Prediction of the hardest BiFeO3 from first-principles calculations

Abstract

BiFeO₃ is the only material with ferroelectric Curie temperature and Néel temperature higher than room temperature, making it one of the most well-studied multiferroic materials. Based on an ab initio evolutionary algorithm, we predicted a new cubic C-type antiferromagnetic structure (Fdm-BiFeO₃) at ambient pressure. It was found that Fdm-BiFeO₃ is the hardest BiFeO₃ (Vickers hardness ∼ 9.12 GPa), about 78% harder than R3c-BiFeO₃ (the well-known multiferroic material), which contributes to extending the life of BiFeO₃ devices. In addition, Fdm-BiFeO₃ has the largest shear modulus (83.74 GPa) and the largest Young's modulus (214.72 GPa). Besides, we found an interesting phenomenon that among the common multiferroic materials (BiFeO₃, BaTiO₃, PbTiO₃, SrRuO₃, KNbO₃, and BiMnO₃), Pnma-BiMnO₃ has the largest bulk modulus, and its bulk modulus is about 15% larger than that of Fdm-BiFeO₃. However, its Vickers hardness (4.47 GPa) is much smaller than that of Fdm-BiFeO₃. This is because the Vickers hardness is proportional to the shear modulus and the shear modulus of Fdm-BiFeO₃ is larger than that of Pnma-BiMnO₃. This work provides a deeper and more comprehensive understanding of BiFeO₃.