Characterization of Cuban and Brazilian natural zeolites by photoacoustic spectroscopy and electron paramagnetic resonance

Abstract

This report describes the photoacoustic and electron paramagnetic resonance investigations of Brazilian and Cuban zeolites. Photoacoustic optical absorption measurements indicate the presence of iron (Fe${}^{3+}$) ions with their respective transition bands for both zeolites. Two species of manganese (Mn${}^{2+}$ and Mn${}^{3+}$) were identified in the Cuban sample and the electronic transitions assigned. Iron and manganese ions were confirmed through nonradiative relaxation ($\tau$) and characteristic diffusion (${\tau }_{\beta }$) times evaluation, whose values were found to be ${\tau }_{\mathrm{\text{BRA}}}$ = 5.40 ms, ${\tau }_{\mathrm{\text{CUB}}}$ = 4.60 ms, ${\tau }_{\beta \mathrm{\text{BRA}}}$ = 387 $\mu$s and ${\tau }_{\beta \mathrm{\text{CUB}}}$ = 305 $\mu$s. Crystal field (Dq-${}_{\mathrm{\text{BRA}}}$/Dq-${}_{\mathrm{\text{CUB}}}$ = 1048 cm${}^{-1}$/945 cm${}^{-1}$) plus Racah (B-${}_{\mathrm{\text{BRA}}}$/B-${}_{\mathrm{\text{CUB}}}$ = 457 cm${}^{-1}$/813 cm${}^{-1}$ and C-${}_{\mathrm{\text{BRA}}}$/C-${}_{\mathrm{\text{CUB}}}$ = 3655 cm${}^{-1}$/2496 cm${}^{-1}$) parameters were assessed as well. Paramagnetic resonance corroborated Fe${}^{3+}$ ions present in the Brazilian zeolite occupying sites showing axial and/or rhombic symmetry distortions. For the Cuban sample, results reveal the characteristic hyperfine sextet lines of Mn${}^{2+}$ overlapping the Fe${}^{3+}$ line. Values of Landé factor and isotropic hyperfine splitting constant were found to be 2.0 and 9.7 mT, respectively. This tells us that the Mn${}^{2+}$ lies in octahedral symmetry probably replacing calcium ions and point towards an ionic bonding character of the Mn${}^{2+}$ and its surroundings.

Key words
Characteristic diffusion time; natural zeolites; nonradiative relaxation time; phase resolved method; Racah parameters