鲁达 晶体的结构在高压下如何演变，是凝聚态物理、材料科学、结构化学及地质科学中非常基础和核心的课题。传统上，人们将晶体定义为具有对称性的晶胞在三维空间的堆垛。此种定义在描述晶体的物性上取得了巨大的成功，如能带结构、晶格振动及光跃迁等。然而，这种定义方式却很难揭示晶体高压相之间的演化及内在关联。以金属二氧化物（MO2）这一家族为例，虽有大量的理论和实验工作来表征它们的高压相结构，然而，对各个相之间的联系及它们的演化模式，目前人们所知甚少。
北京高压科学研究中心刘浩哲研究员带领的合作研究团队，将金属离子的配位多面体选为晶体的基本建构单元，来研究VO2体系高压相的演变及微观机制。在这种新的视角下，他们发现了VO2的高压相之间呈现出一种统一的相变模式。高压原位X射线衍射实验和第一性原理计算表明：强压缩导致的VO2配位数的增加总是以一种有序的方式发生在钒-氧八面体的一个极轴上，而赤道面和另一个极轴在结构拓扑上则保持不变。MO2化合物中的其他一些二氧化物金属阳离子虽然不同，但也表现出类似的相变模式及配位数的演化。此外，配位多面体之间的堆垛方式也朝着增加它们之间的共享来演化，反映出MO2的化学键在高压下由离子性向共价性转化这一共性的规律。本项研究拓展了人们对高压下晶体概念的认识和理解，并找到了解决高压相如何演化这一难题的一个有效途径。该工作近期发表于JPCL 2018, 9 (9)； DOI: 10.1021 。英文标题和摘要如下，点击阅读原文可以自由获取论文PDF。
VO2高压相及钒-氧配位多面体的演化
Correlated High-Pressure Phase Sequence of VO2 under Strong Compression
ABSTRACT Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO2) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO2. Xray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO2 to give away the ionicity in exchange for covalency under pressure.