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

The crystal and magnetic structure of K+ beta-ferrite, K1.33Fe12O17, has be en studied with neutron powder diffraction (lambda = 1.47) at 10, 295 and 9 23 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 magneti c structure is antiferromagnetic consisting of two ferrimagnetic spinel blo cks with opposite spins in the unit cell. At 10 K, the derived magnitudes o f 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 Fe 4: the second octahedral site. The K+ ions in the conduction planes are ref ined 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 wi th 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 diff use scattering is observed at 923 K, (C) 2000 Elsevier Science B.V. All rig hts reserved.