Perovskite phases in the A–R–Fe–O systems, where A – alkaline-earth; R – rare-earth metal

Abstract

AB O 3 phases belonging to the well-known perovskite family constitute an important class of functional materials. Many of them are already used as catalysts, electrodes, sensors, etc. However, a systematic study of perovskite phases remains topical, taking into account the huge tolerance of the CaTiO 3 structure type and its derivatives, as well as the wide range of their physical properties. Ceramic samples of nominal composition A 0.5 R 0.5 FeO 3 , where A – alkaline-earth and R – rare-earth metal, were prepared by solid-state reaction using high-purity CaCO 3 , SrCO 3 or BaCO 3 carbonates, rare-earth metal oxides and Fe 2 O 3 . Firstly, the reagents were mixed manually and heated in corundum crucibles for decomposition of the carbonates. After cooling to room temperature, the final mixtures were ground to achieve homogeneity, pressed into pellets and sintered again. Each stage was carried out at 1 000 °С in air for 24 h. X-ray phase and structure analyses were carried out on powder diffraction data (DRON 2.0M diffractometer, Fe K α radiation). Sample surfaces were studied using a Tescan Vega 3 LMU scanning electron microscope. As a result of the X-ray phase and structure analyses the presence of four-component phases R 1- x Ca x FeO 3 ( x ~ 0.15) with GdFeO 3 -type perovskite structures (Pearson symbol oP 20, space group Pnma ) was discovered in the Ca– R –Fe–O systems. We also showed that Sr 1- x R x FeO 3 (structure type CaTiO 3 , Pearson symbol cP 5, space group Pm -3 m ) and R 1- x Sr x FeO 3 phases (structure type GdFeO 3 , Pearson symbol oP 20, space group Pnma ) are formed on the SrFeO 3 – R FeO 3 cross-sections of the Sr– R –Fe–O systems. The unit-cell volume of the R 1- x Sr x FeO 3 phases with structure type GdFeO 3 decreases monotonically with increasing atomic number of the rare-earth metal. Scanning electron microscopy of the Sr 0.5 Sm 0.5 FeO 3 sample revealed that the size of the crystallites was close to 200 nm. Formation of four-component perovskites in the Ba– R –Fe–O systems was not observed at the conditions of our investigation. Keywords: alkali-earth metal, rare-earth metal, phase analysis, crystal structure, perovskites.