来源：ACS Publications
A series of vanadium-doped orthoniobates of the type La(Nb1–xVx)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.
In this structure, the larger A- and smaller M-site cations occupy the 4b (0, 1/4, 5/8) and 4a (0, 1/4, 1/8) Wyckoff positions. The layered structure is completed by the oxygen anions that occupy the general 16f (x, y, z) position. The structure is built on a network of edge-sharing eight-coordinate A-site polyhedra and unlinked four-coordinate M-site tetrahedra. Representations of the structures are given . The corresponding monoclinic fergusonite-type structure is commonly described in space groups I2/b or I2/a, both of which are alternate settings of the C2/c (#15) space group. This structure can be considered a strained scheelite structure, with displacement of the cations along the z-direction (in I2/b) to the 4e Wyckoff positions (0, 1/4, z), and splitting of the oxygen site into two different 8f sites. Both the A- and M-site polyhedra are more distorted in the monoclinic-fergusonite structure compared to the tetragonal-scheelite structure, with the AO8 polyhedra having four, rather than two, different A–O bond distances. The displacement of the M-site cation can lead to the formation of additional long M–O contacts, and bond valence sum (BVS) calculations indicate that the coordination of the M-type cation is better described as a highly distorted MO6 octahedra rather than MO4 tetrahedra in many mf structured oxides.