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首页> 外文期刊>Physical review >Structural instabilities and sequence of phase transitions in SrBi_2Nb_2O_9 and SrBi_2Ta_2O_9 from first principles and Monte Carlo simulations
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Structural instabilities and sequence of phase transitions in SrBi_2Nb_2O_9 and SrBi_2Ta_2O_9 from first principles and Monte Carlo simulations

机译：根据第一性原理和蒙特卡洛模拟，SrBi_2Nb_2O_9和SrBi_2Ta_2O_9的结构不稳定性和相变序列

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摘要

Despite their structural similarities, SrBi_2Ta_2O_9 (SBT) and SrBi_2Nb_2O_9 (SBN) undergo a different sequence of phase transitions. The phase diagram of SBT as a function of the temperature includes an intermediate phase between the high-temperature phase and the ferroelectric ground state, while in the niobium compound the intermediate phase is suppressed and a direct transition between the high- and low-temperature structures is observed. We present ab initio calculations that reveal the relevance of a trilinear coupling between three symmetry-adapted modes to stabilize the ground state in both compounds. This coupling is much stronger in SBN than in SBT. Within the framework of the phenomenological Landau theory, it is shown that, by solely increasing the strength of the trilinear coupling, the topology of the phase diagram of SBT can change enough to suppress the intermediate phase. Monte Carlo simulations on an idealized φ~4 Hamiltonian confirm that the trilinear coupling is the key parameter that determines the sequence of phase transitions, and that for higher dimensionality of the order parameters the stability region of the intermediate phase is narrower.
机译：尽管它们在结构上相似，但是SrBi_2Ta_2O_9（SBT）和SrBi_2Nb_2O_9（SBN）经历了不同的相变序列。 SBT随温度变化的相图包括高温相和铁电基态之间的中间相，而在铌化合物中，中间相被抑制，高温和低温结构之间直接过渡被观察到。我们提出了从头算的计算，揭示了三个对称适应模式之间的三线性耦合对稳定两个化合物的基态的相关性。 SBN中的耦合比SBT中的耦合强得多。在现象学的朗道理论的框架内，研究表明，仅通过增加三线性耦合的强度，SBT相图的拓扑就可以发生足够的变化以抑制中间相。在理想化的φ〜4哈密顿量上进行的蒙特卡洛模拟证明，三线性耦合是决定相变序列的关键参数，对于阶次参数的维数较大，中间相的稳定性区域较窄。

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