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 4 Mn 2 Si 2 Te 9 , 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)Te 5 units along with traditional Mn(2)Te 4 tetrahedra. The main motifs of the Ba 4 Mn 2 Si 2 Te 9 structure are dimeric Si 2 Te 6 units (with Si-Si single bond), Mn(1)Te 5 , and Mn(2)Te 4 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 2 Te 9 ] 10- layers and 1∞[Mn(2)Si 2 Te 8 ] 8- chains are separated by Ba 2+ cations. The direct bandgap for the polycrystalline Ba 4 Mn 2 Si 2 Te 9 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 4 Mn 2 Si 2 Te 9 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).