ИСТИНА

The static and dynamic magnetic properties of a new layered sodium iron antimonate Na4FeSbO6 were studied comprehensively by performing magnetic susceptibility, heat capacity, electron spin resonance, Mössbauer and XANES spectroscopy measurements as well as density functional theory calculations. In contrast to our recent data on structurally related Li4FeSbO6 [1], which has been classified as a triangular lattice antiferromagnet, Na4FeSbO6 does not show a long-range magnetic order down to 2 K, and demonstrates a complex magnetic behavior, sharing many characteristics with spin-glass materials. The explanation of the magnetic susceptibility, ESR and Mössbauer data requires the presence of some intrinsic antisite defects (Fe3+Sb5+) within the (NaFeSbO6)3- layers of Na4FeSbO6. These antisite defects give rise to three consecutive edge-sharing FeO6 octahedra hence forming Fe3+-O-Fe3+-O-Fe3+ trimer units joined by superexchange pathways in the magnetic sublattice. Density functional theory calculations for Na4FeSbO6 show that the Fe3+-O-Fe3+ superexchange between regular and antisite Fe3+ ions is strongly antiferromagnetic, while the Fe3+-O…O-Fe3+ super-superexchange between regular-site Fe3+ ions is weakly antiferromagnetic. In addition, the calculations suggest that the antisite Fe3+ ion is not in the high-spin state, concomitantly enhancing the magnetization of Na4FeSbO6 due to ferrimagnetic order Fe3+Fe3+Fe3+ in each Fe3+–Fe3+–Fe3+ trimer. The latter is likely to give a reasonable explanation for the positive value of the Curie-Weiss temperature, which is apparently anomalous in the presence of all antiferromagnetic spin exchange interactions in the system. A direct confirmation of the low-spin state for the antisite Fe3+ ions and high-spin state for regular Fe3+ ions has been obtained from synchrotron XANES data.