Synthesis, structure refinement and characterisation of a new oxyphosphate Mg0.50TiO(PO4)

A new titanium oxyphosphate Mg0.50TiO(PO4) has been synthesized and characterized by several physical techniques: X-ray diffraction, 31P MAS-NMR, Raman diffusion, infrared absorption and diffuse reflectance spectroscopy. It crystallizes in the monoclinic system with unit cell parameters: a = 7.367(9), b = 7.385(8), c = 7.373(9) Å, β = 120.23(1), with the space group P21/c (no. 14), Z = 4. The crystal structure has been refined by the Rietveld method using X-ray powder diffraction. The conventional R indices obtained are Rwp = 0.138, Rp = 0.096 and RB = 0.0459. The structure of Mg0.50TiO(PO4) consists of infinite chains of corner-shared [TiO6] octahedra parallel to the c-axis, crosslinked by corner-shared [PO4] tetrahedra. These infinite chains have alternating short (1.74 Å) and long (2.26 Å) TiO bonds and are similar to those found in titanium oxyphosphate MII0.50TiO(PO4) (M2+ = Fe2+, Co2+, Ni2+, Cu2+, Zn2+). The magnesium atom is located in an antiprism between two [TiO6] octahedra. 31P MAS NMR showed only a single 31P resonance line, in a good agreement with the crystal structure. Raman and IR spectra show strong bands respectively at 765 and 815 cm−1, attributed to the vibration of TiOTiO bonds in the infinite chains. The gap due to the Oxygen–Titanium(IV) charge transfer is 3.37 eV.