Oxygen-octahedral distortion and electronic correlation induced semiconductor gaps in ferrimagnetic double perovskite Ca2MReO6 (M = Cr, Fe)

Abstract

Motivated by experimental nonmetallic features and high magnetic Curie temperatures of 360 and 522 K in double perovskite Ca₂CrReO₆ and Ca₂FeReO₆, we systematically investigate the structural, electronic, and magnetic properties of Ca₂MReO₆ (M = Cr, Fe) by combining the modified Becke–Johnson (mBJ) exchange potential with usual generalized gradient approximation (GGA). Our full optimization leads to stable ground-state structures with monoclinic symmetry (P2₁/n) consistent with experiment. The mBJ calculation successfully produces ferrimagnetic phase with semiconductor gaps of 0.38 eV and 0.05 eV, respectively, in contrast with wrong metallic phases from GGA calculations. With the spin–orbit coupling (SOC) taken into account, the Ca₂MReO₆ (M = Cr, Fe) shows high magneto-crystalline anisotropy (MCA) with the magnetic easy axis along the [010] direction. Although reducing to 0.31 and 0.03 eV, the semiconductor gaps remain open in spite of the SOC broadening of the Re t_2g bands. Therefore, our DFT investigation has established the correct ferrimagnetic semiconductor ground states for the double perovskite Ca₂MReO₆ (M = Cr, Fe) materials. Our analysis shows that the semiconductor gaps are due to orbital-selective splitting on Re t_2g bands in the minority-spin channel, originated from the O-octahedral distortion and Coulomb correlation effect. This mechanism, different from that in other double perovskite materials such as Sr₂CrOsO₆, Ca₂CrOsO₆ and Sr₂FeOsO₆, can be useful to fully understand chemical and physical properties of double perovskite compounds.