Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature
Pentacoordinated and hexacoordinated silicon cations in a potassium silicate glass: Effects of pressure and temperature
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DOI：
10.1016/j.jnoncrysol.2018.11.001
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发表时间：
2019-02-01
影响因子：
3.5
通讯作者：
Bistal, Saurav
中科院分区：
文献类型：
--
作者：
Stebbins, Jonathan F.;Bistal, Saurav
具有5或6个氧第一邻域的硅阳离子（SiO5和SiO6，五配位和六配位）主要来自在约12gpa压力下熔融的碱和碱土硅酸盐玻璃，它们的形成被认为是液体致密化的重要组成部分。这些物质，特别是SiO5基团，也被认为是键交换中的高能中间结构，控制着网络阳离子和阴离子的扩散，因此在高温，富硅熔体中，以及在低温液态硅酸盐反应中，粘性传输，因此已经成为许多最近理论研究的主题。尽管有这种兴趣，但是关于压力、温度和组成对五配位硅和六配位硅浓度的控制的实验数据非常有限。在这里，我们报告了具有K2Si4O9模型成分的高压玻璃的Si-29 MAS NMR的新结果，该结果表明，由于淬火过程中的瞬态压降，先前的研究可能严重低估了SiO5和SiO6的浓度。此外，我们改进了在1 bar玻璃中SiO5的早期结果，并证明在更高的有效温度下其浓度会显著增加。形成该物质的反应焓为正，这与它在熔体动力学中作为能量上重要的过渡络合物的作用是一致的。
Silicon cations with five or six oxygen first neighbors (SiO5 and SiO6, pentacoordinated and hexacoordinated) are known primarily from alkali and alkaline earth silicate glasses melted at pressures up to about 12 GPa, where their formation is thought to be an important part of the densification of the liquid. These species, especially SiO5 groups, have also been proposed as high-energy intermediate structures in the bond-swapping that controls network cation and anion diffusion, and hence viscous transport, in high temperature, silica-rich melts, as well as in low-temperature liquid-state silicate reactions, and have thus been the subject of numerous recent theoretical studies. In spite of this interest, the amount of experimental data on controls of pressure, temperature, and composition on concentrations of penta- and hexacoordinated silicon is very limited. Here we report new results from Si-29 MAS NMR on high pressure glasses with the model composition K2Si4O9, which show that because of transient pressure drop during quench, previous studies probably significantly underestimated SiO5 and SiO6 concentrations. In addition, we refine the earlier results on SiO5 even in 1 bar glasses, and demonstrate a strong increase in its concentration at higher fictive temperature. A positive enthalpy for the reaction forming this species is consistent with its role as an energetically important transition complex in melt dynamics.