Difference between mechanochemical and thermal processes of polymorphic transformation of ZnS and PbO

Mechanochemical and thermal processes of the polymorphic transformations, litharge ? massicot and wurtzite ? zinc blende were compared. Massicot transformed readily to litharge with a slight mechanical stimulation at room temperature whereas the mechanochemical transformation in the opposite direction was almost impossible. The wurtzite ? zinc blende transformation was similar. On the other hand, simple heating of litharge at temperatures higher than 673K enabled the transformation into massicot without any difficulty. Wurtzite was also formed on heating zinc blende without any mechanical aids. The reason for this kind of apparent irreversibility of the transformation, being topotactic and including layer structures, is discussed.