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

• Single-phase polycrystalline Ba2Ge2Te5 was synthesized for the first time. • The crystal structure was confirmed by Rietveld refinement and SCXRD studies. • The resistivity measurements show metal to insulator transition below 18 K. • The thermal conductivity study of Ba 2 Ge 2 Te 5 shows a low value of 0.41 Wm–1K–1 at 773 K. • Electronic structure calculations suggest a narrow bandgap for Ba 2 Ge 2 Te 5 . A monophasic polycrystalline sample of Ba 2 Ge 2 Te 5 has been synthesized for the first time using the sealed tube solid-state method. The Rietveld refinement of a polycrystalline Ba 2 Ge 2 Te 5 and a single crystal X-ray diffraction study confirm that Ba 2 Ge 2 Te 5 crystallizes in the orthorhombic polar C 2 v 9 - Pna 2 1 space group. Each of the Ge atoms in the Ba 2 Ge 2 Te 5 structure is covalently connected to one Ge and three Te atoms making one-dimensional (1D) chains of 1 ∞ [Ge 2 Te 5 ] 4− that are separated by Ba 2+ cations. The (Ba 2+ ) 2 (Ge 3+ ) 2 (Te 2− ) 5 can be charge-balanced as per the Zintl-Klemm concept. A resistivity study of Ba 2 Ge 2 Te 5 shows a metallic behavior till 18 K below which metal to insulator transition was observed. Thermal conductivity of Ba 2 Ge 2 Te 5 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 Ba 2 Ge 2 Te 5 with a narrow indirect bandgap of about 0.6 eV.