High pressure synthesis, crystal structure and electronic properties of Ba ₃ Hf(Se _{1– <i>x</i>} Te _<i>x</i> ) ₅ ( <i>x</i> = 0–1)

Abstract Quasi one-dimensional polycrystalline samples of Ba 3 Hf(Se 1− x Te 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, Ba 3 HfSe 5 is identified as having a hexagonal structure with a space group of P 6 3 / mcm (193) and lattice constants of a = 9.5756(1) Å, c = 6.3802(7) Å. The structure is composed of Hf(Se 1 ) 6 chains and Se 2 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 Ba 3 HfTe 5 compared to Ba 3 HfSe 5 . The detailed structural refinements show that the Hf vacancies decrease gradually as Te doping increases in the Ba 3 Hf(Se 1− x Te 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 Ba 3 Hf(Se 1− x Te x ) 5 ( x = 0–1) materials as the Ba 3 HfSe 5 evolves to Ba 3 HfTe 5 .