High pressure synthesis, crystal structure and electronic properties of Ba₃Hf(Se_1–xTe_x)₅ (x = 0–1)

Quasi one-dimensional polycrystalline samples of Ba₃Hf(Se_1−xTe_x)₅ (x = 0–1) are synthesized under high-temperature and high-pressure conditions. Using the powder x-ray diffraction technique and first-principles calculations, Ba₃HfSe₅ is identified as having a hexagonal structure with a space group of P6₃/mcm (193) and lattice constants of a = 9.5756(1) Å, c = 6.3802(7) Å. The structure is composed of Hf(Se₁)₆ chains and Se₂ linear chains extending along the c-axis. As the doping content of Te increases, the lattice expands and leads to 5.8% and 7.3% increases of the a and c values and a 20.1% increase of the unit cell volume of Ba₃HfTe₅ compared to Ba₃HfSe₅. The detailed structural refinements show that the Hf vacancies decrease gradually as Te doping increases in the Ba₃Hf(Se_1−xTe_x)₅ (x = 0–1) materials, which leads to a decrease of electronic localization. In addition, the lower electronegativity of Te and the more extended orbitals with respect to Se contribute to orbital overlap between the inter chains. All these dominate the enhanced electron hopping, leading to a reduction of the bandgap from 1.95 eV to 0.23 eV for Ba₃Hf(Se_1−xTe_x)₅ (x = 0–1) materials as the Ba₃HfSe₅ evolves to Ba₃HfTe₅.