Kinetically Controlled Reduction of β-Vanadyl(V) Orthophosphate: Synthesis and Characterization of New Metastable Polymorphs of Vanadium(III) Phosphate.

Two thermodynamically metastable polymorphs of vanadium(III) phosphate, V III PO 4 - m 1 and VPO 4 - m 2, have been obtained via reduction of β-V V OPO 4 by moist hydrogen. The XRPD pattern of VPO 4 - m 1 can be assigned based on the crystal structure of β-V V OPO 4 , though with distinctly different lattice parameters (VPO 4 - m 1/β-VOPO 4 : Pnma , a = 7.3453(12)/7.7863(5) Å, b = 6.4001(12)/6.1329(3) Å, c = 7.3196(13)/6.9673(5) Å). The XRPD pattern of VPO 4 - m2 was found to be very similar to that of Fe 2 (VO)(P 2 O 7 )(PO 4 ) (VPO 4 - m 2: P 2 1 / m , Z = 2, a = 8.792(4) Å, b = 5.269(2) Å, c = 10.398(6) Å, β = 112.60(4)°). The crystal structure models for VPO 4 - m 1 and VPO 4 - m 2 have been optimized by DFT calculations. Polymorph m 1 contains the unprecedented butterfly shaped [V III O 4 ] chromophore and has been further characterized by magnetic measurements, by powder reflectance spectroscopy (NIR/vis/UV), and IR spectroscopy. For six polymorphic forms of VPO 4 ( m 1', m 1'', m 2, m 3, m 4, and m 5), DFT calculations have been performed. For the existence of VPO 4 - m 1', - m 1'', and - m 2, our experiments provide evidence. VPO 4 - m 3, - m 4, and - m 5 were obtained by structure optimization based on reduced β-VOPO 4 . Their stability is predicted by the DFT calculations.