Molecular Oxygen-Induced Ferromagnetism and Half-Metallicity in α-BaNaO4: A First-Principles Study

Molecular oxygen resembles 3d and 4f metals in exhibiting long-range spin ordering and strong electron correlation behaviors in compounds. Ferromagnetic spin ordering and half-metallicity, however, are quite elusive and have not been well acknowledged. In this Article, we address this issue by studying how spins will interact with each other if the oxygen dimers are arranged in a different way from that in the known superoxides and peroxides by first-principles calculations. Based on the results of a structure search, thermodynamic studies, and lattice dynamics, we show that tetragonal alpha-BaNaO4 is a stable half-metal with a Curie temperature at 120 K, the first example in this class of compounds. Like 3d and 4f metals, the O-2 dimer carries a local magnetic moment of 0.5 mu(B) due to the unpaired electrons in its pi* orbitals. This compound can be regarded as forming from the O-2 dimer layers stacking in a head-to-head way. In contrast to the arrangement in AO(2) (A = K, Rb, Cs), the spins are ferromagnetically coupled both within and between the layers. Spin polarization occurs in pi* orbitals, with spin-up electrons fully occupying the valence band and spin-down electrons partially occupying the conduction band, forming semiconducting and metallic channels, respectively. Our results highlight the importance of geometric arrangement of O-2 dimers in inducing ferromagnetism and other novel properties in O-2-dimer-containing compounds.