Quaternary chalcogenides BaRE2In2Ch7 (RE = La-Nd; Ch = S, Se) containing InCh5 trigonal bipyramids.

Eight new quaternary chalcogenides BaRE2In2Ch7 (RE = La-Nd; Ch = S, Se) have been prepared by reactions of BaCh, In2Ch3, RE, and Ch at high temperatures. They adopt orthorhombic structures (space group Pbam, Z = 2; a = 11.6300(8)-11.5895(7) Å, b = 12.4202(9)-12.3001(8) Å, c = 4.0689(3)-4.0028(2) Å for the sulfides; a = 12.1515(6)-12.1358(10) Å, b = 12.9367(7)-12.8510(11) Å, c = 4.1966(2)-4.1363(4) Å for the selenides) containing one-dimensional anionic [In2Ch7] ribbons of corner-sharing InCh5 trigonal bipyramids, separated by Ba and RE cations. The structure is an ordered variant of the rare Eu3Sn2S7-type with Ba atoms occupying larger sites with nearly cubic geometry and RE atoms occupying smaller sites with bicapped trigonal prismatic geometry. The InCh5 trigonal bipyramids, which are unusual, exhibit four shorter In-Ch bonds and a fifth longer one. Band structure calculations indicate that BaLa2In2S7 is a direct gap semiconductor, and corroborate the description of the In coordination as CN4 + 1. On the basis of the optical absorption spectra, band gaps were estimated to be 1.87(2) eV for BaLa2In2S7 and 1.66(2) eV for BaLa2In2Se7.