Another possible explanation for this order of thermal stability may be due to the relatively low melting temperature of "FeO" (~1640 K ), which is 400-600 degrees lower than the melting temperatures of other 3d transition metal monoxides. In general, the thermal stability threshold of the studied cubic solid solutions is close to the corresponding values for the previously studied related metastable compounds of the ZnO-LiTiO 2 and ZnO-LiFeO 2 systems .