The relaxor material Na_1/2Bi_1/2TiO₃ (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.

中文翻译：

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通过密度泛函理论计算研究了弛豫器Na1 / 2Bi1 / 2TiO3中的相变

弛豫材料Na _1/2 Bi _1/2 TiO ₃（NBT）是开发无铅压电陶瓷的重要基础，但这种材料的许多功能仍未得到很好的理解。在这里，我们通过密度泛函理论计算，利用从头算分子动力学和微动的弹性带计算，研究了NBT中八面体倾斜和A-阳离子位移引起的相变动力学。我们的结果表明，低温菱形，中间斜方和其他亚稳相之间的能量差异接近于室温的热能。因此，很可能在室温以上，仅仅由于氧离子的热振动，在局部尺度上同时存在多个八面体倾斜图案。八面体倾斜变换和A-阳离子位移也显示出类似的高能垒，但是，由于氧气的振动频率较高，因此倾斜变换更频繁地发生。此外，更容易发生氧八面体变形最小的倾斜转变。我们还发现，化学A-阳离子顺序影响能垒，影响旋转和位移模式之间的耦合，并确定某些八面体倾斜顺序的稳定性。我们得出的结论是，这种材料中的所谓极性纳米区域是由局部八面体倾斜转换和随后的A阳离子位移引起的，这些位移由热振动驱动，并由潜在的化学顺序介导。氧八面体变形最少的倾斜变形更有可能发生。我们还发现，化学A-阳离子顺序影响能垒，影响旋转和位移模式之间的耦合，并确定某些八面体倾斜顺序的稳定性。我们得出的结论是，这种材料中的所谓极性纳米区域是由局部八面体倾斜变换和随后的A阳离子位移引起的，这些位移由热振动驱动并由基本的化学顺序介导。氧八面体变形最少的倾斜变形更有可能发生。我们还发现，化学A-阳离子顺序影响能垒，影响旋转和位移模式之间的耦合，并确定某些八面体倾斜顺序的稳定性。我们得出的结论是，这种材料中的所谓极性纳米区域是由局部八面体倾斜转换和随后的A阳离子位移引起的，这些位移由热振动驱动，并由潜在的化学顺序介导。