Continuous Structural Transition in Glass-Forming Molten Titanate BaTi2O5

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• 作者：O. L. G. Alderman ; C. J. Benmore ; A. Tamalonis ; S. Sendelbach ; S. Heald ; R. Weber
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：December 1, 2016
• 年：2016
• 卷：120
• 期：47
• 页码：26974-26985
• 全文大小：625K
• ISSN：1932-7455

文摘

The structure of the model titanate glass former BaTi₂O₅ has been studied over a wide temperature (T) range in the molten, supercooled, and glassy states under conditions of aerodynamic levitation. Both high-energy X-ray diffraction and Ti K-edge X-ray absorption spectroscopy reveal a continuous structural transition involving reduction of the cation–oxygen (and oxygen–cation) average coordination numbers and bond lengths with increasing T. Ti–O coordination in the moderately supercooled and equilibrium melt follows a linear trend n_TiO = 5.4(1) – [3.5(7) × 10^–4]T [K] (1300 ≤ T ≤ 1830 K, T_g = 960 K, T_m = 1660 K). Comparison to the melt-quenched glass implies an increase in ∂n_TiO/∂T at lower T, as T_g is approached from above. Both Ba–O coordination and bond length also decrease at higher T, and the role of Ba addition is to reduce n_TiO below its value in pure molten TiO₂, which is related to the presence of density maxima in molten BaO-TiO₂. Density measurements made by imaging of the levitated melt yielded ρ(T) = 4.82(55) – 0.0004(3)T in units of K and g cm^–3. While BaTi₂O₅ glass likely consists of a fully connected Ti–O network, free of nonbridging oxygen (NBO) [OTi₁] and with at least 13(4)% [OTi₃] triclusters, the 1835(40) K equilibrium melt contains at least 10(4)% NBO along with 90(4)% bridging oxygen [OTi₂]. The results highlight the fact that glasses can be considered as structural analogues of melts only for those melts deeply supercooled into the glass transition region. The results imply possible fictive T dependence of titanate glass structure, suggesting applications as, e.g., laser written waveguides with large refractive indices and refractive index contrasts. The temperature-dependent structure further implies a super-Arrhenian melt viscosity with consequences for glass manufacture, titanate-rich slags produced in iron smelting, TiO₂-bearing magmas, and by analogy silicate melts at high pressures, as found in planetary interiors.