Defects of the crystal structure and Jahn-Teller distortion in BiMnO 3
Defects of the crystal structure and Jahn-Teller distortion in BiMnO 3
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DOI：
10.1103/physrevb.82.245101
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发表时间：
2010-08
影响因子：
3.7
通讯作者：
D. Boukhvalov;I. Solovyev
中科院分区：
文献类型：
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作者：
D. Boukhvalov;I. Solovyev
使用密度泛函理论与现场库仑校正（LDA+U方法），我们进行结构优化的BiMnO 3从不同的实验报道的结构。我们确认，无论在起始条件，所有的计算收敛到相同的中心对称结构，与以前的发现。然而，结构优化大大降低了系统中的Jahn-Teller（JT）失真。我们将这一事实归因于Mn和Bi位置周围局部扭曲的强烈竞争：当局部Mn环境经历JT不稳定性时，Bi位置之一有利于偏离中心对称的位移，这涉及相同的氧原子。第二种机制的存在解释了BiMnO 3和更典型的JT锰氧化物如LaMnO 3之间的差异。最后，由实验研究的动机，我们已经调查了不同类型的缺陷的形成，并得到了BiMnO 3（相反，其他考虑的系统，如LaMnO 3和BiFeO 3）可以相对容易地形成氧杂质在间隙网站。杂质氧原子倾向于与主体氧形成电子对，这解释了氧过量的BiMnO 3 +x的绝缘特性。此外，我们发现，BiMnO 3 +x样品的经验的“记忆效应”，其中优化的晶体结构强烈依赖于初始配置。我们认为，这样的记忆效应可以解释一些BiMnO 3的晶体结构的稳定性，这是以前报道的实验。
Using density-functional theory with the on-site Coulomb correction (the LDA+U method), we perform the structural optimization of BiMnO3 by starting from different experimentally reported structures. We confirm that irrespectively on the starting condition, all calculations converge to the same centrosymmetric structure, in agreement with the previous finding. Nevertheless, the structural optimization substantially reduces the Jahn-Teller (JT) distortion in the system. We attribute this fact to the strong competition of local distortions around the Mn- and Bi-sites: while the local Mn-environment experiences the JT instability, the one of the Bi-sites favours the off-centrosymmetric displacements, which involves the same oxygen atoms. The existence of the second mechanism explains the difference between BiMnO3 and more canonical JT manganites, such as LaMnO3. Finally, being motivated by experimental studies, we have investigated the formation of different types of defects and obtained that BiMnO3 (contrary to other considered systems, such as LaMnO3 and BiFeO3) can relatively easily form oxygen impurities at interstitial sites. The impurity oxygen atom tends to form a pair with the host oxygen, which explains the insulating character of the oxygen-excessive BiMnO3+x. Moreover, we found that the BiMnO3+x samples experience the "memory effect", where the optimized crystal structure strongly depends on the starting configuration. We suggest that such a memory effect may explain stability of some of the crystal structures of BiMnO3, which have been previously reported experimentally.