Influence of dopant concentration on the phase transition and ionic conductivity in BIHFVOX system

BIHFVOX, formulated as Bi4HfxV2−xO11−(x/2)−δ, is a new member of BIMEVOX family. The system with various dopant concentrations (0⩽x⩽0.40) was prepared by the solid state reaction. The phase stability as a function of composition was investigated using FT-IR, X-ray powder diffraction, differential scanning calorimetry and conductivity measurements. For x=0.05, the α-polymorph was noticed at room temperature with clear evidence for two successive transitions; α↔β↔γ at 436 and 561 °C, respectively. For relatively higher dopant concentrations, 0.15⩽x⩽0.20, the β↔γ transition was clearly evident. The variation of unit cell parameters as a function of temperature for x=0.20 exhibited a subtle change in c parameter around 400 °C due to the vacancy ordering phenomenon which is associated with occurrence of such transition. However, the existence of order-disorder, γ′↔γ transition was clearly confirmed for x⩾0.25. It was also noticed that the increased dopant concentration of highly sized and charged Hf4+ ion has a significant role in collapsing the fully disordered tetragonal into orthorhombic β′-domain at higher temperatures. AC impedance spectroscopy showed that the ionic conductivity is principally due to the grain contribution as clearly evident in the increased short-range diffusibility of oxide ion vacancy in the grains with increasing temperature. The composition dependence of conductivity exhibited a maximum for x=0.25 at lower temperatures.