Refinement of the crystal structure of fresnoite, Ba2TiSi2O8, from L ö hley (Eifel district, Germany); Gladstone – Dale compatibility, electronic polarizability and vibrational spectroscopy of minerals and inorganic compounds with pentacoordinated TiIV and a titanyl bond

Most known compounds with five-coordinated Ti4+ are natural and synthetic titanosilicates. The crystal structure of natural fresnoite, Ba2TiSi2O8 [tetragonal, space group P4bm, a = 8.510   (1)   Å , c = 5.197   (1)   Å , V = 376.4   (1)   Å 3, Z = 2], has been refined to R = 0.011 on the basis of 807 unique single-crystal reflections with I> 2 σ (I). Titanium has fivefold coordination with one short (`titanyl') bond of 1.692   (5)   Å . Bonds in the TiO5 polyhedron are discussed in comparison to analogous coordination polyhedra in other minerals and compounds. A review of all known compounds with Ti4+O5 polyhedra shows that most of them are titanosilicates in which titanium forms a short Ti — O bond ( ∼ 1.61 to ∼ 1.77   Å ). Poor Gladstone – Dale compatibility between chemical composition, optical characteristics and density of these compounds is explained by the anomalous contribution of [5]Ti4+ to the optical properties as shown by calculations based on the relationship between electronic polarizabilities and refractive indices. An improved Gladstone – Dale coefficient of 0.29 is suggested for TiO2 with [5]Ti4+. A negative correlation between `titanyl' bond lengths and wavenumbers of the bands of Ti — O stretching vibrations (in the range of 890 – 830   cm − 1) in infrared and Raman spectra is observed.
Source: Acta Crystallographica Section B - Category: Chemistry Authors: Tags: fresnoite titanyl bond pentacoordinated titanium electronic polarization optical properties research papers Source Type: research