Synthesis and characterisation of metal oxides and fluorinated perovskite-related oxides

• Main Author: Ren, Xiaolin
• Published: Open University 2005
• Subjects: 541.39
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412395

Perovskite-related materials of composition LaFe_1-xCo_xO₃ prepared by conventional calcination methods and mechanical milling are shown by temperature programmed reduction to be more susceptible to reduction in a flowing mixture of hydrogen and nitrogen by the incorporation of cobalt. X-ray powder diffraction and Mossbauer spectroscopy show that in iron-rich systems the limited reduction of iron and cobalt leads to the segregation of discrete metallic phases without destruction of the perovskite structure. In cobalt-rich systems, the reduction of Co³⁺ to Co^o precedes complete reduction of Fe³⁺ and the segregation of alloy and metal phases is accompanied by destruction of the perovskite structure. Phases made by milling techniques were of smaller particle size and are more susceptible to hydrogen reduction than their counterparts made by conventional techniques. Materials of the type La_0.5Sr_0.5MO₃ (M= Fe, Co) made by calcination methods are more susceptible to reduction when the transition metal M is cobalt as compared to iron. Perovskite-related oxides of composition La_1-xSr_xFe_1-yCo_yO₃ have been fluorinated by reaction with poly(vinylidene fluoride). The materials have been characterised by X-ray powder diffraction and Mossbauer spectroscopy. Fluorination induces a reduction in the oxidation state of iron from Fe⁴⁺ to Fe³⁺. The fluorinated materials were magnetically ordered at 298 K. Compounds of the type SrFe_1-xSn_xO₃ were found to contain Fe⁵⁺ and Fe³⁺. Fluorination resulted in reduction of the transition metal to Fe³⁺ and, in iron-rich systems, magnetic order. The compound Ba₂SnO₄ which adopts the K₂NiF₄-type structure has also been fluorinated by reaction with zinc fluoride. X-ray powder diffraction shows an enlargement of the unit cell of the fluorinated phase along the c-axis. Small particle iron- and vanadium- antimonate have been prepared by mechanical milling methods. The phases have been examined by M6ssbauer spectroscopy and can be formulated M³⁺Sb⁵⁺O₄ (M = Fe, V). Thermal analysis suggests that the vanadium animonate formed by milling V₂O₅ and Sb₂O₃ in an inert atmosphere may be oxygen deficient. X-ray powder diffraction shows that milling also induces the phase transformation of the cubic senarmontite Sb₂O₃ form to the orthorhombic valentinite Sb₂O₃ form and of a-Sb₂O₄ to B-Sb₂O₄.