SYNTHESIS AND RIETVELD WHOLE-PATTERN ANALYSIS OF Sr-DOPED PEROVSKITE BISMUTH FERRITE

Abstract

Doping of perovskite bismuth ferrite (BiFeO 3 ) affect greatly its microstructure characteristics. X-ray diffraction analysis using Reitveld method of the whole-pattern fitting is a very useful approach to study the structure variation in relation to doping level. Phase identification, crystallite size, residual microstrain, lattice parameters and bond lengths have been investigated. The conventional solid state reaction method at 650 and 800 °C for 1 h with intermediate grinding was used for the preparation of a series of Bi 1-x Sr x FeO 3 (x = 0.1 -0.5). FULLPROF program was used and modified THOMPSON Cox-HASTING PESUDO-VOIGT function was applied and the background was modeled with fourth order. The sample of x = 0.1 consists of two polymorphic phases perovskite cubic and rhombohedral, the cubic one as the major phase and the rhombohedral one as the minor one. For samples of x ═ 0.2 -0.45, only single phase of cubic perovskite structure was identified while for x 0.5 the sample consists of cubic perovskite and traces of unidentified phase. A continuous decrease in the lattice parameter as well as in the octahedral and dodechaderal bond lengths was observed with increasing Sr 2+ content and this may be attributed to the increasing number of oxygen vacancies with increasing doping level. The micostrain was found to increase with increasing the Sr 2+ content, which was attributed also to the formation of point defects (oxygen vacancies) as well as to ionic size mismatch between host and substituent cations that is led to local structural disorder. The crystallite size increases with increasing the doping level which seems to be due to the increase in the surface free energy resulting in the enhancement of the size growth by the presence of oxygen vacancies