Existence of La-site antisite defects in LaMO₃ (M=Mn, Fe, Co, and Ni) predicted with many-body diffusion quantum Monte Carlo

The properties of LaMO₃ (M: 3d transition metal) perovskite crystals are significantly dependent on point defects. The most studied defects in La perovskites are the oxygen vacancies and doping impurities on the La and M sites. Based on results of an accurate many-body and ab initio theoretical approach, here we suggest that another intrinsic antisite defect, the replacement of lanthanum by the transition metal, M, can be formed under certain growth conditions. Fixed-node diffusion Monte Carlo (FNDMC) calculations of LaMO₃ (M=Mn, Fe, Co, and Ni) find that such antisite defects can have low or negative formation energies and are magnetized. Complementary density functional theory (DFT) based calculations show that these antisite defects improve the p-type electronic conductivity.  Our bulk validation studies establish that FNDMC reproduces the antiferromagnetic (AFM) state of LaMnO₃, while DFT with PBE and SCAN functionals and LDA+U method all favor ferromagnetic (FM) states, in variance with experiment. For LaNiO₃, the experiments have reported the antiferromagnetic (AFM) or non-magnetic (NM) ground state. Our FNDMC predict that R-type AFM state and FM state (1 bohr/f.u.) are equally stable: E(FM)-E(AFM)=-0.036(26) eV/f.u.