Effect of Nonmagnetic Ion Deficiency on Magnetic Structure: Density Functional Study of Sr₂MnO₂Cu_2–xTe₂, Sr₂MO₂Cu₂Te₂ (M = Co, Mn), and the Oxide-Hydrides Sr₂VO₃H, Sr₃V₂O₅H₂, and SrVO₂H

Two seemingly puzzling observations on two magnetic systems were analyzed. For the oxide-hydrides Sr₂VO₃H, Sr₃V₂O₅H₂, and SrVO₂H, made up of VO₄H₂ octahedra, the spin orientations of the V³⁺ (d², S = 1) ions were reported to be different, namely, perpendicular to the H-V-H bond in Sr₂VO₃H but parallel to the H-V-H bond in Sr₃V₂O₅H₂ and SrVO₂H, despite that the d-state split patterns of the VO₄H₂ octahedra are similar in the three oxide-hydrides. Another puzzling observation is the contrasting magnetic structures of Sr₂CoO₂Cu₂Te₂ and Sr₂MnO₂Cu_1.58Te₂, consisting of the layers made up of corner-sharing MO₄Te₂ (M = Co, Mn) octahedra. The Co²⁺ spins in each CoO₂Te₂ layer are antiferromagnetically coupled with spins perpendicular to the Te–Co–Te bond, whereas the Mn³⁺/Mn²⁺ ions of each MnO₂Te₂ layer are ferromagnetically coupled with spins parallel to the Te-Mn-Te bonds. We investigated the cause for these observations by performing first-principles density functional theory (DFT) calculations for stoichiometric phases Sr₂VO₃H, Sr₃V₂O₅H₂, SrVO₂H, Sr₂CoO₂Cu₂Te₂, and Sr₂MnO₂Cu₂Te₂, as well as nonstoichiometric phase Sr₂MnO₂Cu_1.5Te₂. Our study reveals that the V³⁺ ions in all three oxide-hydrides should have the spin orientation parallel to the H–V–H bond. The unusual magnetic structure of the MnO₂Te₂ layers of Sr₂MnO₂Cu_1.52Te₂ arises from the preference of a Mn³⁺ spin to be parallel to the Te-Mn-Te bond, the ferromagnetic spin exchange between adjacent Mn³⁺ and Mn²⁺ ions, and the nearly equal numbers of Mn³⁺ and Mn²⁺ ions in each MnO₂Te₂ layer. We show that the spin orientation of the magnetic ions in an antiferromagnetically coupled perovskite layer, expected in the absence of nonmagnetic ion vacancies, cannot be altered by the magnetic ions of higher oxidation that result from trace vacancies at the nonmagnetic ion sites.