Five coordinated Mn in Ba₄Mn₂Si₂Te₉: synthesis, crystal structure, physical properties, and electronic structure

We report the synthesis of single-crystals of a new transition metal-containing quaternary chalcogenide, Ba₄Mn₂Si₂Te₉, synthesized by the solid-state method at 1273 K. A single-crystal X-ray diffraction study shows that it crystallizes in the orthorhombic crystal system (space group: Pbam) with cell constants of a = 13.4690(6) Å, b = 8.7223(4) Å, and c = 10.0032(4) Å. The asymmetric unit of the structure consists of eight unique crystallographic sites: one Ba, two Mn, one Si, and four Te sites. In this structure, the two Mn sites, Mn(1) and Mn(2), are disordered, each with fractional occupancy of 50%. The short distance of 2.170(3) Å between Mn(1) and Mn(2) implies that both Mn sites are not occupied simultaneously. The Mn atoms show two types of polyhedra: unique Mn(1)Te₅ units along with traditional Mn(2)Te₄ tetrahedra. The main motifs of the Ba₄Mn₂Si₂Te₉ structure are dimeric Si₂Te₆ units (with Si-Si single bond), Mn(1)Te₅, and Mn(2)Te₄ polyhedra. The structure can be described as pseudo-two-dimensional if only Mn(1) atoms are present and one-dimensional when only Mn(2) atoms are filled in the structure. The extended 2∞[Mn(1)Si₂Te₉]^10- layers and 1∞[Mn(2)Si₂Te₈]^8- chains are separated by Ba²⁺ cations. The direct bandgap for the polycrystalline Ba₄Mn₂Si₂Te₉ sample is 0.6(1) eV, as determined from an optical absorption study consistent with the sample's black color. The resistivity study of the polycrystalline Ba₄Mn₂Si₂Te₉ also confirms the semiconducting behavior. The thermal conductivity (κ) values are extremely low and decrease with increasing temperature up to 0.46 W m^-1 K^-1 at 773 K. The DFT studies suggest that the computed bandgap depends on the magnetic ordering of Mn magnetic moments, and the value varies from ∼0.3-1.0 eV. Relative inter-atomic bond strengths of pertinent atom pairs have been analyzed using the crystal orbital Hamilton populations (COHP).