Role of Disorder on the Unusual Composition Dependence of the Structures in V-Doped LaNbO4

A series of vanadium-doped orthoniobates of the type La(Nb_1–xV_x)O₄ (x = 0.00–0.40) were synthesized using a solid-state method, and their long-range average and short-range local structures comprehensively investigated using a combination of high-resolution neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) that produced accurate and precise structures of several members of the series. The structures evolve from a monoclinic fergusonite-type to a tetragonal scheelite-type structure as the V content increases. Substituting Nb⁵⁺ by the smaller V⁵⁺ cations induces an atypical expansion in lattice parameters, most notably an increase in unit cell volume within the monoclinic fergusonite phase, attributed to pronounced elongation of the longer crystallographic c-axis. A progressive shift from distorted six-coordinate octahedra to four-coordinate tetrahedra of the mixed Nb/V site upon V doping was established through X-ray absorption and Raman spectroscopy. Variable-temperature S-XRD measurements were employed to assess the influence of V incorporation on the thermally induced ferroelastic monoclinic fergusonite-to-tetragonal scheelite phase transition, including quantification of the coefficients of thermal expansion. The anomalous compositional and thermal dependence of the unit cell parameters is correlated with cation displacements.