The crystal structure of BaPO sub(3)F revisited - a combined X-ray diffraction and solid-state super(19)F, super(31)P MAS NMR study

Barium monofluorophosphate, BaPO sub(3)F, 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 (BaSO sub(4)) by replacing the SO sub(4) super(2-) anions with isoelectronic PO sub(3)F super(2-) anions in two orientations. BaPO sub(3)F 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 PO sub(3)F super(2-) anions. The single crystals [P2 sub(1)/c, Z= 4, a= 11.3105(7) Aa, b= 8.6934(5) Aa, c= 9.2231(4) Aa, beta = 127.819(3) degree , R[F super(2) > 2 sigma (F super(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 super(19)F and super(31)P MAS NMR spectra of polycrystalline BaPO sub(3)F acquired at magnetic fields of 7.05 T and 14.09 T resolve resonances from two distinct super(19)F sites and super(31)P sites, in accordance with the local symmetry of the OD description. From these spectra, the super(19)F chemical shift anisotropies and super(1)J sub(P-F ) couplings are determined for the two distinct super(19)F sites and an average set of chemical shift anisotropy parameters for the two super(31)P sites.