Mechanism of structure formation in samarium and holmium titanates prepared from mechanically activated oxides

Abstract

We have studied the formation mechanism and phase transitions of samarium and holmium titanates prepared from mechanically activated oxide mixtures with the overall compositions Sm2(Ho2)Ti2O7 and Sm2TiO5. Mechanical activation of oxide mixtures leads to the formation of amorphous solid phases which crystallize in a distorted pyrochlore-like structure and contain OH groups on the oxygen site and structural vacancies up to 1000°C. In the range 800–1000°C, Sm2−x Ti1−y O5−δ (OH) n (x < 0.02; y < 0.08; δ, n < 0.19) converts to a distorted orthorhombic phase as a result of the relaxation of internal stress and removal of OH groups. Above 1000°C, the phases studied have the compositions Sm2(Ho2)Ti2O7 and Sm2TiO5 and ordered pyrochlore-like and orthorhombic structures, respectively. The lattice parameters of the titanates have been measured in the range 800–1350°C. The internal stress produced by mechanical activation in the phases studied here fully relaxes by ∼1300°C.