Electron spin echo of Cu2+ in the triglycine sulfate crystal family (TGS, TGSe, TGFB): electron spin–latticerelaxation, Debye temperature and spin–phonon coupling

Abstract

The electron spin–lattice relaxation ofCu2+ has been studied by the electron spin echo technique in the temperaturerange 4.2–115 K in triglycine sulfate (TGS) family crystals. Assuming thatthe relaxation is due to Raman relaxation processes the Debye temperatureΘD was determined as 190 K for TGS, 168 K for triglycine selenate (TGSe)and 179 K for triglycine fluoroberyllate (TGFB). We also calculated theΘD values from the sound velocities derived from available elastic constants. The elastic Debyetemperatures were found as 348 K for TGS, 288 K for TGSe and 372 K for TGFB.The results shown good agreement with specific heat data for TGS. The elasticΘD are considerably larger than those determined from the Raman spin–lattice relaxation. Thepossible reasons for this discrepancy are discussed. We propose to use a modifiedexpression describing two-phonon Raman relaxation with a single variable only(ΘD) after elimination of the sound velocity. Moreover, we show that the relaxation data can befitted using the elastic Debye temperature value as a constant with an additional relaxationprocess contributing at low temperatures. This mechanism can be related to a local mode of theCu2+ defect in the host lattice. Electron paramagnetic resonanceg-factors and hyperfine splitting were analysed in terms of the molecularorbital theory and the d-orbital energies and covalency factors of theCu(gly)2 complexes were found. Using the structural data and calculated orbital energies thespin–phonon coupling matrix element of the second-order Raman process was calculated as553 cm−1 forTGS, 742 cm−1 forTGSe and 569 cm−1 for TGFB.