TY - JOUR
T1 - Coordination chemistry of Ti(IV) in silicate glasses and melts
T2 - II. Glasses at ambient temperature and pressure
AU - Farges, François
AU - Brown, Gordon E.
AU - Navrotsky, Alexandra
AU - Gan, Hao
AU - Rehr, John J.
PY - 1996/8
Y1 - 1996/8
N2 - The coordination environment of Ti(IV) in a number of Ti-silicate and Ti-aluminosilicate glasses has been determined by x-ray absorption fine structure (XAFS) spectroscopy at the Ti K-edge at ambient temperature and pressure. These glasses contain 2.7-30.5 wt% TiO2 and varying amounts of Na2O, K2O, or CaO (5.0-38.7 wt%) and Al2O3 (0-11.9 wt%), and have NBO/T ratios ranging from 0.07-0.81. Quantitative analysis of the Ti XANES spectra, based on ab initio multiple-scattering calculations for a variety of Ti-containing clusters, and anharmonic analysis of the normalized XAFS oscillations suggest the presence of three types of atoms around Ti: O first neighbors, (Si, Ti)-second neighbors, and alkali third neighbors. Five-coordinated Ti, [5]Ti, is the dominant Ti species in the glasses most concentrated in Ti (>16 wt% TiO2) and is located in distorted square pyramids ([5]TiO)O4), with one short Ti=O titanyl distance (1.67-1.70 ± 0.03 Å) and four long Ti - O distances (1.94-1.95 ± 0.02 Å). In addition, minor amounts of [4]Ti were detected, the proportion of [4]Ti increasing in the order: Na glasses < K glasses. [4]Ti is the dominant Ti species in the potassic glasses with the lowest TiO2 contents (≈3-6 wt%) and highest NBO/T ratio. The relative amount of [4]Ti increases in the order: Ca glass < K glass. Finally, [6]Ti is a minor species (<20%) when detected in these glasses. The presence of Ti-(Si, Ti) correlations near 3.2-3.4 ± 0.1 Å, as in crystalline Na2([5]TiO)SiO4, is consistent with [5]TiO5 and SiO4/TiO5 polyhedra sharing corners in these glasses, with Ti - O-(Si, Ti) angles of ≈120°-130° ± 10°. Quantitative analysis of the Ti K-edge XANES for the K-bearing glasses suggests the presence K around Ti, in good agreement with bond-valence predictions, which indicate that [5]Ti is most likely to bond to both nonbridging oxygens (one O in short Ti=O titanyl bonds) and bridging oxygens (four O in long Ti - O bonds), thus can act as a new type of Q4 specie with one additional nonbridging oxygen. Then, we propose [5]Ti to behave simultaneously a network former and a network modifier, with the network former role dominant. Bond valence models explain why the relative proportions of [4]Ti and [5]Ti change when the type of low field strength cation or the type of network-forming cation (Si vs. P) changes in oxide glasses. These models also provide a structural basis for the study of glasses and melts at higher temperatures (see Part III of this study).
AB - The coordination environment of Ti(IV) in a number of Ti-silicate and Ti-aluminosilicate glasses has been determined by x-ray absorption fine structure (XAFS) spectroscopy at the Ti K-edge at ambient temperature and pressure. These glasses contain 2.7-30.5 wt% TiO2 and varying amounts of Na2O, K2O, or CaO (5.0-38.7 wt%) and Al2O3 (0-11.9 wt%), and have NBO/T ratios ranging from 0.07-0.81. Quantitative analysis of the Ti XANES spectra, based on ab initio multiple-scattering calculations for a variety of Ti-containing clusters, and anharmonic analysis of the normalized XAFS oscillations suggest the presence of three types of atoms around Ti: O first neighbors, (Si, Ti)-second neighbors, and alkali third neighbors. Five-coordinated Ti, [5]Ti, is the dominant Ti species in the glasses most concentrated in Ti (>16 wt% TiO2) and is located in distorted square pyramids ([5]TiO)O4), with one short Ti=O titanyl distance (1.67-1.70 ± 0.03 Å) and four long Ti - O distances (1.94-1.95 ± 0.02 Å). In addition, minor amounts of [4]Ti were detected, the proportion of [4]Ti increasing in the order: Na glasses < K glasses. [4]Ti is the dominant Ti species in the potassic glasses with the lowest TiO2 contents (≈3-6 wt%) and highest NBO/T ratio. The relative amount of [4]Ti increases in the order: Ca glass < K glass. Finally, [6]Ti is a minor species (<20%) when detected in these glasses. The presence of Ti-(Si, Ti) correlations near 3.2-3.4 ± 0.1 Å, as in crystalline Na2([5]TiO)SiO4, is consistent with [5]TiO5 and SiO4/TiO5 polyhedra sharing corners in these glasses, with Ti - O-(Si, Ti) angles of ≈120°-130° ± 10°. Quantitative analysis of the Ti K-edge XANES for the K-bearing glasses suggests the presence K around Ti, in good agreement with bond-valence predictions, which indicate that [5]Ti is most likely to bond to both nonbridging oxygens (one O in short Ti=O titanyl bonds) and bridging oxygens (four O in long Ti - O bonds), thus can act as a new type of Q4 specie with one additional nonbridging oxygen. Then, we propose [5]Ti to behave simultaneously a network former and a network modifier, with the network former role dominant. Bond valence models explain why the relative proportions of [4]Ti and [5]Ti change when the type of low field strength cation or the type of network-forming cation (Si vs. P) changes in oxide glasses. These models also provide a structural basis for the study of glasses and melts at higher temperatures (see Part III of this study).
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U2 - 10.1016/0016-7037(96)00145-7
DO - 10.1016/0016-7037(96)00145-7
M3 - Article
AN - SCOPUS:0030499549
SN - 0016-7037
VL - 60
SP - 3039
EP - 3053
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 16
ER -