Effects of Vibronic Dynamics of Cu(H2 O)6 Complexes and Electron Spin Relaxation in Temperature Dependence of EPR Linewidth in Diamagnetic Tutton Salt Single Crystals

S.K. Hoffmann and J. Goslar Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland

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Received: 2 10 2006;

EPR linewidth of Cu2+ in the Tutton salt crystals weakly depends on temperature at low temperatures and then it rapidly grows above 60 K. We present detailed results of measurements and analysis for 63Cu2+ in K2 Zn(SO4 )2 · 6 H2 O, K2 Zn(SO4 )2 · 6D2 O, (NH4 )2 Mg(SO4 )2 · 6H2 O and Cs2 Zn (SO4 )2 · 6H2 O in a temperature range of 4.2-300 K and compare them with already published electron spin-lattice relaxation data. The relaxation contributes weakly to the linewidth which is dominated by molecular dynamics and grows exponentially with temperature. To describe this we are discussing the influence of jumps between two sites of Cu2+ complexes in a slow motion region where the sites are differently thermally populated. This case has not been considered so far. We have derived appropriate expressions describing the contribution of jumps to the linewidth which allows the determination of the jump rate and energy difference δA,B between the two sites being two Jahn-Teller distorted configurations of the vibronic Cu(H2 O)6 complexes. The jump rate 1/τ strongly depends on temperature and reaches 109 s-1 at room temperature, whereas the δA,B varies from 117 cm-1 for K2 Zn(SO4 )2 · 6D2 O to 422 cm-1 for Cs2 Zn(SO4 )2 · 6 H2 O. The comparison with vibronic level splitting, which varies in the range of 67-102 cm-1, indicates that the reorientation mechanism involves phonon induced tunnelling via excited vibronic levels. These reorientations do not contribute, however, to the spin-lattice relaxation which is governed by ordinary two-phonon relaxation processes in the whole temperature range. Thus, the reorientations and spin relaxation are two independent phenomena contributing to the total linewidth.

DOI: 10.12693/APhysPolA.110.807 PACS numbers: 63.90.+t, 76.30.-v