Elastic properties of tetragonal BiFeO3 from first-principles calculations

Multiferroic BiFeO3 can exist in tetragonal G-type and C-type antiferromagnetic phases with a giant c/a ratio and polarizability. In this letter, the elastic constants cij's of these tetragonal BiFeO3 phases are studied as the function of pressure using first-principles density-functional theory. We find that, except for c44, the predicted cij's decrease with decreasing pressure (or increasing volume). When the volume is less than 11 Å3/atom (or greater than 17 Å3/atom), the c44 of these tetragonal phases tend to zero and the structures become unstable. The tetragonal phases are predicted to be softer than the rhombohedral antiferromagnetic phase. Other elastic properties, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and elastic anisotropy ratios are also investigated.