Metal to insulator transition in Ba2Ge2Te5: Synthesis, crystal structure, resistivity, thermal conductivity, and electronic structure

A monophasic polycrystalline sample of Ba2Ge2Te5 has been synthesized for the first time using the sealed tube solid-state method. The Rietveld refinement of a polycrystalline Ba2Ge2Te5 and a single crystal X-ray diffraction study confirm that Ba2Ge2Te5 crystallizes in the orthorhombic polar C-2v(9)-Pna2(1) space group. Each of the Ge atoms in the Ba2Ge2Te5 structure is covalently connected to one Ge and three Te atoms making one-dimensional (1D) chains of 1 infinity [Ge2Te5](4-) that are separated by Ba2+ cations. The (Ba2+)(2)(Ge3+)(2)(Te2-)(5) can be charge-balanced as & INFIN; per the Zintl-Klemm concept. A resistivity study of Ba2Ge2Te5 shows a metallic behavior till 18 K below which metal to insulator transition was observed. Thermal conductivity of Ba2Ge2Te5 was found to decrease gradually on heating the sample with a minimum of about 0.41 Wm(-1)K(-1) at 773 K. The DFT studies predict semiconducting nature for Ba2Ge2Te5 with a narrow indirect bandgap of about 0.6 eV.