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

Abstract

Abstract BIHFVOX, formulated as Bi 4 Hf x V 2− x O 11−( 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 Hf 4+ 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.