Stacking disorder in Mo1+xV2−xO8 phase (0.12⩽x⩽0.18). Solid state chemistry-X-rays-TEM-physical properties

The title phase and its homogeneity range (0.12⩽x⩽0.18) have been established via several controlled solid state reactions at 630 °C. X-ray diffraction and TEM investigations indicate that the diffraction patterns are dominated by diffuse scattering due to heavily faulted layer stacking. After several attempts crystals of better quality were obtained and studied. The structural refinements carried out on one selected crystal from a batch treated at 700 °C and annealed at 600 °C during 4 days presents a formula slightly richer in Mo6+ and V4+, corresponding to x≈0.5, i.e., Mo1.5V1.5O8. The main structural features are reported and analysed. The structure of these crystals is still affected by stacking defects, but in lower concentrations, leading to twinning phenomena. A detailed investigation by TEM and HREM of crystals corresponding to x=0.15 confirms the drastic stacking disorder of these materials. Magnetic and electric measurements allow to ascribe the mixed valence formula, Mo6+1+xV5+2−2xV4+xO8 for these various compositions which shows a semi-conducting behaviour with an activation energy for x=0.15 phase being Ea=0.22eV.