Polymorphous nature of cubic halide perovskites
Polymorphous nature of cubic halide perovskites
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DOI：
10.1103/physrevb.101.155137
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发表时间：
2019-05
影响因子：
3.7
通讯作者：
Xingang Zhao;G. Dalpian;Zhi Wang;A. Zunger
中科院分区：
文献类型：
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作者：
Xingang Zhao;G. Dalpian;Zhi Wang;A. Zunger
人们早就知道，许多卤化物和氧化物钙钛矿可能具有非理想的八面体，表现出倾斜、旋转和金属原子位移。还已知的是，在低温下具有单一结构基序（“单态结构”）的化合物在较高温度下可能变得无序，从而导致非理想的八面体作为熵效应。这里所示的是，在许多立方卤化物钙钛矿和一些氧化物化合物中，不同低对称性八面体（“多态网络”）的分布已经从系统内能的最小化中出现，即，它们代表了化学键合的固有的、优选的低温模式。热无序效应在升高的温度下在这种低温多晶型网络的顶部上积累。与单态对应物相比，多晶型网络具有较低的预测总能量（增强的稳定性），较大的带隙和介电常数，现在主要由离子部分，并同意更密切地与观察到的对分布函数。从X射线衍射推导出的标称立方钙钛矿（Pm-3 m）结构实际上是宏观平均的高对称性配置，其不应用于模拟电子性质，因为后者反映了低对称性局部配置。
It has been long known that numerous halide and oxide perovskites can have non-ideal octahedra, showing tilting, rotation, and metal atom displacements. It has also been known that compounds that have at low temperatures a single structural motif ("monomorphous structures") could become disordered at higher temperatures, resulting in non-ideal octahedra as an entropy effect. What is shown here is that in many cubic halide perovskites and some oxides compounds a distribution of different low-symmetry octahedra ("polymorphous networks") emerge already from the minimization of the systems internal energy, i.e., they represent the intrinsic, preferred low temperature pattern of chemical bonding. Thermal disorder effects build up at elevated temperatures on top of such low temperature polymorphous networks. Compared with the monomorphous counterparts, the polymorphous networks have lower predicted total energies (enhanced stability), larger band gaps and dielectric constants now dominated by the ionic part, and agrees much more closely with the observed pair distribution functions. The nominal cubic perovskites (Pm-3m) structure deduced from X-Ray diffraction is actually a macroscopically averaged, high symmetry configuration, which should not be used to model electronic properties, given that the latter reflect a low symmetry local configuration.