Strain analysis of phase transitions in (Ca,Sr)TiO3 perovskites
Strain analysis of phase transitions in (Ca,Sr)TiO3 perovskites
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(Ca,Sr)TiO3 钙钛矿相变的应变分析
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
F. Seifert
中科院分区：
文献类型：
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作者：
M. Carpenter;A. I. Becerro;F. Seifert
摘要本文用一个单一的朗道自由能展开式描述钙钛矿的相变，从立方母体结构到四方晶系和正交晶系结构，空间群与Pm 3 π m倒易晶格的M3和R25点有关。这种扩展允许的自发应变和个别序参量分量的自适应形式之间的关系进行预测。从文献中的数据（Ca，Sr）TiO 3钙钛矿进行了分析，根据这些预测。剪切应变I4/mcm，Pnma，和Cmcm结构往往符合预测的模式。在CaTiO_3中，Pm_3 → m_Particl_4/mcm相变具有接近三临界的特征，而在固溶体的富Sr端，相变具有接近二阶的特征。与体积应变的耦合似乎是温度和组成依赖性的，这可能是钙钛矿中相变的一般特征。重整化的四阶项通过改变体积耦合系数可以负责不寻常的顺序参数的演变所示的CaTiO 3和相变的热力学性质的变化作为一个功能的组合物。应变变化的模式也密切相关的文献中的热容量的变化模式，建议修订的（Ca，Sr）TiO 3的固溶体在室温以上的亚固相线相图。
Abstract A single Landau free energy expansion is used to describe phase transitions in perovskites, from a cubic parent structure to tetragonal and orthorhombic structures with space groups related to the M3 and R25 points of the Pm3̅m reciprocal lattice. This expansion permits relationships between symmetry-adapted forms of the spontaneous strain and individual order parameter components to be predicted. Data from the literature for (Ca,Sr)TiO3 perovskites are analyzed in the light of these predictions. Shear strains for I4/mcm, Pnma, and Cmcm structures tend to conform to the predicted pattern. The Pm3̅m↔I4/mcm transition has nearly tricritical character as a function of temperature in CaTiO3 and more nearly second-order character as a function of composition at the Sr-rich end of the solid solution. Coupling with the volume strain appears to be both temperature and composition dependent, which may be a general feature of phase transitions in perovskites. Renormalization of fourth-order terms by changing the volume coupling coefficients could be responsible for the unusual order parameter evolution shown by CaTiO3 and for changes in thermodynamic character of the phase transitions as a function of composition. The pattern of strain variations also correlates closely with patterns of variations in heat capacity from the literature, suggesting revisions to the subsolidus phase diagram for the (Ca,Sr)TiO3 solid solution above room temperature.