Ferroelectricity promoted by cation/anion divacancies in SrMnO3

We investigate the effect of polar Sr-O vacancy pairs on the electric polarization of SrMnO3 (SMO) thin films using density functional theory (DFT) calculations. This is motivated by indications that ferroelectricity in complex oxides can be engineered by epitaxial strain but also via the defect chemistry. Our results suggest that intrinsic doping by cation and anion divacancies can induce a local polarization in unstrained non-polar SMO thin films and that a ferroelectric state can be stabilized below the critical strain of the stoichiometric material. This polarity is promoted by the electric dipole associated with the defect pair and its coupling to the atomic relaxations upon defect formation that polarize a region around the defect. This suggests that polar defect pairs affect the strain-dependent ferroelectricity in semiconducting antiferromagnetic SMO. For metallic ferromagnetic SMO we find a much weaker coupling between the defect dipole and the polarization due to much stronger electronic screening. Coupling of defect-pair dipoles at high enough concentrations along with their switchable orientation thus makes them a promising route to affect the ferroelectric transition in complex transition metal oxide thin films.

本研究采用密度泛函理论（Density Functional Theory, DFT）计算，探究极性Sr-O空位对锰酸锶（SrMnO3, SMO）薄膜的电极化特性的影响。本研究的动机源于已有研究表明：复杂氧化物中的铁电性既可通过外延应变进行调控，也可借助缺陷化学策略实现工程化设计。研究结果显示，阳离子与阴离子双空位的本征掺杂，可在无应变的非极性SMO薄膜中诱导局域电极化；且在低于化学计量比材料临界应变的条件下，可稳定铁电态。该电极化现象由与缺陷对相关联的电偶极子所促进，且该偶极子与缺陷形成过程中引发的原子弛豫发生耦合，进而在缺陷周边区域诱导极化。这表明极性缺陷对会影响半导体性反铁磁SMO中依赖应变的铁电性。而对于金属性铁磁SMO，由于电子屏蔽效应更强，缺陷偶极子与电极化之间的耦合作用显著更弱。综上，当缺陷对偶极子浓度足够高且取向可切换时，其有望成为调控复杂过渡金属氧化物薄膜中铁电相变的有效途径。