Quaternary Rare-Earth Oxyselenides RE ₄Ga₂Se₇O₂ (RE = Pr, Nd) with Trigonal Bipyramidal GaSe₅ Units: Evaluation of Optical, Thermoelectric, and Electrocatalytic Properties

Phase-pure samples and single crystals of the rare-earth oxyselenides RE4Ga2Se7O2 (RE = Pr, Nd) were prepared by reactions at 950 degrees C. They adopt a new structure type (orthorhombic, space group Pnma, a = 11.721(2)-11.683(2) & Aring;, b = 3.9882(7)-3.9667(7) & Aring;, c = 29.644(5)-29.581(5) & Aring;, Z = 4) consisting of RESe6 trigonal prisms, GaSe4 tetrahedra, and GaSe5 trigonal bipyramids linked to form corrugated layers between which strips of edge-sharing RE4O tetrahedra are inserted. The bonding character is mostly ionic within RE-O and RE-Se blocks but mostly covalent with Ga-Se blocks, as supported by electronic structure calculations. Both compounds show a nearly direct optical band gap of 1.7 eV. Their Seebeck coefficients and electrical conductivities are predicted to be similar to other oxychalcogenides, with the thermoelectric power factors being improved by n-doping. They exhibit high electrocatalytic performance for the oxygen evolution reaction, with the Pr member demonstrating superior activity, as characterized by a low overpotential of 257 mV (at a current density of 10 mA cm(-2)), a high mass activity of 70.1 A g(-1), and a turnover frequency of 0.0234 s(-1) in 1 M KOH electrolyte.