Aluminum solubility in rutile (TiO2)

Abstract

The solubility of aluminum in rutile has been studied in the systems Al 2 O 3 -TiO 2 -H 2 O and Al 2 O 3 -SiO 2 -TiO 2 -H 2 O at 700-1200 °C and 0.075-3.3 GPa. Electron probe microanalysis (EPMA) measurements of rutile crystals grown in equilibrium with corundum show that the concentration of Al increases with increasing temperature, pressure, and oxygen fugacity. Solubility is enhanced by the addition of Nb and reduced by the addition of trivalent cations. These results are consistent with the substitution of Al 3+ for Ti 4+ on normal cation sites and the simultaneous incorporation of positively charged hydrous defects for charge compensation. Parameters for a thermodynamic model of Al substitution in rutile are calculated and the merits of using the model as a geochemical tool are discussed.

Crystal Structure, Vibrational Spectra on Trivalent-Cation-Doped Rutile and Application in Optical Materials

“…All the synthetic single crystals of rutile are well indexed in the tetragonal structure of P 4 2 / mnm , and the refined unit-cell volumes of the seven M 3+ -doped samples are 0.1–0.5% larger than that for the Ti-pure one, even in the case for the incorporation of smaller Al 3+ cations. Interestingly, this phenomenon is consistent with a previous thermodynamic model for aluminum substitution in hydrous rutile . While in dry synthetic conditions, Al 3+ incorporation would decrease the volume of rutile instead. , Hence, even a small amount of incorporated OH – groups could have significant impact on the crystal structure of rutile.…”

“…11 When the population of M 3+ is higher than that of H + , 12 the excess M 3+ cations may prefer to occupy the interstitial sites, like in the case of Al-doped rutile. 6,13,14 This is consistent with the experimental finding that the concentrations % bulk oxygen vacancies (V O ) are very low in rutile and can be entirely replaced by OH − groups even under modest pH 2 O conditions. 15−17 In addition, incorporation of metallic cations further affects the hydrogen behaviors as reflected by Fourier transform infrared (FTIR) spectra.…”

“…In the rutile phase, hydrogen could be incorporated by the reduction of Ti 4+ (Ti 4+ = Ti 3+ + H + ) or substitution of lower charges (like M 3+ ) which is generally identified as Ti 4+ = M 3+ + H + . , Water (as hydroxyl defects) concentrations in rutiles range from hundreds of ppmw (by H 2 O weight) to a few thousand ppmw with incorporation of Nb 5+ and Ta 5+ . When the population of M 3+ is higher than that of H + , the excess M 3+ cations may prefer to occupy the interstitial sites, like in the case of Al-doped rutile. ,, This is consistent with the experimental finding that the concentrations % bulk oxygen vacancies ( V O ) are very low in rutile and can be entirely replaced by OH – groups even under modest p H 2 O conditions. − In addition, incorporation of metallic cations further affects the hydrogen behaviors as reflected by Fourier transform infrared (FTIR) spectra. For example, the intrinsic OH-stretching mode in hydrous rutile is typically observed at 3279 cm –1 as caused by the reduction of Ti 4+ , while incorporation of Fe 3+ introduces another OH band at a higher frequency of 3297 cm –1 …”

“…Rutile, the most common form of naturally occurred titanium dioxide (TiO 2 ), is widely found in igneous and metamorphic rocks, such as in ultrahigh-pressure eclogite formed during continental deep subduction. , In the tetragonal crystal structure, each Ti 4+ cation is coordinated with six O 2– anions, forming TiO 6 octahedra stacked in the c direction. Rutile can incorporate significant amounts of divalent (Mg 2+ , Ca 2+ ), trivalent (e.g., Al 3+ , Fe 3+ , Cr 3+ , Mn 3+ ), tetravalent (Zr 4+ ), and pentavalent (V 5+ , Nb 5+ , Ta 5+ ) cations. − For instance, coupled substitution of two Ti 4+ by trivalent and pentavalent cations maintains neutral charge.…”

“…In this study, we synthesized rutile samples doped with various trivalent cations (Al 3+ , Ni 3+ , Co 3+ , Cr 3+ , Ga 3+ , Fe 3+ , and Mn 3+ ) under high-pressure ( P ) and high-temperature ( T ) conditions, and checked the M 3+ effect on the crystal structure, vibrational, and optical properties. There are several factors affecting the incorporation of cations, including pressure, temperature, oxygen fugacity, , and the sizes of the doped cations . First-principles calculation is also utilized in the present study to examine the role of the intra-octahedral distortion in M 3+ substitution, , as well as its effect on the optical properties.…”