Solid Solution (V1–x Nb x )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 αII-VOPO4 has been achieved by meticulous control of the seed formation and water vapor. The solid solution (V1–x Nb x )­OPO4 (0.1 ≤ x < 1.0) (αII-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 (V0.2Nb0.8)­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 [(VV ≡ 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 [(VV ≡ O)­O4O] and [(NbV ≡ O)­O4O] polyhedra are present as in the boundary phases in addition to just one type of geometrically slightly modified [(VV ≡ O)­O4O] and [(NbV ≡ 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 (V1–x Nb x )­OPO4 are possible at 400 °C. Thus, (V0.5 VNb0.5 V)­OPO4 yields (V0.5 IIINbV)­O0.5PO4, which decomposes at 800 °C in a sealed ampule to VIIIPO4 and NbVOPO4.