Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations
Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations
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DOI：
10.1111/jace.15207
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发表时间：
2018
影响因子：
3.9
通讯作者：
K. Meyer;L. Koch;K. Albe
中科院分区：
文献类型：
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作者：
K. Meyer;L. Koch;K. Albe
弛豫材料Na_（1/2）Bi_（1/2）TiO_3（NBT）是无铅压电陶瓷的重要基础材料，但对该材料的许多特性还不清楚。在这里，我们研究的动力学相变八面体倾斜和A-阳离子位移NBT通过密度泛函理论计算，采用从头算分子动力学和轻推弹性带计算。结果表明，低温菱方相、中间斜方相和其它亚稳相的能量差接近于室温热能。因此，很可能在室温以上，几个八面体倾斜图案在局部尺度上同时存在，只是因为氧离子的热振动。八面体倾斜变换和A-阳离子位移显示出类似的高能量势垒，然而，由于氧的振动频率更高，倾斜变换更频繁地发生。此外，更可能发生氧八面体变形最少的倾斜转变。我们还发现，化学A-阳离子的顺序影响的能量障碍，影响旋转和位移模式之间的耦合，并确定某些八面体倾斜订单的稳定性。我们的结论是，在这种材料中的所谓的极性nanoregions从本地八面体倾斜变换和随后的A-阳离子位移，这是由热振动驱动，并介导的基础化学秩序的结果。
The relaxor material Na_1/2Bi_1/2TiO_3(NBT) is an important basis for the development of lead-free piezoceramics, but still many features of this material are not well understood. Here, we study the kinetics of phase transformations by octahedral tilts and A-cation displacements in NBT by means of density functional theory calculations, employing ab initio molecular dynamics and nudged elastic band calculations. Our results show that the energetic differences between the low temperature rhombohedral, intermediate orthorhombic and other metastable phases are close to the room temperature thermal energy. Therefore, it is likely that above room temperature, several octahedral tilt patterns are present simultaneously on the local scale, just because of thermal vibration of the oxygen ions. Octahedral tilt transformations and A-cation displacements show similarly high energy barriers, however, since the vibrational frequency of oxygen is higher, tilt transformations occur more frequently. Further, tilt transformations in which the oxygen octahedra get deformed the least are more probable to occur. We also find that the chemical A-cation order affects energy barriers, influences the coupling between rotational and displacive modes and determines the stability of certain octahedral tilt orders. We conclude that the so-called polar nanoregions in this material result from local octahedral tilt transformations and subsequent A-cation displacements, which are driven by thermal vibration and are mediated by the underlying chemical order.