Infrared Properties of Solid Titanium Oxides: Exploring Potential Primary Dust Condensates

We present optical constants and opacities of solid TiO₂, Ti₂O₃, magnesium and calcium titanates, largely based on laboratory measurements. These dust species deserve interest as potential primary condensates in oxygen-rich dusty environments. Of the three known solid TiO₂ phases, only one (rutile) has been extensively studied so far with respect to its mid-IR optical properties. We compare these with our measurements of the optical constants of anatase, brookite, and CaTiO₃. Furthermore, for several Mg-Ti-oxides, powder transmission spectra are shown. While all known TiO₂ modifications have their strongest bands between 13 and 13.5 μm (for spherical particles), CaTiO₃, MgTiO₃, and other Mg titanates have principal maxima of their absorption coefficients between 14 and 19 μm. This makes a spectroscopic identification of circumstellar Ti oxide particles rather difficult, because both the 13 and the 14-19 μm region are crowded with other features in the spectra of oxygen-rich circumstellar shells.