Defects and Impurities Influences on Phase Transformations in Rare-Earth Oxides

Abstract

Several types of defects are observed on thin films (up to 2500 A) of pure rare-earth oxides of type A-Ln₂ O₃ and B-Ln₂O₃. They are (a) structural steps connected with the layered character of the structures which contain the (LnO)_n ⁿ⁺ entities parallel to the thin-film surface, (b) reciprocal twins in B-Ln₂ O₃ of type I [twin plane (313)] and of type II [twin axis (132)] whose combination yields microtwinned zones; (c) planar defects; (d) dislocations; and possibly (e) antiphase boundaries. The twinning in B-Ln₂ O₃ is directly seen, under the electron microscope, securing the syntaxy conditions between the two phases when the A → B-Ln₂ O₃ or B → A-Ln₂ O₃ transition takes place. A small amount of yttrium in A-Nd₂ O₃ induces the formation of domains of B-Nd₂ O₃. The transformation is, from X-ray investigation, complete at 10 atomic percent yttrium, and consequently the B-Nd₂ O₃ phase is stabilized. An electron microscopy investigation of thin films of mixed oxides containing up to 10 atomic percent yttrium suggests the possibility that microdomains of C-Ln₂ O₃ are operative in the transition from A to B-Nd₂ O₃, which can be produced in the pure phase by a mechanical stress. A well-defined epitaxy between C and B-Ln₂ O₃ was determined in the mixed oxides thin films.