Quaternary Rare-Earth Oxyselenides RE4Ga2Se7O2 (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 °C. They adopt a new structure type (orthorhombic, space group Pnma, a = 11.721(2)–11.683(2) Å, b = 3.9882(7)–3.9667(7) Å, c = 29.644(5)–29.581(5) Å, 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.