利用互补电子与X射线技术解决Ca0.4Sr0.6TiO3中的竞争畸变问题
Resolving competing distortions in Ca0.4Sr0.6TiO3 using complementary electron and X-ray techniques
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中文总结 AI 辅助
本研究利用变温TEM与PXRD技术，明确Ca0.4Sr0.6TiO3的结构相变机制，揭示其纳米级片晶微结构是拉曼与衍射测量差异的成因。
中文摘要 AI 辅助
我们对钙钛矿Ca0.4Sr0.6TiO3开展研究，采用变温透射电子显微镜（TEM）与粉末X射线衍射（PXRD）技术。室温和更低温度下，PXRD显示该材料采用正交晶系Pbcm结构，与NaNbO3的P相类似；温度高于380 K时，该材料转变为四方晶系I4/mcm相。这些相的结构畸变可描述为布里渊区M、T、Δ和R点对应的模式与序参量的组合，每个点对应一组不同的超结构反射，可在X射线和电子衍射图中观测到。对于I4/mcm相，PXRD中仅观测到预期的R点反射，而电子衍射中同时观测到M和R反射。利用Δ和R暗场TEM图像，我们表明该相变通过沿c轴的TiO6八面体倾斜失去相干性进行，形成由薄纳米级片晶构成的微结构，其局域对称性与宏观结构不同。这些片晶在相变温度以上仍稳定存在，可能是该材料体系中拉曼光谱与衍射测量长期存在差异及其他次级效应的原因。
英文摘要
We present a study of the perovskite Ca0.4Sr0.6TiO3 using variable temperature transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). At room temperature and below, PXRD shows that the material adopts an orthorhombic Pbcm structure analogous to the P-phase of NaNbO3. Above 380 K the material transforms to a tetragonal I4/mcm phase. The structural distortions of these phases can be described as a combination of modes and order parameters associated with the M, T, $Δ$ and R-points of the Brillouin zone, each of which can be associated with a different set of superstructure reflections visible in X-ray and electron diffraction patterns. For the I4/mcm phase only the expected R-point reflections are observed in PXRD while both M and R- reflections are observed in electron diffraction. Using $Δ$ and R dark field TEM images we show that the phase transition proceeds by a loss of coherence of TiO6 octahedral tilting along the c-axis, leading to a microstructure of thin nanoscale platelets with a different local symmetry to the macroscopic structure. These persist well above the phase transition temperature and are probably responsible for the long-standing discrepancy between Raman spectroscopy and diffraction measurements in this materials system, as well as other secondary effects.
