Crystal structure of non-stoichiometric β″ K–Ba-Na mixed ferrite K0.30Na0.62Ba0.62Fe10.44Mg0.29O17

Crystals of non-stoichiometric K–Na-Ba ferrite, with β″-alumina structure type and chemical formula K0.30Na0.62Ba0.62Fe10.44Mg0.29O17, were prepared by a partial ionic exchange from a K–Na-β″-ferrite. The ion exchange was performed in a Ba(NO3)2-KNO3 eutectic melt at 650°C for 15 min. The study of a single crystal with X-ray diffraction (MoKα radiation), resulted in the R3̄m space group, with a=b=5.9436(7) Å, c=35.743(7) Å and Z=3. The structure determination was based on the β″-alumina structure model (Na1+xAl11−xMgxO17), where the Na atom sites are occupied by (Ba, K, Na) and the Al ones by Fe. The investigation on the electron density of the conduction plane (z=1/6) 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 R1=0.057.