Single-crystal structure and Raman spectroscopy of synthetic titanite analog CaAlSiO sub(4)F

Synthetic CaAlSiO sub(4)F, the Al-F analog of titanite, has been investigated using single-crystal synchrotron diffraction experiments at Beamline X06DA (Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland) and Raman spectroscopy. The presented structural model with 40 parameters was refined against 506 unique reflections to a final R o b s of 0.026 (space group A2/a, a = 6.9120(11), b = 8.5010(10), c = 6.435(2) Aa, beta = 114.670(11) degree , and Z = 4) and exhibits less distorted coordination polyhedra than earlier models from powder data. Vibrational spectra were calculated in harmonic approximation at the Gamma point from fully relaxed energy optimisations of the crystal structure, using 3D-periodic density functional theory with Gaussian basis sets and the software CRYSTAL06. The lattice parameters of the fully relaxed structure were in good agreement with the experimental values, with the calculated values 0.8 plus or minus 0.4 % too large; the monoclinic angle was calculated 0.4 degree too large. The agreement of the calculated Raman frequencies with the observed ones was very good, with standard deviation plus or minus 3 cm super(-1) and maximum deviations of plus or minus 7 cm super(-1). Furthermore, a detailed discussion of the atomic displacements associated with each Raman mode is given.