Department of Physics, University of Bath, Bath BA2 7AY, UK.
Phys Chem Chem Phys. 2013 Oct 7;15(37):15286-308. doi: 10.1039/c3cp51741a.
The structure of several network-forming liquids and glasses is considered, where a focus is placed on the detailed information that is made available by using the method of neutron diffraction with isotope substitution (NDIS). In the case of binary network glass-forming materials with the MX2 stoichiometry (e.g. GeO2, GeSe2, ZnCl2), two different length scales at distances greater than the nearest-neighbour distance manifest themselves by peaks in the measured diffraction patterns. The network properties are influenced by a competition between the ordering on these "intermediate" and "extended" length scales, which can be manipulated by changing the chemical identity of the atomic constituents or by varying state parameters such as the temperature and pressure. The extended-range ordering, which describes the decay of the pair-correlation functions at large-r, can be represented by making a pole analysis of the Ornstein-Zernike equations, an approach that can also be used to describe the large-r behaviour of the pair-correlation functions for liquid and amorphous metals where packing constraints are important. The first applications are then described of the NDIS method to measure the detailed structure of aerodynamically-levitated laser-heated droplets of "fragile" glass-forming liquid oxides (CaAl2O4 and CaSiO3) at high-temperatures (~2000 K) and the structure of a "strong" network-forming glass (GeO2) under pressures ranging from ambient to ~8 GPa. The high-temperature experiments show structural changes on multiple length scales when the oxides are vitrified. The high-pressure experiment offers insight into the density-driven mechanisms of network collapse in GeO2 glass, and parallels are drawn with the high-pressure behaviour of silica glass. Finally, the hydrogen-bonded network of water is considered, where the first application of the method of oxygen NDIS is used to measure the structures of light versus heavy water and a difference of approximately equal to 0.5% is found between the O-D and O-H intra-molecular bond lengths. The experimental data are best matched by using path integral molecular dynamics simulations with a flexible anharmonic water model, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions.
考虑了几种形成网络的液体和玻璃的结构，重点介绍了使用中子衍射与同位素取代（NDIS）方法获得的详细信息。对于具有 MX2 化学计量比的二元网络玻璃形成材料（例如 GeO2、GeSe2、ZnCl2），在大于最近邻距离的距离处，通过测量衍射图案中的峰，可以表现出两种不同的长度标度。网络性质受到这些“中间”和“扩展”长度标度上的有序性之间竞争的影响，可以通过改变原子成分的化学性质或通过改变状态参数（例如温度和压力）来操纵这种竞争。扩展范围有序性描述了在大半径处对关联函数的衰减，可以通过对奥恩斯坦-泽尔尼克方程进行极点分析来表示，这种方法也可用于描述对液体和非晶态金属中大半径处对关联函数的行为，其中包装约束很重要。然后描述了 NDIS 方法的首次应用，以测量在高温（约 2000 K）下“易碎”玻璃形成氧化物（CaAl2O4 和 CaSiO3）的空气动力学悬浮激光加热液滴的详细结构以及在从环境到约 8 GPa 的压力范围内的“强”网络形成玻璃（GeO2）的结构。高温实验表明，氧化物玻璃化时，多个长度标度上的结构发生变化。高压实验提供了对 GeO2 玻璃中网络崩溃的密度驱动机制的深入了解，并与二氧化硅玻璃的高压行为进行了比较。最后，考虑了氢键网络的水，首次应用氧 NDIS 方法来测量轻水和重水的结构，发现 O-D 和 O-H 分子内键长之间存在大约 0.5%的差异。实验数据与使用具有灵活非谐水模型的路径积分分子动力学模拟得到的最佳匹配，结果支持了一种竞争的量子效应模型，其中水的结构和动力学性质由分子内和分子间量子贡献之间的偏移来控制。
