Elucidating a Series of the Quaternary Selenides BaRECuSe₃ with a Rich Library of Optical Properties

We report a series of novel selenides BaRECuSe3 (RE = Nd, Tb-Ho and Tm-Lu), which were readily fabricated with high yields (91-100%) from a mixture of quaternary barium oxides using a reductive selenidation method. Using the same synthetic procedure, polycrystalline samples of the previously reported selenides BaRECuSe3 (RE = Y, Gd and Er) were produced. All the obtained selenides formed in orthorhombic space group Cmcm and exhibited the same KZrCuS3 structure type, constructed from distorted RESe6 octahedra, trigonal prisms BaSe6 and CuSe4 tetrahedra. The overall crystal structures of the reported selenides are fabricated from 2D layers produced by alternate 1D chains of RESe6 and CuSe4 polyhedra, separated by 1D chains of the BaSe6 trigonal prisms. The resulting selenides were discussed in comparison with MRECuCh(3) (M = Sr and Eu; Ch = S and Se). In MRECuCh(3), an increase in r[M2+] or r[Ch(2-)] leads to a more rapid formation of the structure with Cmcm symmetry. Optical properties of the obtained selenides were studied by means of diffuse reflectance spectroscopy, which revealed values of 1.48-2.20 and 1.05-1.45 eV for direct and indirect band gaps, respectively. An increase in the band gap in MRECuCh(3) has been established with a decrease in r[M2+] or r[Ch(2-)].