Multi-frequency Electron Paramagnetic Resonance Study Research Articles

Variable-frequency EPR study of Mn 2+-doped NH 4Cl 0.9I 0.1 single crystal at 9.6, 36, and 249.9 GHz: structural phase transition

Multifrequency electron paramagnetic resonance studies on the Mn 2+ impurity ion in a mixed single crystal NH 4Cl 0.9I 0.1 were carried out at 9.62 (X-band) in the range 120–295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn 2+ ions with their respective magnetic Z-axes oriented along the crystallographic [1 0 0], [0 1 0], [0 0 1] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn 2+ spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of λ-transitions in the mixed NH 4Cl 0.9I 0.1 crystal at 242 and 228 K consistent with those observed in the pure NH 4Cl and NH 4I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn 2+ ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn 2+ ion substitutes for a monovalent NH 4 + ion in the NH 4Cl 0.9I 0.1 crystal. This reorganization is almost the same as that observed in NH 4Cl and NH 4I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.

Multi-frequency EPR study of radiation-induced radicals in tooth enamel

Human tooth enamel powders, unheated as well as heated prior to X -irradiation at room temperature, have been investigated by means of Q - and W -band Electron Paramagnetic Resonance (EPR). Upon irradiation of enamel, carbonate-derived radicals are generated. The simplest acquired EPR spectra in this study consist mainly of a group of three different \\hbox{CO}_3^{3-} signals, with a very weak \\hbox{CO}_2^{-} contribution. The characterisation of the paramagnetic species in enamel is quite important for the reliability of EPR applications ( e.g. , EPR retrospective dosimetry). The spectra from the heated samples reveal a striking resemblance with spectra from certain irradiated synthetic apatite powders. The spin Hamiltonian parameters obtained from the computer simulations of the Q - and W -band spectra are compared with those reported in the literature.