Solid Solution\n(V<sub>1–<i>x</i></sub>Nb<sub><i>x</i></sub>)OPO<sub>4</sub> (0.1 ≤ <i>x</i> < 1.0): A Combined\nExperimental and Theoretical Study\nto Understand Substitution

Single-phase synthesis of α<sub>II</sub>-VOPO<sub>4</sub> has been achieved by meticulous control of the seed formation\nand\nwater vapor. The solid solution (V<sub>1–<i>x</i></sub>Nb<sub><i>x</i></sub>)OPO<sub>4</sub> (0.1 ≤ <i>x</i> < 1.0) (α<sub>II</sub>-VOPO<sub>4</sub>/MoOPO<sub>4</sub>-type structure) has been obtained by solution combustion\nsynthesis. It is thermodynamically stable only for <i>x</i> ≥ 0.8; for smaller <i>x</i> values, equilibration\nleads to β-VOPO<sub>4</sub> and (V<sub>0.2</sub>Nb<sub>0.8</sub>)OPO<sub>4</sub>. With increasing niobium content, the <i>a</i>-axis of the tetragonal unit cell increases, while the <i>c</i>-axis decreases. Calculated model structures (density functional\ntheory (DFT) using CRYSTAL17, PW1PW hybrid functional, and D3 dispersion\ncorrection) suggest that the dopant Nb<sup>5+</sup> cations lead to\nless distorted [(V<sup>V</sup> ≡ O)O<sub>4</sub>O] octahedra\nin their vicinity. The experimental vibrational (IR, Raman) and electronic\nultraviolet/visible (UV/vis) spectra show considerable variation with\ncomposition, perfectly reflecting the calculated dopant effects. These\nresults suggest that within the solid solution [(V<sup>V</sup> ≡\nO)O<sub>4</sub>O] and [(Nb<sup>V</sup> ≡ O)O<sub>4</sub>O]\npolyhedra are present as in the boundary phases in addition to just\none type of geometrically slightly modified [(V<sup>V</sup> ≡\nO)O<sub>4</sub>O] and [(Nb<sup>V</sup> ≡ O)O<sub>4</sub>O]\ngroups. The continuous variation of lattice parameters with <i>x</i> is related to the concentration of these four types of\npolyhedra. Reversible reduction (hydrogen) and reoxidation (air) of\n(V<sub>1–<i>x</i></sub>Nb<sub><i>x</i></sub>)OPO<sub>4</sub> are possible at 400 °C. Thus, (V<sub>0.5</sub><sup>V</sup>Nb<sub>0.5</sub><sup>V</sup>)OPO<sub>4</sub> yields (V<sub>0.5</sub><sup>III</sup>Nb<sup>V</sup>)O<sub>0.5</sub>PO<sub>4</sub>, which decomposes at 800 °C in a sealed ampule to V<sup>III</sup>PO<sub>4</sub> and Nb<sup>V</sup>OPO<sub>4</sub>.