ESR study of molecular jumps of manganese(II) impurity in single crystals of Cs2HfCl6 and Cs2ZrCl6

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Abstract
Electron spin resonance spectra at 35 and 9.3 GHz of Mn2+ impurity ions in single crystals of Cs2HfCl6 and Cs2ZrCl6, from 12 to about 400 K are reported. Below ∼60 K the spectrum consists of a superposition of a number of anisotropic subspectra due to crystallographically equivalent Mn2+ species. Each subspectrum consists of five fine structure bands, due to the different (m−1↔m) transitions, split into six hyperfine components. The spin Hamiltonian of the Mn2+ species in this temperature range is characterized by isotropic g and hyperfine tensors (g=2.004, A=77.0 G) and an almost axially symmetric zero field splitting (ZFS) tensor (D=470 G and E=3 G). It is suggested that the Mn2+ impurity that gives rise to these subspectra consists of MnCl3−5 ions with nearly tetragonal symmetry. The various subspectra correspond to MnCl3−5 ions whose principal ZFS tensor components point along different crystallographic axes. As the temperature is raised above 60 K conspicuous changes occur in the spectrum over an extremely wide range of temperatures: The m≠1/2 fine structure components first broaden and eventually disappear in the noise, while the m=1/2 transitions, due to the different sites, coalesce to give a singly, slightly anisotropic, hyperfine sextet. At high temperatures the full intensity of the spectrum emerges again to give a sharp isotropic sextet. The temperature dependence of the spectrum over the whole temperature range studied can be quantitatively interpreted in terms of a dynamic model in which the major ZFS tensor axis undergoes random jumps between the three crystallographic cubic directions. A general theory for the ESR line shape in the presence of such a process is derived for the case that the ZFS interactions is small compared to the Zeeman energy. Simple and explicit equations are derived for the linewidth and frequency shift in the slow jump limit for the m≠1/2 as well as the m=1/2 fine structure bands. For the fast jump limit the appropriate relaxation matrix is also derived. This theory is then used for a quantitative analysis of the experimental spectra in the CsHfCl6 and CsZrCl6 crystals, and values for the jump rate and the magnetic parameters as function of temperature are derived. It is found that as the temperature is raised from 60 to about 400 K the g value changes from 2.004 to 2.006 and D decreases monotonically from 470 to 330 G. In this temperature range the jump rate changes by about four orders of magnitude and it can be characterized by an Arrhenius plot with the following kinetic parameters: (1.τ)(300 K) =3.5×1011 s−1, ΔE=1.4 kcal/mol. Possible mechanisms for the jump process are discussed.