ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
While the CrO3−4 and SeO3−4 radicals have been used as spin probes in numerous electron spin resonance (ESR) studies of phase transitions in the KH2PO4-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 well-defined activation energy, ΔE. Our new ESR data on these probes show that their linewidth, Γ, varies as Γ=Γ0+A exp(ΔE/kT)+B exp(−ΔE/kT). Here Γ0 is the temperature-independent, residual linewidth and A and B are parameters that depend on the ESR line splittings and the Zeeman field. ΔE has been estimated to be (approximate)0.22±0.02 eV (20±2 kJ/mol) for CrO3−4 in KD2AsO4, KD2PO4, NH4H2AsO4, ND4D2AsO4, and ND4D2PO4, and SeO3−4 in KD2PO4. The results suggest that the omission of the term B exp(−ΔE/kT) in all of the earlier analyses of the ESR linewidth data from these probes was the major cause of the reported non-Arrhenius behavior, especially in the high temperature (T(very-much-greater-than)Tc) regime. Measurements utilizing externally applied electric fields indicate that this new, Orbach-type, ESR line broadening process originates from thermally activated spin–flips via vibronic transitions, and ΔE provides a measure of the barrier height for the intrabond hydrogen transfer along the O–H⋅ ⋅ ⋅O bonds.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.456480
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