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Hyperfine Interactions

, Volume 198, Issue 1–3, pp 273–293 | Cite as

Microscopic 57 Fe electric-field-gradient and anisotropic mean-squared-displacement tensors: ferrous chloride tetrahydrate

  • James N. Bull
  • Christopher M. Fitchett
  • W. Craighead TennantEmail author
Article

Abstract

This paper reports the determination of the electric-field-gradient and mean-squared-displacement tensors in 57Fe symmetry-related sites of \(\bar {1}\) Laue class in monoclinic FeCl2.4H2O at room temperature by single-crystal Mössbauer spectroscopy. Contrary to all previous work, the mean-squared-displacement matrix (tensor), <msd>, is not constrained to be isotropic resulting in the determination of physically meaningful estimates of microscopic (local) electric-field gradient (efg) and <msd> tensors. As a consequence of anisotropy in the <msd> tensor the absorber recoilless fractions are also anisotropic. As expected of a low-symmetry site, Laue class \(\bar{1}\) in this case, no two principal axes of the efg and <msd> tensors are coaxial, within the combined errors in the two. Further, no principal direction of the efg tensor seems related to bond directions in the unit cell. Within error, and in agreement with an earlier study of sodium nitroprusside, it appears that the <msd> tensor principal directions lie close to the crystallographic axes suggesting that they are determined by long wavelength (phonon) vibrations in the crystal rather than by approximate local symmetry about the 57Fe nucleus. Concurrent with the Mössbauer measurements, we determined as part of a new X-ray structural determination, precise atomic displacement parameters (ADPs) leading to an alternative determination of the <msd> matrix (tensor). The average of the eigenvalues of the Mössbauer-determined <msd> exceeds that of the average of the X-ray-determined eigenvalues by a factor of around 2.2. Assuming isotropic absorber recoilless fractions leads to substantially the same (macroscopic) efg tensor as had been determined in earlier work. Taking \(\frac{1}{3}\times\) the trace of the anisotropic absorber recoilless fractions leads to an isotropic value of 0.304 in good agreement with earlier single crystal studies where isotropy was assumed.

Keywords

Mössbauer spectroscopy Single crystals Symmetry-related sites Microscopic and macroscopic efg tensors Atomic displacement parameters Lamb-Mössbauer factors 

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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  • James N. Bull
    • 1
  • Christopher M. Fitchett
    • 1
  • W. Craighead Tennant
    • 1
    Email author
  1. 1.Department of ChemistryUniversity of CanterburyChristchurchNew Zealand

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