Novel synthesis methods have been investigated for the preparation of Bi4Ti3O12 ferroelectric Aurivillius-type powders, namely, the wet no-coprecipitated chemistry using a n-butylamine aqueous solution and two mechanochemical activation techniques (vibrating and planetarium mills). For the sake of comparison, Bi4Ti3O12 powders were also produced by the traditional ceramic route. The syntheses of the crystalline phases were obtained by annealing of the precursors at different temperatures and times. Both the precursors and the thermally treated products were characterized by means of X-ray powder diffraction, thermal analysis, and scanning electron microscopy. The Aurivillius phase was found to appear at 600 °C for vibrating and 500 °C for planetarium-milled precursors. These temperatures are 250−350 °C lower than those reported for the traditional ceramic route (850 °C). Moreover, the stabilization at room temperature of a new fluorite Bi−Ti−O structure was obtained by use of the planetarium milling precursor. The microstructure of the powder investigated by SEM was correlated with the XRD results. In particular, monolithic Bi4Ti3O12 grains of a few microns in size were achieved by means of the wet chemistry method.

Phase transitions experienced by Mn x Co 3-x O 4 (1.4 < x < 2.0) materials synthesized at low temperature have been followed by X-ray powder diffraction and thermal analysis. A tetragonal single phase (space group I41/amd) has been isolated for the first time for samples quenched from T > 700 °C, with a distortion parameter ranging from 1.0263(1) for x ) 1.45 to 1.1353(1) for x ) 1.9. The existence at T > 725 °C of a nonisolable cubic spinel single phase (space group Fd3m) in the whole compositional range has been shown by hightemperature X-ray diffraction techniques. It readily transforms on quenching into the new tetragonal single phase. When the cubic high-temperature single phase is allowed to cool slowly, it decomposes into a mixture of tetragonal (x ≈ 2) and cubic (x ≈ 1.3) spinel-type phases. The composition x ) 1.4 is found to be the upper compositional limit of existence of the cubic Fd3m spinel, when it is synthesized by a low-temperature procedure.

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