钙钛矿结构锗酸盐奇特高压行为研究

The unusual high-pressure behaviors of the perovskite-structured germanates, which are the analogues of silicates, can be used to discuss the phase transitions of silicates, the crystal structures and properties of the Earth's mantle minerals. It is an important progress in understanding the high-pressure behaviors of the perovskite-structured compounds that transition metal oxide of PbCrO3 appears the pressure-induced isostructural phase transition accompanying a large volume collapse, a kind of rare electronic transition phenomenon. Recently, we found a clue by preliminary experimental research that this kind of special electronic transition also appears possibly in PbGeO3, a kind of main group element oxide with cubic perovskite structure. In this research project, in order to verify the electronic phase transition by investigating systematically the high pressure behaviors in germanate perovskite, we intend to carry out the experimental research on the high pressure and high temperature synthesis on PbGeO3 and BaGeO3 perovskites, and the Ti-doped CaGeO3, SrGeO3 and BaGeO3 perovskite solid solutions by both large volume press and diamond anvil cell techniques. In-situ high pressure synchrotron X-Ray diffraction and resistance (between low to room temperatures) experiments by diamond anvil cell are conducted on the synthesized samples of germanate perovskites and Ti-doped germanate perovskite solid solutions to understand their high-pressure properties and behaviors, for instance, pressure-induce phase transition between the possible perovskite-structured polymorphs, equations of state, and electrical properties. These experimental data would reveal the phase relationship between the low-pressure phase and high-pressure phase of PbGeO3 cubic perovskites, and uncover the possible polymorphic phase transitions existing in other germanate perovskites. The experimental results would help to understand the electronic transition nature of the pressure-induced polymorphic phase transitions in germanate perovskites. Based on the experimental results, we also intend to carry out the first principle calculation to understand the electronic band structures of several germanate perovskites, and furthermore, to reveal the physical mechanism of the unusual electronic transitions in germanate perovskites. After carrying out this project, we predict that, new findings of special electronic transition phenomenon existing in main group element oxides with perovskite structure would be confirmed for the first time, the unknown high-pressure nature of Ge-O bond would be revealed, and new high-pressure phase of germanate perovskite would be discovered.

钙钛矿结构锗酸盐奇特高压行为的研究结果可应用于类比硅酸盐矿物相变、地幔矿物结构和性质的探讨。过渡金属氧化物立方钙钛矿在高压下会发生等结构相变，即电子相变，是该类研究最近取得的一个重要进展。我们最近在PbGeO3立方钙钛矿中也发现存在上述奇特电子结构相变的线索。本申请拟利用大压机和金刚石压腔设备开展钙钛矿结构PbGeO3和BaGeO3，以及钙钛矿结构CaGeO3、SrGeO3、BaGeO3掺钛固溶体的高温高压合成实验，并通过高压原位同步辐射X射线衍射和高压变温电阻测量，探索PbGeO3两个立方钙钛矿多形间的相关系，并系统研究锗酸盐钙钛矿结构多形相变、状态方程、电学性质等高压性质和行为。开展第一原理理论研究，了解钙钛矿结构锗酸盐电子结构及其多形相变物理机制。本申请有望在主族元素氧化物钙钛矿的奇特电子相变研究中取得新发现，揭示Ge-O键尚未认识的高压属性，发现新的锗酸盐高压相。

钙钛矿多形相变由于涉及丰富的物理内涵和潜在的应用价值而受到广泛关注。最近我们实验发现PbCrO3立方钙钛矿奇特的压致等结构相变现象，并在PbGeO3立方钙钛矿中发现类似相变的线索。锗酸盐钙钛矿可能存在奇特多形相变现象的实验证实，将进一步丰富钙钛矿多形相变物理机制，并为探索下地幔硅酸盐钙钛矿是否存在类似相变现象提供线索。为此本项目设计了高温高压实验方案来合成锗酸盐钙钛矿并探索异常多形相变现象。我们在高温高压下合成了Ba(Ge0.5,Ti0.5)O3立方钙钛矿固溶体，发现其存在异常的结构行为和多形相变现象。常压时Ba(Ge0.5,Ti0.5)O3立方钙钛矿固溶体的晶格体积比BaTiO3钙钛矿还要大，高压时其晶格体积比BaTiO3钙钛矿要小。虽然其晶体结构随压力没有发生变化，但在~5GPa时其体积-压力关系存在拐点，显示样品发生了一种未见报道的二级等结构相变现象。为进一步揭示锗酸盐钙钛矿的异常多形相变行为，我们对BaGeO3进行了高温高压研究，首次合成了BaGeO3立方钙钛矿，该样品在8-10GPa压力区间发生类似于PbCrO3立方钙钛矿的体积不连续（~8%体积变化）多形相变，低压相除显示立方钙钛矿结构衍射线条外，还出现一些弱的衍射线条，显示为一种立方钙钛矿的调制结构。该低压相在接近常压时发生非晶化而不能保持。. 上述实验现象的实验发现丰富了钙钛矿结构多形相变现象，为揭示这类凝聚态物质电子相互作用、电子－晶格相互作用提供了新的研究对象。事实上，在我们发表PbCrO3奇特相变现象后，引起了国际上相关理论和实验物理学家对这一现象的关注，并发表了一系列进一步的研究成果，但对其物理机制仍有不同的观点。这也反映了我们进一步发现更多类似现象物质的科学意义。为进一步探索下地幔硅酸盐钙钛矿中是否存在类似的多形相变现象，我们对SrSiO3进行了高温高压实验研究，首次合成了SrSiO3立方钙钛矿，该样品在低压时发生非晶化现象，反映了钙钛矿多形相变与非晶化之间的竞争关系。

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