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

Abstract

Two thermodynamically metastable polymorphs of vanadium(III) phosphate, VIIIPO4-m1 and VPO4-m2, have been obtained via reduction of β-VVOPO4 by moist hydrogen. The XRPD pattern of VPO4-m1 can be assigned based on the crystal structure of β-VVOPO4, though with distinctly different lattice parameters (VPO4-m1/β-VOPO4: 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 VPO4-m2 was found to be very similar to that of Fe2(VO)(P2O7)(PO4) (VPO4-m2: P21/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 VPO4-m1 and VPO4-m2 have been optimized by DFT calculations. Polymorph m1 contains the unprecedented butterfly shaped [VIIIO4] chromophore and has been further characterized by magnetic measurements, by powder reflectance spectroscopy (NIR/vis/UV), and IR spectroscopy. For six polymorphic forms of VPO4 (m1', m1'', m2, m3, m4, and m5), DFT calculations have been performed. For the existence of VPO4-m1', -m1'', and -m2, our experiments provide evidence. VPO4-m3, -m4, and -m5 were obtained by structure optimization based on reduced β-VOPO4. Their stability is predicted by the DFT calculations.