NMR Study of BaFPO3: 31P and 19F Chemical-Shift Anisotropies and the Absolute Sign of the 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, $\mathrm{(\sigma \Vert \hspace{0.17em}-\hspace{0.17em}\sigma \perp )}$ , 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. Further‐more, 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 K₂FPO₃, then there is a high degree of disorder in the orientation of the FPO₃⁼ ions in the crystal.