Solid State Ionics 136–137 (2000) 513–517 www.elsevier.com / locate / ssi Crystal structure of non-stoichiometric b0 K–Ba-Na mixed ferrite K Na Ba Fe Mg O 0.30 0.62 0.62 10.44 0.29 17 a, a b b * A.C. Stergiou , D.K. Gioureli , D. Samaras , G. Litsardakis a Department of Physics, Aristotle University, 54006 Thessaloniki, Greece b Department of Electrical Engineering, Aristotle University, 54006 Thessaloniki, Greece Abstract Crystals of non-stoichiometric K–Na-Ba ferrite, with b0-alumina structure type and chemical formula K Na Ba Fe Mg O , were prepared by a partial ionic exchange from a K–Na-b0-ferrite. The ion exchange 0.30 0.62 0.62 10.44 0.29 17 was performed in a Ba(NO ) -KNO eutectic melt at 6508C for 15 min. The study of a single crystal with X-ray diffraction 3 2 3 ¯ ˚ ˚ (MoKa radiation), resulted in the R3m space group, with a 5 b 55.9436(7) A, c 535.743(7) A and Z 53. The structure determination was based on the b0-alumina structure model (Na Al Mg O ), where the Na atom sites are occupied by 11x 112x x 17 (Ba, K, Na) and the Al ones by Fe. The investigation on the electron density of the conduction plane ( z 51/6) gave trigonal distribution around the middle position (0,0,1 / 6) for (K–Na-Ba) atoms. This distribution can be described by a partial occupation of two sites (18 h) with coordinates 0.056, 0.028, 1/6 and 0.26, 0.13, 1/6, by (K, Na, Ba) atoms with a consequent decrement of the site occupation factors. Refinement of the cation population parameters, and positional and anisotropic thermal parameters for all atoms, resulted in the final residual factor R 50.057.  2000 Elsevier Science B.V. 1 All rights reserved. Keywords: Crystal structure; b9 K–Ba-Na mixed ferrite 1. Introduction BaCl –KCl melt at T .7008C [3]. If the exchange 2 conditions are milder (e.g. at T 56508C in a melt or Fast ion exchange by both monovalent and di- at T 540–808C in Ba(NO ) aqueous solution), 3 2 valent ions has been observed in K or Na ferrites neither the exchange is complete, nor the phase with b0-alumina type structure [1,2]. In exchange transformation takes place; the result is mixed Ba–K reactions by monovalent ions the b0-type structure is b0-type ferrite, which transforms to mixed Ba–K non maintained, while the exchange by divalent ions, stoichiometric hexaferrite after annealing [4]. Non- 21 21 such as Ba , Sr , leads to a transformation from stoichiometric hexaferrites have been investigated, as b9-ferrite to hexaferrite structure, and produces non- an alternative to (Co, Ti) modified Ba hexaferrite stoichiometric hexaferrites. This structure trans- particles, because they present interesting magnetic formation takes place during the exchange in a and morphological characteristics, which find appli- cations as magnetic recording media [5]. We have previously described the crystal structures and ion *Corresponding author. Fax: 130-31-998-054. E-mail address: stergiou@ccf.auth.gr (A.C. Stergiou). distribution of completely [6] and partially [7] 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(00)00479-3