High-Temperature Synchrotron X-Ray Powder Diffraction Study of the Orthorhombic–Tetragonal Phase Transition in La0.63(Ti0.92,Nb0.08)O3

The zinc fluoro phosphate Zn2F(PO4) has been produced by hydrothermal synthesis employing hydrofluoric acid as a mineralizer in a H2O or D2O medium. A single-crystal X-ray synchrotron diffraction analysis of Zn2F(PO4) shows that the zinc fluoro phosphate is monoclinic, a=9.690(1), b=12.793(1), and c=11.972(1) ?, beta=108.265(1)°, space group P21/c, No. 14, Z=16. Reflections hkl with k=2n+1 are weak but significant and the structure shows pseudosymmetry. Zn2F(PO4) has the wagnerite-type M2F(XO4) structure with four Zn atoms each coordinated to four O atoms and one F atom while four other Zn atoms are coordinated to four O atoms and two F atoms. A difference Fourier map, calculated from the singlecrystal X-ray data, shows additional electron density close to the four fluorine atoms, indicating a possible partial substitution of F- by OH- ions. This is unambiguously confirmed by 31P–{1H} cross-polarization magic-angle spinning (MAS) and by 1H/2H MAS NMR spectroscopy. The narrow line width observed for the 1H resonance and the unique set of 2H quadrupole coupling parameters (obtained for the Zn2F(PO4) sample using D2O as medium) show that 1H/2H is present as OH(D) groups rather than as water of crystallization in the structure. Quantitative 1H MAS NMR analysis shows that the composition of the sample is Zn2(OH)0.14(3)F0.86(3)(PO4). The high-speed 19F MAS NMR spectrum exhibits two resolved resonances with equal intensity, which are ascribed to an overlap of resonances from the four distinct fluorine sites in Zn2(OH)0.14(3)F0.86 (3)(PO4).