Substitution of a part of the iron cations by diamagnetic zinc cations leads to a significant decrease of T N (Fig. , Table ), similar to the previously reported case of equilibrium cubic solid solutions of the FeO-MgO system . At a small degree of substitution of Fe 2+ cation for Mg 2+ cation (x ≤ 0.4), the decrease of T N is almost linear, and such a state is usually called homogeneous or classical antiferromagnetism . At higher degrees of substitution (x > 0.4), the decrease of T N slows down and becomes nonlinear , as in our case. Such a state is commonly referred to as frustrated antiferromagnetism . Thus, metastable rs-Fe 1-x Zn x O (0 < x ≤ 0.5) solid solutions synthesized at high pressure and high temperature generally exhibit antiferromagnetic properties similar to other related binary oxide systems such as FeO-MgO.