Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb; x = Ln vacancy): syntheses, crystal structures, optical, resistivity, and electronic structure

Abstract

Three new isostructural quaternary tellurides, Ba₂Ln_1−xMn₂Te₅ (Ln = Pr, Gd, and Yb), have been synthesized by the molten-flux method at 1273 K. The single-crystal X-ray diffraction studies at 298(2) K showed that Ba₂Ln_1−xMn₂Te₅ crystallize in the space group –C2/m of the monoclinic crystal system. There are six unique crystallographic sites in this structure's asymmetric unit: one Ba site, one Ln site, one Mn site, and three Te sites. The Ln site in the Ba₂Ln_1−xMn₂Te₅ structure is partially filled, which leaves about one-third of the Ln sites vacant (□) for Pr and Gd compounds. These structures do not contain any homoatomic or metallic bonding and can be charge-balanced as (Ba²⁺)₂(Gd/Pr³⁺)_2/3(Mn²⁺)₂(Te²⁻)₅. The refined composition for the Yb compound is Ba₂Yb_0.74(1)Mn₂Te₅ and can be charge-balanced with a mixed valence state of Yb²⁺/Yb³⁺. The crystal structures of Ba₂Ln_1−xMn₂Te₅ consist of complex layers of ²_∞[Ln_1−xMn₂Te₅]⁴⁻ stacked along the [100] direction, with Ba²⁺ cations separating these layers. The Ln atoms are bound to six Te atoms that form a distorted octahedral geometry around the central Ln atom. Each Mn atom in this structure is coordinated to four Te atoms in a distorted tetrahedral fashion. These LnTe₆ and MnTe₄ units are the main building blocks of the Ba₂Ln_1−xMn₂Te₅ structure. The optical absorption study performed on a polycrystalline Ba₂Gd_2/3Mn₂Te₅ sample reveals a direct bandgap of 1.06(2) eV consistent with the DFT study. A semiconducting behavior was also observed for polycrystalline Ba₂Gd_2/3Mn₂Te₅ from the resistivity study. The temperature-dependent magnetic studies on a polycrystalline sample of Ba₂Gd_2/3Mn₂Te₅ did not reveal any long-range magnetic order down to 5 K. The effective magnetic moment (μ_eff) of 10.37μ_B calculated using the Curie–Weiss law is in good agreement with the theoretical value (μ_cal) of 10.58μ_B.