Mn–Zn ferrite powders were prepared by hydrothermally aging the coprecipitates of compositional metal ions using ammonium hydroxide as a precipitant. R value (alkalinity) = (moles of added OH−)/[(moles of added Zn2+) × 2 + (moles of added Mn2+) × 2 + (moles of added Fe3+) × 3] was introduced to adjust the amount of added ammonia. The results show that the R value of starting suspension and hydrothermal time have similar and dominant effects on the composition, spinel ratio, and crystallite size of synthesized powders. From the analyses of x-ray diffraction (XRD) and inductively-coupled plasma (ICP), it notes that no α–Fe2O3 peak in the XRD patterns of powders synthesized at R = 2–3, 150 °C × 2 h, may be due to lower degree of crystallinity and less amount of α–Fe2O3 existing in these powders. Both the increase of hydrothermal time and of R value can promote the crystallinity of powders and also cause a significant loss of zinc, hinting that in the hydrothermal process, the loss of zinc may play a crucial role in the crystallinity of hydrothermally synthesized powders.

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