Soper A K
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK.
J Phys Condens Matter. 2007 Aug 22;19(33):335206. doi: 10.1088/0953-8984/19/33/335206. Epub 2007 Jul 4.
X-ray diffraction data on liquids and disordered solids often provide useful complementary structural information to neutron diffraction data. Interpretation of the x-ray diffraction pattern, which is produced by scattering from the atomic electrons rather than from the atomic nuclei as in the case of neutron diffraction, is, however, complicated by the Q-dependent electronic form factors, which cause the x-ray diffraction signal to decline rapidly with increasing Q, where Q is the wave vector change in the diffraction experiment. The problem is particularly important in cases such as water where there is a significant molecular polarization caused by charge transfer within the molecule. This means that the electron form factors applicable to the molecule in the condensed environment often deviate from their free atom values. The technique of empirical potential structure refinement (EPSR) is used here to focus on the problem of forming a single atomistic structural model which is simultaneously consistent with both x-ray and neutron diffraction data. The case of liquid water is treated explicitly. It is found that x-ray data for water do indeed provide a powerful constraint on possible structural models, but that the Q-range of the different x-ray data sets (maximum Q ranges from 10.8 to ∼17.0 Å(-1) for different x-ray experiments), combined with variations between different data sets, means that it is not possible to rigorously define the precise position and height of the first peak in the OO radial distribution function. Equally, it is found that two different neutron datasets on water, although measured to a maximum Q of at least 30 Å(-1), give rise to further small uncertainties in the position of the hydrogen bond peaks. One general conclusion from the combined use of neutron and x-ray data is that many of the classical water potentials may have a core which is too repulsive at short distances. This produces too sharp a peak in r-space at too short a distance. A softer core potential is proposed here.
液体和无序固体的X射线衍射数据通常能为中子衍射数据提供有用的补充结构信息。然而，X射线衍射图案是由原子电子散射产生的，与中子衍射中由原子核散射不同，其解释因与波矢Q相关的电子形状因子而变得复杂，这会导致X射线衍射信号随Q的增加而迅速下降，其中Q是衍射实验中的波矢变化。在诸如水这种因分子内电荷转移而产生显著分子极化的情况下，该问题尤为重要。这意味着适用于凝聚态环境中分子的电子形状因子常常偏离其自由原子值。这里使用经验势结构精修（EPSR）技术来关注构建一个同时与X射线和中子衍射数据一致的单原子结构模型的问题。具体处理了液态水的情况。结果发现，水的X射线数据确实对可能的结构模型提供了有力约束，但不同X射线数据集的Q范围（不同X射线实验的最大Q范围从10.8到约17.0 Å⁻¹），再加上不同数据集之间的差异，意味着无法严格定义氧-氧径向分布函数中第一个峰的精确位置和高度。同样，发现关于水的两个不同中子数据集，尽管测量的最大Q至少为30 Å⁻¹，但在氢键峰的位置上仍存在进一步的小不确定性。结合使用中子和X射线数据得出的一个普遍结论是，许多经典的水势在短距离处可能有一个过于排斥的核心。这在r空间中会在过短的距离处产生一个过于尖锐的峰。这里提出了一个更软的核心势。
