NUCLEATION OF ULTRAFINE PARTICLES DURING IRREVERSIBLE SOLID-SOLID PHASE-TRANSFORMATION IN OXIDES

We have studied the nucleation and growth processes that occur during the irreversible structural transformation of metastable oxide phases to their thermodynamically stable polymorphs. The transformations exam ined were: (i) cubic pyrochlore Pb2Ti2O6 (metastable) to tetragonal pe rovskite PbTiO3; (ii) hexagonal BaFe2O4 (metastable) to orthorhombic B aFe2O4; and (iii) orthorhombic stibiotantalite BiNbO4 (Metastable) to triclinic BiNbO4. In each case the irreversible transformation was acc ompanied by a marked reduction in the particle size, which can be corr elated to the change in the specific volume at the transition. Solids which -undergo a relatively large change in specific volume during str uctural transformation can therefore be obtained in fine-particle form by controlled heating of their metastable polymorphs just above the t ransformation temperature.