Salzmann Christoph G, Slater Ben, Radaelli Paolo G, Finney John L, Shephard Jacob J, Rosillo-Lopez Martin, Hindley James
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom.
Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.
J Chem Phys. 2016 Nov 28;145(20):204501. doi: 10.1063/1.4967167.
The DO ice VI to ice XV hydrogen ordering phase transition at ambient pressure is investigated in detail with neutron diffraction. The lattice constants are found to be sensitive indicators for hydrogen ordering. The a and b lattice constants contract whereas a pronounced expansion in c is found upon hydrogen ordering. Overall, the hydrogen ordering transition goes along with a small increase in volume, which explains why the phase transition is more difficult to observe upon cooling under pressure. Slow-cooling ice VI at 1.4 GPa gives essentially fully hydrogen-disordered ice VI. Consistent with earlier studies, the ice XV obtained after slow-cooling at ambient pressure is best described with P-1 space group symmetry. Using a new modelling approach, we achieve the atomistic reconstruction of a supercell structure that is consistent with the average partially ordered structure derived from Rietveld refinements. This shows that C-type networks are most prevalent in ice XV, but other structural motifs outside of the classifications of the fully hydrogen-ordered networks are identified as well. The recently proposed Pmmn structural model for ice XV is found to be incompatible with our diffraction data, and we argue that only structural models that are capable of describing full hydrogen order should be used.
利用中子衍射对常压下DO冰VI到冰XV的氢有序相变进行了详细研究。发现晶格常数是氢有序化的敏感指标。氢有序化时，a和b晶格常数收缩，而c晶格常数则有明显膨胀。总体而言，氢有序化转变伴随着体积的小幅增加，这解释了为什么在压力下冷却时更难观察到相变。在1.4 GPa下缓慢冷却冰VI得到的基本上是完全氢无序的冰VI。与早期研究一致，常压下缓慢冷却后得到的冰XV最好用P-1空间群对称性来描述。使用一种新的建模方法，我们实现了与从Rietveld精修得到的平均部分有序结构一致的超晶胞结构的原子结构重建。这表明C型网络在冰XV中最为普遍，但也识别出了完全氢有序网络分类之外的其他结构基序。发现最近提出的冰XV的Pmmn结构模型与我们的衍射数据不兼容，我们认为应该只使用能够描述完全氢有序的结构模型。
