Structural and Elastic Properties of Post-Perovskite Related Phases at High PT

D”层的地震观测揭示了地幔与液态外核界面存在大尺度的非均匀结构和地震各向异性。最近对地球下地幔主要相MgSiO 3钙钛矿的实验表明，D”不连续性可能是由钙钛矿（pv）到CaIrO 3结构类型的后钙钛矿（ppv）的相变引起的。虽然矿物物理学界的这一发现已经导致了对地震数据解释的快速应用，但关于ppv的原子结构和弹性性质，还有几个悬而未决的问题。 构造和弹性性质是检验理论模型和解释地震观测结果的关键数据。 虽然硅酸盐ppv结构和弹性的研究受到合成所需的极端条件的阻碍，但最近有理论预测和实验证据表明，NaMgF 3中的pv-ppv相变起始于低至P~18 GPa。 NaMgF 3通常用作硅酸盐pv相的类似物。 准确的原位研究NaMgF 3和CaIrO 3在高压和高温下提供了一种方法，系统的理论/实验研究的PPV结构的晶体化学和弹性在环境和高PT条件下。 此外，ppv-NaMgF 3在室温下从40 GPa以上淬火时可以恢复到环境条件，从而允许在环境和高PT下精确研究晶体学和弹性，以解决我们对这种新材料的知识中的许多空白，直到最近，这类材料相对模糊。 通过pv-ppv转变的pv形式的NaNgF 3的压缩机制的比较将有助于在理论和实验上都易于处理的系统中测试所提出的机制的有效性。 虽然理论上具有挑战性，但CaIrO 3本身需要使用现代衍射技术重新检查，因为氧的位置仅在60年代中期以图形方式确定。 由于中子在重元素（如铱）存在下对氧比X射线更敏感，因此需要使用神经元散射在高PT下对该阶段进行系统研究。 这些研究将得到在明尼苏达州进行的NaMgF 3和CaIrO 3理论工作的支持。 衍射工作将在具有大容量高压设备的中子和同步加速器X射线设施中进行，同时收集衍射和超声波数据，从而使晶体结构和弹性直接相关，因为pv-NaMgF 3转化为ppv-NaMgF 3。 这些结果将适用于核幔边界可能的pv-ppv转变。

Seismic observations of the D" layer reveal large-scale heterogeneous structures and seismic anisotropy existing at the interface of Earth's mantle and liquid outer core. Recent experiments on MgSiO3 perovskite, the dominant phase in Earth's Lower Mantle, indicate the D" discontinuity might result from a phase transition from perovskite (pv) to a post-perovskite (ppv) of CaIrO3 structure-type. While this discovery by the mineral physics community has led to rapid application to interpretation of the seismic data, there are several open questions concerning the atomic structure and elastic properties of ppv. The structure and elastic properties are data crucial to the testing of theoretical models and to interpretation of the seismic observations. While studies of silicate ppv structure and elasticity is stymied by the extreme conditions required for synthesis, there are recent theoretical predictions and experimental evidence of pv-ppv phase transitions in NaMgF3 starting at as low as P~18 GPa. NaMgF3 is often used as an analogue for the silicate pv phase. Accurate in situ studies of NaMgF3 and of CaIrO3 at high pressures and temperatures offer a way forward for systematic theoretical/experimental studies of the crystal chemistry and elasticity of the ppv structure at ambient and at high PT conditions. Further, the ppv-NaMgF3 can be recovered to ambient conditions when quenched from above 40 GPa at room temperature, allowing accurate studies of crystallography and elasticity at ambient and high PT to address many of the gaps in our knowledge for this new, and until recently, relatively obscure class of materials. Comparisons of the compression mechanism for the pv form of NaNgF3 through the pv-ppv transition would aid in testing the validity of proposed mechanisms in a system that is tractable both theoretically and experimentally. Although challenging theoretically, CaIrO3 itself needs to be reexamined using modern diffraction techniques, since positions of oxygen were only determined graphically in the mid 60's. Because neutrons are more sensitive to the oxygen in the presence of heavy elements such as iridium than are X-rays, a systematic study of this phase at high PT using neuron scattering is required. These studies will be underpinned by theoretical work being carried out in Minnesota on both NaMgF3 and CaIrO3. The diffraction work will be carried out at neutron and synchrotron X-ray facilities with large volume high-pressure devices, where diffraction and ultrasonic data are collected simultaneously, allowing direct correlation of crystal structure and elasticity as pv-NaMgF3 transforms to ppv-NaMgF3. These results will be applicable to the possible pv-ppv transition at the core mantle-boundary.