The crystal structure of BaPO3F revisited – a combined X-ray diffraction and solid-state 19F, 31P MAS NMR study

Abstract

Barium monofluorophosphate, BaPO3F, was prepared in a polycrystalline form by fast precipitation, whereas single crystals were obtained using a long-lasting gel growth method. The samples were characterized by powder- and single-crystal X-ray diffraction, which revealed orthorhombic symmetry for the polycrystalline material and monoclinic symmetry for the single crystals. Both phases belong to the same order-disorder (OD) family and can be derived from the baryte structure (BaSO4) by replacing the SO4(2-) anions with isoelectronic PO3F(2-) anions in two orientations. BaPO3F can crystallize in an infinite number of locally equivalent layer stacking sequences. The polycrystalline material represents the family structure and is characterized by a purely random stacking of layers. Its structure is isotypic with the baryte structure and contains disordered PO3F(2-) anions. The single crystals [P21/c, Z = 4, a = 11.3105(7) Å, b = 8.6934(5) Å, c = 9.2231(4) Å, β = 127.819(3)°, R[F(2) > 2σ(F(2))] = 0.036, 4146 structure factors, 110 parameters] crystallize as a polytype with a maximum degree of order (MDO), which can be regarded as a two-fold superstructure of the baryte-structure type. Fragments of the second MDO polytype are evidenced by systematic twinning by mirroring at (100). Solid-state (19)F and (31)P MAS NMR spectra of polycrystalline BaPO3F acquired at magnetic fields of 7.05 T and 14.09 T resolve resonances from two distinct (19)F sites and (31)P sites, in accordance with the local symmetry of the OD description. From these spectra, the (19)F chemical shift anisotropies and (1)J(P-F) couplings are determined for the two distinct (19)F sites and an average set of chemical shift anisotropy parameters for the two (31)P sites.