摘要
通过第一性原理计算方法，大多数理论研究显示LaCoO3中（1102）-CoO2终结面不能稳定存在，这与实验观察结果不相符．在之前的研究当中除了LaCoO3的晶体结构以外，Co3＋离子的自旋对其晶面稳定性影响并没有被很好地考虑．通过测试六方相LaCoO3中不同Co3＋离子的自旋态：低自旋（LS）、中自旋（IS）和高自旋（HS），计算并比较了这些晶面在铁磁和反铁磁两种组态下的表面自由能．计算结果表明Co3＋离子的自旋对LaCoO3晶面稳定性有着很大的影响．不同于前人的研究结果，稳定性相图显示（1102）．CoO2终结面在O-rich的情况下能够稳定存在，这使理论计算与实验结果达成一致．另外，计算了稳定晶面在不同自旋组态下的氧空位生成能（Eov）．这些可能暴露终结面的（Eov）主要取决于Co3＋离子的自旋态：高自旋（HS）的（Eov）比其他自旋态的要低．这表明可以通过直接调变Co3＋离子的自旋态来调变LaCoO3材料的性质．
By the first-principles calculations, most studies indicated that the （1102）-CoO2 termination of LaCoO3 cannot be stabilized, which disagrees with the experimental observation. Besides the crystal structure, we found that the spin states of Co3＋ ions could affect surface stability, which previously were not well considered. By examining the different states of Co3＋ ions in hexagonal-phase LaCoO3, including low spin, intermediate spin, and high spin states, the surface grand potentials of these facets are calculated and compared. The results show that the spin states of Co3＋ ions have an important influence on stability of the LaCoO3 facets. Different from the previous results, the stability diagrams demonstrate that the （1102）- CoO2 termination can stably exist under O-rich condition, which can get an agreement with the experimental ones. Furthermore, the surface oxygen vacancy formation energies （Eov） of stable facets are computed in different spin states. The Eov of these possible exposed terminations strongly depend on the spin state of Co3＋ ions： in particular, the Eov of the HS states is lower than that of other spin states. This indicates that one can tune the properties of LaCoO3 by directly tuning the spin states of Co3＋ ions.
基金
This work was supported by the National Natural Science Foundation of China （No.U1232118, No.21203099）, the National Basic Research Program （No.2014CB932403）, the Program of Introducing Talents of Disciplines to China Universities （No.B06006）, Research Program for Advanced and Applied Technology of Tianjin （No.13JCYBJC36800）, Doctoral Fund of Ministry of Education of China （No.20120031120033）, Fok Ying Tung Education Foundation （No.151008）, and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund （the second phase）. We appreciate the supports from the National Super-Computing Center at Tianjin and Guangzhou.
