Control of Uniaxial Negative Thermal Expansion in Layered Perovskites by Tuning Layer Thickness.
Control of Uniaxial Negative Thermal Expansion in Layered Perovskites by Tuning Layer Thickness.
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DOI：
10.3389/fchem.2018.00455
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发表时间：
2018
影响因子：
5.5
通讯作者：
Senn MS
中科院分区：
文献类型：
--
作者：
Ablitt C;Mostofi AA;Bristowe NC;Senn MS
单轴负热膨胀（NTE）已知发生在An+1BnO 3 n +1 Ruddlesden-Popper（RP）层状钙钛矿系列的低n成员中，其中BO 6八面体围绕分层轴冻结旋转。先前的工作已经表明，这种NTE的出现是由于一个紧密接近的过渡到竞争相，所谓的“对称捕获”，和高度各向异性的弹性顺应性特定的NTE相的对称性的组合效应。我们将这种分析扩展到更广泛的RP族（n = 1，2，3，4，...，∞），表明通过改变结构中的层界面的分数（即，1/n）的值可以控制在这些系统中观察到的显著单轴NTE所必需的各向异性柔量。更详细的分析如何与1/n发展的组件的依从性矩阵，使我们能够识别不同的制度，链接增强这些制度之间的晶体学自由度的结构。我们进一步讨论了钙钛矿层厚度如何影响具有大的负Grüneisen参数的软区边界模式的频率，与上述相变相关，构成了NTE的热力学驱动力。这一新的见解补充了我们以前的工作，表明化学控制可用于在这些系统中从正热膨胀切换到负热膨胀，因为它使层厚度n成为一个额外的设计参数，可用于设计具有可调热膨胀的层状钙钛矿。在这些方面，我们预测，与适当的化学取代，n = 1相将是最明显的NTE可以实现的系统。
Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the An+1BnO3n+1 Ruddlesden–Popper (RP) layered perovskite series with a frozen rotation of BO6 octahedra about the layering axis. Previous work has shown that this NTE arises due to the combined effects of a close proximity to a transition to a competing phase, so called “symmetry trapping”, and highly anisotropic elastic compliance specific to the symmetry of the NTE phase. We extend this analysis to the broader RP family (n = 1, 2, 3, 4, …, ∞), demonstrating that by changing the fraction of layer interface in the structure (i.e., the value of 1/n) one may control the anisotropic compliance that is necessary for the pronounced uniaxial NTE observed in these systems. More detailed analysis of how the components of the compliance matrix develop with 1/n allows us to identify different regimes, linking enhancements in compliance between these regimes to the crystallographic degrees of freedom in the structure. We further discuss how the perovskite layer thickness affects the frequencies of soft zone boundary modes with large negative Grüneisen parameters, associated with the aforementioned phase transition, that constitute the thermodynamic driving force for NTE. This new insight complements our previous work—showing that chemical control may be used to switch from positive to negative thermal expansion in these systems—since it makes the layer thickness, n, an additional design parameter that may be used to engineer layered perovskites with tuneable thermal expansion. In these respects, we predict that, with appropriate chemical substitution, the n = 1 phase will be the system in which the most pronounced NTE could be achieved.