Solid Solution (V1–xNbx)OPO4 (0.1 ≤ x < 1.0): A Combined Experimental and Theoretical Study to Understand Substitution Effects on the Structure, Bonding, and Catalytic Behavior

Single-phase synthesis of α_II-VOPO₄ has been achieved by meticulous control of the seed formation and water vapor. The solid solution (V_1–xNb_x)OPO₄ (0.1 ≤ x < 1.0) (α_II-VOPO₄/MoOPO₄-type structure) has been obtained by solution combustion synthesis. It is thermodynamically stable only for x ≥ 0.8; for smaller x values, equilibration leads to β-VOPO₄ and (V_0.2Nb_0.8)OPO₄. With increasing niobium content, the a-axis of the tetragonal unit cell increases, while the c-axis decreases. Calculated model structures (density functional theory (DFT) using CRYSTAL17, PW1PW hybrid functional, and D3 dispersion correction) suggest that the dopant Nb⁵⁺ cations lead to less distorted [(V^V ≡ O)O₄O] octahedra in their vicinity. The experimental vibrational (IR, Raman) and electronic ultraviolet/visible (UV/vis) spectra show considerable variation with composition, perfectly reflecting the calculated dopant effects. These results suggest that within the solid solution [(V^V ≡ O)O₄O] and [(Nb^V ≡ O)O₄O] polyhedra are present as in the boundary phases in addition to just one type of geometrically slightly modified [(V^V ≡ O)O₄O] and [(Nb^V ≡ O)O₄O] groups. The continuous variation of lattice parameters with x is related to the concentration of these four types of polyhedra. Reversible reduction (hydrogen) and reoxidation (air) of (V_1–xNb_x)OPO₄ are possible at 400 °C. Thus, (V_0.5^VNb_0.5^V)OPO₄ yields (V_0.5^IIINb^V)O_0.5PO₄, which decomposes at 800 °C in a sealed ampule to V^IIIPO₄ and Nb^VOPO₄.