The crystal and magnetic structure of nonstoichiometric K+ β-ferrite

The crystal and magnetic structure of K+ beta-ferrite, K1.33Fe12O17, has been studied with neutron powder diffraction (lambda = 1.47) at 10, 295 and 923 K. Parameters describing the crystal and magnetic structure were refined with the Rietveld method. A magnetic phase transition at 713 K occurs from an antiferromagnetic to a paramagnetic state. The nuclear structure (space group P6(3)/mmc, Z = 2) consists of spinel blocks of iron and oxygen atoms interleaved by conduction layers where the K+ ions reside. The net magnetic structure is antiferromagnetic consisting of two ferrimagnetic spinel blocks with opposite spins in the unit cell. At 10 K, the derived magnitudes of the moments for the four crystallographically independent iron atoms are: 4.37(4) mu(B) for the octahedral site Fe1, 3.82(10) mu(B) and 4.09(11)mu(B), for the tetrahedral sites Fe2 and Fe3, respectively, and 3.89(15) for Fe4: the second octahedral site. The K+ ions in the conduction planes are refined in two 6h-sites (0.70, - x, 1/4 and 0.83, - x, 1/4) with an occupation 0.92 and 0.41 for a total of 1.33 K+ ions at both 10 and 295 K. Above the Neel temperature an ordering of the column oxygen ion O5 occurs together with an ordering of the K+ ions into the 6h-positions (0.71, - x, 1/4) and (0.84, - x, 1/4) with the occupations 0.64 and 0.73. Magnetic disordered diffuse scattering is observed at 923 K, (C) 2000 Elsevier Science B.V. All rights reserved.