Structure evolution and the relationship between high-pressure phases of VO2 – Dr. Haozhe Liu

APRIL 28, 2018

Pressure-induced phase transition always involves symmetry and coordination change in a crystal. While researchers most focus on symmetry change instead of considering its coordination or local bonding varying when a crystal structure changed. A team of scientists led by Dr. Haozhe Liu of HPSTAR find a general coordination increase in VO2 phase transitions and state that this structure-evolution rule may commonly exist in other dioxides. This work is published on JPCL.

When we describe a crystal, we usually think of the stacking of a unit cell with special symmetry. While only symmetry will not give us the detailed connection between different structures.

“Instead of considering the possible change of the symmetry, our focus is the evolution of the coordination polyhedral of a crystal when the structure change happened”, said Dr. Haozhe Liu, the lead author.

At ambient pressure, VO₂ exhibits a rutile structure with the vanadium located in the center of the oxygen octahedron. Upon compression, VO2 shows a serial of structure evolution which is connected by a simple and orderly coordination increase from x-ray diffractions together with theoretical calculations.

“It’s interesting that the coordination increase is orderly and only happens at one corner of the V-centered octahedron”, said Dr. Liu. “Under compression, an oxygen atom previously belonging to other octahedrons is compelled to insert itself into the bottom apex of the octahedron”, exained Dr. Liu.

Though Si, Ti, and V have different atomic orbitals, their dioxides display similar phases and coordination polyhedra. During the coordination increase, strong compression also forces the coodination polyhedron to increase sharing with its neighboring coordination polyhedral in metal dioxides, which increases the covalent character of the metal−oxygen bond and weakens the ionicity of crystal.

“This unified and correlated structure transition pattern suggests a common structural evolution in metal dioxides”, Liu added. “The evolution of metal-centered polyhedron studied in this work provides a new way to describe the phase transitions and the relations of these phases, which could be the key to understanding their chemistry of the bonding, structures and phase sequence under strong compression”, Dr. Liu added.

Caption: The crystal structures and the stacking of the polyhedra in the CN6 to CN10 structures and related unit cells of these structures during compression.

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搜狐：http://www.sohu.com/a/230953330_199523

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的刘浩哲研究员带领的研究团队反常规地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体系高压相直接的演变及微观机制。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。