Semiconducting Sm₃GaSe₅O with trigonal bipyramidal GaSe₅ units

The oxyselenide Sm3GaSe5O was prepared by reaction of Sm2O3, Sm, Ga2Se3, and Se at 950 degrees C. It adopts a new monoclinic structure type (space group P21/m, a = 11.042(9) angstrom, b = 3.9761(3) angstrom, c = 21.2458(17) angstrom, beta = 92.2436(14)degrees, Z = 4) built up of Sm-Se, Sm-O, and Ga-Se blocks. GaSe5 trigonal bipyramids, which are un-precedented, are the most unusual feature of this structure. Electronic structure calculations corroborate the occurrence of primarily ionic bonding within the Sm-Se and Sm-O blocks and covalent bonding within the Ga-Se blocks. Optical measurements reveal a nearly direct band gap of 1.2 eV, narrower than those of many other oxychalcogenides.