Coordination chemistry of Ti(IV) in silicate glasses and melts .3. Glasses and melts from ambient to high temperatures
Coordination chemistry of Ti(IV) in silicate glasses and melts .3. Glasses and melts from ambient to high temperatures
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DOI：
10.1016/0016-7037(96)00146-9
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发表时间：
1996-08-01
影响因子：
5
通讯作者：
Rehr, JR
中科院分区：
文献类型：
--
作者：
Farges, F;Brown, GE;Rehr, JR
用原位X射线吸收精细结构（XAFS）方法，在293-1650 K温度范围内，测定了Ti在5种Na-、K-和Ca-钛硅酸盐玻璃/熔体（TiO_2含量为2.7- 30.5wt%）中的局部结构环境。在平行，两个钛模型化合物（Ni2.6Ti0.7O4尖晶石和TiO 2金红石）在相同的条件下进行了研究，以更好地了解温度（非谐性）的XAFS光谱的影响。发现温度诱导的非谐性变化，主要是作为Ti配位的函数，并且当以Ti-[6]存在时，随着温度的升高，在Ti周围显著增加。相反，在低于1200 K的温度下，Ti-[4]周围的非谐性似乎可以忽略不计。我们预测在Ti-[5]附近的非谐性也很弱。没有发现明显的证据表明，在Na-和K-钛硅酸盐玻璃和熔体中，Ti的平均最近邻配位环境发生了显著变化，这些玻璃和熔体在玻璃化转变温度T-g（860-930 K）以上表现出反常的热容变化。在高温下，这些系统中（（TiO 2+）-Ti-[5]）-O键的小（预测和测量）线性热膨胀预计在玻璃-过冷液体转变期间对Ti-[5]的局部环境影响不显著。在所研究的最稀的Ti-玻璃（KS 1; 2.7重量% TiO 2）中，Ti-[4]周围的局部环境（尤其是第二邻居碱金属）在环境温度下相对有序，但该顺序在T-g以上急剧降低。较低的淬冷速率似乎有利于Ti-[4]而不是Ti-[5]。这些熔体热容的异常正变化可能与Ti周围的中程环境在T-g以上的显著变化有关，包括在T-g结构弛豫期间，涉及富碱区和富网络形成物区之间界面的逾渗畴消失;这些渗滤域与Ti在硅酸盐玻璃/熔体中的双重结构作用有关（同时充当网络形成剂和网络改性剂）。
The local structural environment of Ti in five Na-, K-, and Ca-titanosilicate glass/melts with TiO2 concentrations ranging from 2.7-30.5 wt% has been determined by in situ Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at temperatures ranging from 293-1650 K. In parallel, two Ti-model compounds (Ni2.6Ti0.7O4 spinel and TiO2 rutile) were studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induced anharmonicity was found to vary, largely as a function of the Ti-coordination, and increases significantly around Ti with increasing temperature when present as Ti-[6]. In contrast, anharmonicity appears negligible around Ti-[4] at temperatures below 1200 K. We predict that anharmonicity should be weak around Ti-[5] as well.No clear evidence was found for a significant change in the average nearest-neighbor coordination environment of Ti in the Na- and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above their glass transition temperatures, T-g (860-930 K). The small (predicted and measured) linear thermal expansion of the ((TiO2+)-Ti-[5])-O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of Ti-[5] during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO2), the local environment around Ti-[4] (especially the second-neighbor alkalis) is relatively ordered at ambient temperature, but this order decreases dramatically above T-g. Lower quench rates appear to favor Ti-[4] over Ti-[5].The origin of the observed anomalous positive variations in heat capacities of these melts may be related to significant changes in the medium-range environment around Ti above T-g including the disappearance of percolation domains involving interfaces between alkali-rich and network-former rich regions during structural relaxation at T-g; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneously as network former and network modifier).