EPR study of Fe3+- and Ni2+-doped macroporous CaSiO3 ceramics

Thermally stable macroporous CaSiO3, Fe3+- and Ni2+-doped (0.5 to 5 mol%) ceramics have been prepared by solution combustion process by mixing respective metal nitrates (oxidizers), fumed silica. Diformol hydrazine is used as a fuel. The combustion products were identified by their X-ray diffraction and thermal gravimetry/differential thermal analysis. Single phases of beta-CaSiO3 and alpha-CaSiO3 were observed at 950 and 1200C, respectively. The phase transition temperatures of combustion-derived CaSiO3 were found to be lower compared to those obtained via solid-state reaction method. It is interesting to note that with an increase in the calcination temperature the samples become more porous with an increase in the pore diameter from 0.2 to 8 mu m. The electron paramagnetic resonance (EPR) spectrum of Fe3+ ions in CaSiO3 exhibits a weak signal at g = 4.20 +/- 0.1 followed by an intense signal at g = 2.0 +/- 0.1. The signal at g = 4.20 is ascribed to isolated Fe3+ ions at rhombic site. The signal at g = 2.0 is due to Fe3+ coupled together with dipolar interaction. In Ni2+-doped CaSiO3 ceramics the EPR spectrum exhibits a symmetric absorption at g = 2.23 +/- 0.1. This deviation from the free electron g-value is ascribed to octahedrally coordinated Ni2+ ions with moderately high spin-orbit coupling. The number of spins participating in resonance and the paramagnetic susceptibilities have been evaluated from EPR data as a function of Fe3+ as well as Ni2+ content. The effect of alkali ions (Li, Na and K) on the EPR spectra of these ceramics has also been studied.