EPR Study of Fe³⁺- and Ni²⁺-Doped Macroporous CaSiO₃ Ceramics

Abstract.

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 β-CaSiO₃ and α-CaSiO₃ were observed at 950 and 1200 °C, 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 µm. 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.