LAUSR

The tunable magnetism and reversible phase transformation between SrCoO2.5 (SCO) and HSrCoO2.5 (HSCO) have attracted vast research interest; however, the physical origin of the weakly ferromagnetism of the hydrogenated phase is still unclear. Various point defects, especially H related ones, may play important roles in the magnetic order of SCO and HSCO. In this study, we performed first-principles calculations combined with bond orbital model analysis to investigate the stabilities and magnetic effects of these defects and their complexes in both phases. We find that Hi, VO, CoSr, and Oi are relatively stable in SCO, while VH, CoSr, Hi, and Oi are relatively stable in HSCO. Additionally, these defects show significant differences of formation energy in these two phases because the charge transfer mechanisms from defects to nearby Co atoms are different. The different mechanisms also lead to different local reconstructions and crystal field splitting of the Co 3d states, affected by the interaction between Co–O bond orbital and surrounding bonding environment. Single defects of VH, Hi, and CoSr contribute significantly to the total magnetic moment of the system for HSCO or SCO. However, a ferromagnetic coupling is discovered in the two VH configurations only in HSCO, which may explain the experimental observation of the weakly ferromagnetism of HSCO.