For the Ni 0.3 Zn 0.7 O composition the onset temperature of cubic phase decomposition (T d ) is 1100 K (the highest temperature for the experimental setup used), and for rs-Ni 0.5 Zn 0.5 O solid solution no changes were observed in the whole temperature range studied. Thus, rs-Ni 1-x Zn x O solid solutions exhibit high kinetic and phase stability, i.e., up to ~½T melting , which is quite unique for metastable high-pressure phases. Previously, such high phase stability has only been observed for coesite and stishovite, high-pressure phases of SiO 2 , which are kinetically stable up to 973-1073 K at ambient pressure .