Electron spin relaxation governed by Raman processes both for Cu2+ ions and carbonate radicals in KHCO3 crystals: EPR and electron spin echo studies

• Published in: Journal of magnetic resonance (1997) 2012-08, Vol.221, p.120-128
• Main Authors: Hoffmann, Stanislaw K., Goslar, Janina, Lijewski, Stefan
• Format: Article
• Language: English
• Subjects: Carbonate radical EPR; Cu2+ EPR; Electron spin–lattice relaxation; KHCO3 single crystal; Raman relaxation processes
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2012.06.001

[Display omitted] ► Radiation defects were identified as CO3- and HOCO radical molecules. ► Localization of Cu2+ and the radicals in the crystal unit cell was determined. ► Raman processes govern the spin–lattice relaxation for Cu2+ and the radicals. ► The Debye temperature was calculated from elastic constants as 246K. EPR studies of Cu2+ and two free radicals formed by γ-radiation were performed for KHCO3 single crystal at room temperature. From the rotational EPR results we concluded that Cu2+ is chelated by two carbonate molecules in a square planar configuration with spin-Hamiltonian parameters g||=2.2349 and A||=18.2mT. Free radicals were identified as neutral HOCO with unpaired electron localized on the carbon atom and a radical anion CO3·- with unpaired electron localized on two oxygen atoms. The hyperfine splitting of the EPR lines by an interaction with a single hydrogen atom of HOCO was observed with isotropic coupling constants ao=0.31mT. Two differently oriented radical sites were identified in the crystal unit cell. Electron spin–lattice relaxation measured by electron spin echo methods shows that both Cu2+ and free radicals relax via two-phonon Raman processes with almost the same relaxation rate. The temperature dependence of the relaxation rate 1/T1 is well described with the effective Debye temperature ΘD=175K obtained from a fit to the Debye-type phonon spectrum. We calculated a more realistic Debye temperature value from available elastic constant values of the crystal as ΘD=246K. This ΘD-value and the Debye phonon spectrum approximation give a much worse fit to the experimental results. Possible contributions from a local mode or an optical mode are considered and it is suggested that the real phonon spectrum should be used for the relaxation data interpretation. It is unusual that free radicals in KHCO3 relax similarly to the well localized Cu2+ ions, which suggests a small destruction of the host crystal lattice by the ionizing irradiation allowing well coupling between radical and lattice dynamics.