EPR Study of $Fe^{3+}$- and $Ni^{2+}$-Doped Macroporous $CaSiO_3$ Ceramics

Abstract

Thermally stable macroporous $CaSiO_3$, $Fe^{3+}-$ and $Ni^{2+}-$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-$CaSiO_3$ and \alpha-$CaSiO_3$ were observed at 950 and 1200 °C, respectively. The phase transition temperatures of combustion-derived $CaSiO_3$ 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 $Fe^{3+}$ ions in $CaSiO_3$ 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 $Fe^{3+}$ ions at rhombic site. The signal at g = 2.0 is due to $Fe^{3+}$ coupled together with dipolar interaction. In $Ni^{2+}$-doped $CaSiO_3$ 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 $Ni^{2+}$ 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 $Fe^{3+}$ as well as $Ni^{2+}$ content. The effect of alkali ions (Li, Na and K) on the EPR spectra of these ceramics has also been studied