S.C.M. Chan

Phase stability, structure and electrical conductivity in the system Bi2ZrxV1−xO5.5−(x/2)−δ

Abstract The BIMEVOX system, Bi 2 Zr x V 1− x O 5.5− x /2 , has been investigated using ac impedance spectroscopy and X-ray powder diffraction, in order to examine the effects of vacancy and dopant cation concentration on γ-phase stability and ionic conductivity. Four crystallographically distinct phases are observed at ambient temperature over the composition range 0.05≤ x ≤0.50. Below x =0.10, the orthorhombic α-phase is seen. Between x =0.10 and 0.16, the β-phase is stabilised. The stabilisation of orthorhombic α and β phases at lower compositions is typical for BIMEVOXes and results from an ordering of oxide ion vacancies. At x =0.19, the data were modelled on a tetragonal γ-phase cell, while at compositions where x ≥0.22 a mixture of the tetragonal γ-phase and Bi 8 V 2 O 17 is observed. The electrical conductivity is correlated with the stabilisation of the various polymorphs within the system. The observed γ-phase stabilisation region and the solid solution limit are discussed with respect to the defect structure.

Phase stabilisation in the pseudo-binary system Bi2MgO4–Bi2VO5.5−δ

Abstract The structure and electrical conductivity of compositions in the pseudo-binary system Bi 2 MgO 4 –Bi 2 VO 5.5− δ were investigated beyond the BIMEVOX stabilisation region using X-ray powder diffraction and ac impedance spectroscopy. Compositions of the general formula Bi 2 V 1− x Mg x O 5.5−3 x /2 were studied and several discrete phases were identified, viz: x =0.30, orthorhombic BIMGVOX; x =0.50, Bi 8 V 2 O 17 ; x =0.60, Bi 12 V 2 O 23 ; x =0.80, a δ-Bi 2 O 3 -type phase; and x =0.90, a γ-Bi 2 O 3 -type phase. In between these compositions, mixtures of phases are observed. High-temperature diffraction reveals phase separation at x =0.30 and 0.90. The structural data correlate well with variations in activation energy and conductivity.