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Abstract:
Two thermodynamically metastable polymorphs of vanadium(III) phosphate, (VPO4)-P-III-m1 and VPO4-m2, have been obtained via reduction of beta-(VOPO4)-O-V by moist hydrogen. The XRPD pattern of VPO4-m1 can be assigned based on the crystal structure of beta-(VOPO4)-O-V, though with distinctly different lattice parameters (VPO4-m1/beta-VOPO4: Pnma, a = 7.3453(12)/ 7.7863(5) angstrom, b = 6.4001(12)/6.1329(3) angstrom, c = 7.3196(13)/6.9673(5) angstrom). The XRPD pattern of VPO4-m2 was found to be very similar to that of Fe-2(VO)(P2O7)(PO4) (VPO4-m2: P21/m, Z = 2, a = 8.792(4) angstrom, b = 5.269(2) angstrom, c = 10.398(6) angstrom, beta= 112.60(4)degrees). The crystal structure models for VPO4-m1 and VPO4-m2 have been optimized by DFT calculations. Polymorph m1 contains the unprecedented butterfly shaped [(VO4)-O-III] 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 beta-VOPO4. Their stability is predicted by the DFT calculations.
