Deviations from Vegard's Law and Intense Raman Scattering in Low-Doped Bi₂Mo_1−xW_xO₆ (x = 0–10%) Ceramics

Hume-Rothery rules along with Vegard's law form the basis of systematic doping variation resulting into similar changes in unit cell parameters and volume. Many recent reports have highlighted positive–negative departure from Vegard's law in binary semiconducting systems and a few oxide alloys as well. The current work targets investigations on the missing response of bismuth molybdate on low tungsten doping onto molybdenum sites (Bi₂Mo_1−xW_xO₆). Bismuth molybdate and bismuth tungstate have been widely reported for lattice oxygen migration-driven photocatalytic response and pressure-sensitive Raman scattering. Octahedral tilting in low-tungsten-doped Bi₂Mo_0.94W_0.06O₆ carries an interesting lattice oxygen folding toward a–c plane. As a result, unit cell polarity increases drastically and becomes a source for intense Raman scattering phase that is normally observable at pressures around 8.3 GPa. An interesting departure from Vegard's law behavior at around 5–6% tungsten doping confirms nearly identical unit cell structure rather than mix unit cell-based alloy formation.