Five coordinated Mn in Ba 4 Mn 2 Si 2 Te 9 : synthesis, crystal structure, physical properties, and electronic structure

We report the synthesis of single-crystals of a new transition metal-containing quaternary chalcogenide, Ba Mn Si Te , 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: ) with cell constants of = 13.4690(6) Å, = 8.7223(4) Å, and = 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)Te units along with traditional Mn(2)Te tetrahedra. The main motifs of the Ba Mn Si Te structure are dimeric Si Te units (with Si-Si single bond), Mn(1)Te , and Mn(2)Te 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)Si Te ] layers and 1∞[Mn(2)Si Te ] chains are separated by Ba cations. The direct bandgap for the polycrystalline Ba Mn Si Te 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 Ba Mn Si Te also confirms the semiconducting behavior. The thermal conductivity ( ) values are extremely low and decrease with increasing temperature up to 0.46 W m K 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).