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 SO₄²⁻ anions with isoelectronic PO₃F²⁻ 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 PO₃F²⁻ 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[F² > 2σ(F²)] = 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 ¹⁹F and ³¹P MAS NMR spectra of polycrystalline BaPO₃F acquired at magnetic fields of 7.05 T and 14.09 T resolve resonances from two distinct ¹⁹F sites and ³¹P sites, in accordance with the local symmetry of the OD description. From these spectra, the ¹⁹F chemical shift anisotropies and ¹J_P–F couplings are determined for the two distinct ¹⁹F sites and an average set of chemical shift anisotropy parameters for the two ³¹P sites.