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A New Organic–Inorganic Hybrid Oxyfluorotitanate [Hgua]2·(Ti5O5F12) as a Transparent UV Filter

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journal contribution
posted on 20.06.2011, 00:00 authored by J. Lhoste, X. Rocquefelte, K. Adil, R. Dessapt, S. Jobic, M. Leblanc, V. Maisonneuve, M. Bujoli-Doeuff
A new generation UV absorber is obtained by microwave-heating-assisted hydrothermal synthesis: [Hgua]2·(Ti5O5F12). The structure of this hybrid titanium(IV) oxyfluoride is ab initio determined from powder X-ray data by combining a direct space method, Rietveld refinement [orthorhombic, Cmm2, a = 22.410(1) Å, b = 11.191(1) Å, c = 3.802(1) Å], and density functional theory geometry optimization. The three-dimensional network is built up from infinite inorganic layers (Ti5O5F12) separated by guanidinium cations. The theoretical optical gap (3.2 eV) estimated from density of state calculations is in good agreement with the experimental gap (3.3 eV) obtained by UV–vis diffuse reflectivity. The optical absorption is mainly due to O(2p) → Ti(3d) and F(2p) → Ti(3d) transitions at higher energies. The refraction index is low in the visible range (n ≈ 1.9) compared to that of TiO2 and, consequently, [Hgua]2·(Ti5O5F12) shows a good transparency adapted to UV shielding. Under UV irradiation at 254 nm for 40 h, the white microcrystalline powder turns to light purple-gray. This color change is caused by the reduction of TiIV to TiIII, confirmed by magnetic measurements.