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

Abstract

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.