Polarization fluctuations as a source of electron spin resonance linewidths and relaxation of spin probes in KH2PO4hyphen;type of polar solids

摘要

While the CrO3minus;4and SeO3minus;4radicals have been used as spin probes in numerous electron spin resonance (ESR) studies of phase transitions in the KH2PO4hyphen;type of solids, discrepancies exist regarding the nature of molecular reorientations sensed by these probes, in particular, on whether the detected motion is thermally activated, with a wellhyphen;defined activation energy, Dgr;E. Our new ESR data on these probes show that their linewidth, Ggr;, varies as Ggr;=Ggr;0+Athinsp;exp(Dgr;E/kT)+Bthinsp;exp(minus;Dgr;E/kT). Here Ggr;0is the temperaturehyphen;independent, residual linewidth andAandBare parameters that depend on the ESR line splittings and the Zeeman field. Dgr;Ehas been estimated to be ap;0.22plusmn;0.02 eV (20plusmn;2 kJ/mol) for CrO3minus;4in KD2AsO4, KD2PO4, NH4H2AsO4, ND4D2AsO4, and ND4D2PO4, and SeO3minus;4in KD2PO4. The results suggest that the omission of the termBthinsp;exp(minus;Dgr;E/kT) in all of the earlier analyses of the ESR linewidth data from these probes was the major cause of the reported nonhyphen;Arrhenius behavior, especially in the high temperature (TGt;Tc) regime. Measurements utilizing externally applied electric fields indicate that this new, Orbachhyphen;type, ESR line broadening process originates from thermally activated spinndash;flips via vibronic transitions, and Dgr;Eprovides a measure of the barrier height for the intrabond hydrogen transfer along the Ondash;Hsdot;thinsp;sdot;thinsp;sdot;O bonds.