Reactive molten-flux assisted syntheses of single crystals of Cs19Ln19Mn10Te48 (Ln = Pr and Gd) crystallizing in a new structure type

Abstract

Two new quaternary tellurides, Cs₁₉Ln₁₉Mn₁₀Te₄₈ (Ln = Pr and Gd), were synthesized by the reactive molten-flux method at 1273 K using an excess of CsCl as a reactive flux. The single-crystal X-ray diffraction studies reveal that these compounds are isostructural and crystallize in a new structure type in the space group C³_2h–C2/m of the monoclinic crystal system with two formula units. There are forty-nine unique crystallographic sites in the unit cell of the structure: ten Cs sites, ten Ln sites, five Mn sites, and twenty-four Te sites. All atoms have a site symmetry of .m. except for Cs(10) and Ln(10), which have a site symmetry of 2/m. The unique crystal structure of Cs₁₉Ln₁₉Mn₁₀Te₄₈ consists of complex layers of ²_∞[Ln₁₉Mn₁₀Te₄₈]¹⁹⁻ that are separated by filling of Cs⁺ cations between the layers. The Ln atoms are bonded to six Te atoms that form a distorted octahedral geometry around the central Ln atom, whereas Mn atoms are coordinated to four Te atoms in a distorted tetrahedral fashion. The building blocks of these structures (LnTe₆ octahedra and MnTe₄ tetrahedra) are fused in a complex fashion to create ²_∞[Ln₁₉Mn₁₀Te₄₈]¹⁹⁻ layers. These structures do not contain any homoatomic bonding and hence, can be easily charge-balanced as (Cs¹⁺)₁₉(Ln³⁺)₁₉(Mn²⁺)₁₀(Te²⁻)₄₈ as per the Zintl–Klemm concept. The optical absorption study on finely ground single crystals of the Gd-compound reveals a direct bandgap of about 0.5(1) eV. The DFT studies also suggest both systems to be semiconductors with bandgaps of ∼0.5 eV consistent with the experimental value.