Structural Frustration and Occupational Disorder: The Rare Earth Metal Polysulfides Tb8S14.8, Dy8S14.9, Ho8S14.9, and Y8S14.8

Abstract

Dark red crystals of Y₈S(14.8), Tb₈S(14.8), Dy₈S(14.9), and Ho₈S(14.9) have been obtained following different reaction routes. The isostructural title compounds adopt the Gd₈Se₁₅ type, a 24-fold superstructure of the ZrSSi-type and can be described in space group A112 (non standard setting of C121, no. 5) with lattice parameter of a = 11.505(1) Å, b = 15.385(1) Å, c = 15.726(1) Å, and γ = 90.21(2)° for Y₈S(15-x); a = 11.660(1) Å, b = 15.468(2) Å, c = 15.844(2) Å, and γ = 90.19(2)° for Tb₈S(15-x); a = 11.584(1) Å, b = 15.340(2) Å, c = 15.789(2) Å, and γ = 90.34(2)° for Dy₈S(15-x); and a = 11.538(1) Å, b = 15.288(2) Å, c = 15.740(2) Å, and γ = 90.23(1)° for Ho₈S(15-x), respectively. The structure consists of an alternating stacking of puckered [RES] (RE, rare-earth metals) double slabs and planar sulfur layers along [001]. The planar sulfur layers have a complex arrangement of S₂²⁻ dinuclear dianions, isolated S²⁻ ions, and vacancies. All compounds contain trivalent rare-earth metal ions, for Tb₈S(15-x) and Dy₈S(15-x) antiferromagnetic order was found at T(N) = 5.4(2) K and 3.8(1) K, respectively. Short wavelength cutoff optical band gaps of 1.6 to 1.7 eV were determined.