NMR STUDY OF BAFPO3 - 31P AND 19F CHEMICAL-SHIFT ANISOTROPIES AND ABSOLUTE SIGN OF F-P COUPLING CONSTANT

The FPO3- ion in polycrystalline BaFPO3 is investigated from the point of view that it is a very slightly distorted, axially symmetric two-spin system. NMR line-shape analysis for the 31P spectra as a function of magnetic field yielded the following information: (1) the chemical-shift anisotropy, (σ∥- σ⊥), is -145± 20 ppm for phosphorus; (2) the absolute sign of the F-P indirect dipolar coupling constant is negative; (3) the internuclear F-P distance is 1.63±0.035 Å. Investigation of the 19F spectra was carried out through a second-moment analysis as a function of magnetic field. It was found that the fluorine chemical-shift anisotropy is +182±22 ppm, where the sign is deduced from the sense of the line-shape asymmetry. Furthermore, an estimate of the intermolecular dipolar contribution to the second moment was obtained by extrapolating the total second moment to low field and then subtracting the intramolecular dipolar contribution which is known from the 31P spectra. The intermolecular dipolar contribution is such that if BaFPO3 has a structure analogous to BaSO4, K2SO4, and K2FPO 3, then there is a high degree of disorder in the orientation of the FPO3- ions in the crystal.