solution, with members at increments of x = 0.14 prepared as phase-pure samples. Based on their powder X-ray diffraction (XRD) patterns, the cell volume increases with greater Se substitution, but the cell parameters vary in a nonmonotonic way. Single-crystal XRD studies at room temperature indicated that they adopt noncentrosymmetric hexagonal structures (in space group P6₃) containing stacks of Ga-centered tetrahedra and stacks of octahedra with partially occupied Ga sites. Strong preferences of S vs Se atoms within three types of chalcogen sites lead to unusual structural changes in the intermediate members of the solid solution and can be understood in terms of the need to satisfy optimum bonding requirements. Structure determination of the selenide end-member La₃Ga_1.67Se₇ at low temperature (100 K) revealed a supercell (in space group P6₁) with a tripled c-axis characterized by distortions of the partially occupied Ga sites and the coordinating Se atoms, which help relieve bond strain. Two Ga sites in tetrahedral and roughly octahedral geometry were assigned by ⁷¹Ga solid-state nuclear magnetic resonance spectroscopy. The experimental optical band gaps vary from 2.6 eV for La₃Ga_1.67S₇ to 2.0 eV for La₃Ga_1.67Se₇.

The ternary rare-earth sulfides RE2SnS5 (RE = La-Nd) and the partial solid solutions RE2Sn(S1-xSex)(5) (RE = La, Ce; x = 0-0.8) were prepared in the form of polycrystalline samples by reaction of the elements at 900 degrees C and as single crystals in the presence of KBr flux. They adopt the La2SnS5-type structure (orthorhombic, space group Pbam, Z = 2) consisting of chains of edge-sharing SnCh(6) octahedra separated by RE atoms. Although the cell parameters evolve smoothly in RE2Sn(S1-xSex)(5), detailed structural analysis by single-crystal X-ray diffraction revealed a pronounced preference for the Se atoms to occupy two out of the three chalcogen sites, which offers a rationalization for why the all-selenide end-members RE2SnSe5 do not form. Solid-state Sn-119 NMR spectra confirmed the nonrandom distribution of SnS6-nSen local environments, which could be resolved into individual resonances. The Raman spectra of RE2SnS5 compounds show an intense peak at 307-320 cm(-1) assigned to a symmetric A(1g) mode, which is dominated by Sn-S bonds; the Raman peak intensities varied with Se substitution in La2Sn(S1-xSex)(5). Optical diffuse reflectance spectra, band structure calculations, and electrochemical impedance spectra indicated that these compounds are narrow band gap semiconductors; the optical band gaps are insensitive to RE substitution in RE2SnS5 (0.7 eV) but they gradually decrease with greater Se substitution in RE2Sn(S1-xSex)(5) (0.7-0.4 eV).