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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