The effect of oxygen deficiency on a phase transition from cubic to tetragonal phase has been investigated for the spinels with by measurements of differential scanning calorimetry (DSC), magnetic susceptibility nuclear magnetic resonance ( NMR) and electronic resistivity . According to the DSC measurements, the transition temperature for a stoichiometric compound is determined to be K. As increases from zero to 0.066, the magnitude of is found to decrease by about 4 K; then, the magnitude of increases by about 41 K with further increase of up to 0.10; thus, the curve exhibits a broad minimum at around . This relationship between and is also observed by the measurements of NMR and . The dependence of on is considered to be a result of a competition between a decrease in the average valence of Mn ions and a dilution of the nearest-neighbour interaction between Mn ions. Both effects are caused by the oxygen deficiency; the former raises , whereas the latter reduces it.

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