Structural diversities in centrosymmetric La8S4Cl8La12S8Cl4[SbS3]8 and non-centrosymmetric Ln12S8Cl8[SbS3]4 (Ln = La and Ce): syntheses, crystal and electronic structures, and optical properties

Abstract

Three thioantimonides charge compensated by Ln/S/Cl cationic layers, namely, La₈S₄Cl₈La₁₂S₈Cl₄[SbS₃]₈ and Ln₁₂S₈Cl₈[SbS₃]₄ (Ln = La and Ce), have been discovered by conventional solid-state reactions. The former crystallizes in the centrosymmetric space group Pbcm (no. 57), while the latter adopts the polar non-centrosymmetric space group Cc (no. 9). Both of them contain isolated SbS₃ trigonal-pyramidal units, which are connected either with the alternating centric [La₈S₄Cl₈]⁸⁺ and mirror-symmetric [La₁₂S₈Cl₄]¹⁶⁺ cationic layers perpendicular to the [001] direction in La₈S₄Cl₈La₁₂S₈Cl₄[SbS₃]₈ or with the acentric [Ln₁₂S₈Cl₈]¹²⁺ cationic layers perpendicular to the [100] direction in Ln₁₂S₈Cl₈[SbS₃]₄. Interestingly, the discrete SbS₃ trigonal pyramids pack in a centrosymmetric and non-centrosymmetric fashion in La₈S₄Cl₈La₁₂S₈Cl₄[SbS₃]₈ and La₁₂S₈Cl₈[SbS₃]₄, respectively, which can be ascribed to the different compositions and packing fashions in Ln/S/Cl cationic layers. In addition, optical gaps of 2.31 and 2.60 eV for La₁₂S₈Cl₈[SbS₃]₄ and La₈S₄Cl₈La₁₂S₈Cl₄[SbS₃]₈, respectively, were determined by UV/vis reflectance spectroscopy, showing a blue shift with respect to La₇Sb₉S₂₄, which can be attributed to the greater contributions of La to the bottom of the CB as confirmed by the DFT study.