Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction
Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction
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DOI：
10.2138/am-2022-8458
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发表时间：
2022-07
影响因子：
3.1
通讯作者：
Yanyao Zhang;S. Chariton;Jiaming He;S. Fu;Fang Xu;V. Prakapenka;Jung‐Fu Lin
中科院分区：
文献类型：
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作者：
Yanyao Zhang;S. Chariton;Jiaming He;S. Fu;Fang Xu;V. Prakapenka;Jung‐Fu Lin
摘要后辉钛矿相变是一种典型的具有对称性破缺自发应变的伪真性铁弹性相变。在朗道模型的基础上，利用高压自发应变、光学模式和弹性模量(CIJ)研究了这一转变，但对其原子信息和结构变形的了解还很少。在这里，我们进行了同步加速器单晶X射线衍射测量高达75.3 Gpa的辉石晶体在钻石顶压室中。对数据的分析揭示了高压下SiO_6八面体的原子位置、键长、键角和变化。结果表明，O坐标在~51.4 GPa时分裂，此时顶端和赤道Si-O键长交叉，SiO_6八面体变形消失，SiO_6八面体开始绕c轴旋转。此外，变形模式分析表明，在高压下，辉钛矿结构发生面内拉伸变形(GM1+模)，而后辉钛矿结构则以旋转变形(GM2+模)为主。这些结果被用来关联弹性模量和朗道参数(对称破缺应变e1-e2和序参数Q)，以提供对铁弹性相变的原子洞察。当两个Si-O键的键长相等时，由于GM1+伸缩模式的贡献，C11与C12收敛，沿[110]极化并沿[110]传播的剪切波Vs1[110]消失。E1-e2和q值与后辉钛矿结构中Gm1+旋转模出现时的SiO6旋转角成正比。我们的结果还与钠长石的伪真类型和钙钛矿的真类型以及钙钛矿的真类型进行了比较。在所有这些类型的相变中，对称性破缺应变是低对称性铁弹性相中结构角度(如SiO_6旋转或晶格常数角)及其相应的变形模式所产生的主要影响。
Abstract The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase.