BARIUM HEXAALUMINOFERRITES WITH NEW STRUCTURAL FEATURES

Barium hexaferrite and hexaaluminate are of hexagonal symmetry (P6(3)/ mmc) with a structure respectively of magnetoplumbite (MP) and beta-al umina (beta) type. The BaAlxFe12-xO19 phases (BAF) have been synthesiz ed by solid state reactions at high temperatures (1350-degrees-C - 150 0-degrees-C). The X-ray powder diffraction patterns are close to those of a MP or beta structure type but for 4 < x < 8 they also exhibit so me extralines. These new lines can be interpreted as 001,1 = 2n + 1 wh ich are ''forbidden'' reflexions in the P6(3)/mmc space group. Their i ntensities are particularly strong for BaAl6Fe6O19 (x = 6) and decreas e apart from this composition. The a unit cell constant shows a linear decrease with increasing x, whereas the c vs. x curve presenTs a mark ed inflection point for intermediate x values. The C/a ratios show tha t for x less-than-or-equal-to 4 the structure should be of MP-type whi le compounds with x greater-than-or-equal-to 9 would adopt the beta st ructure. For the intermediate compositions, corresponding to the appea rance of the extra lines, a new structural type appears. Models are pr oposed to describe these new phases.