The Thermal Behavior at Low Temperature

In systems containing a number of physically inequivalent sites, Mossbauer effect spectroscopy (MES) can often allow the determination of the properties of the individual sites. This also proved to be the case here [24], 

Analysis of Au MES measurements as a function of temperature between 1.25 K and 60 K on the AU55 system, employing the transmission integral 

The four sites of Aujj exhibit different line intensities, and from the relative site occupations, the Mossbauer f-factors for the different sites could be calculated [24], using standard techniques [91]. These, in turn, could be related to effective Einstein (or Debye) temperatures 0 (or 0 ) associated with the vibrations of the individual sites. An unexpected consequence was that the three surface sites could not be described by a single meaning that the use of 

A further result of this analysis, as shown in Fig. 4, is that while the relative spectral intensities are determined by the individual site f-factors, the temperature dependence of all the sub-spectra together in the temperature range studied is determined by the motion of the center of mass of the whole Auj, cluster. This can be seen by the uniform decrease of the total intensity with increasing temperature, without any visible change in the general shape of the spectrum. In effect, this means that the f-factors for the individual sites must be multiplied by an f-factor due to the motion of the whole particle [24]. See also Refs. [95,96,97], where this concept was originally developed. The use of such an inter-cluster f-factor, in addition to the usual intra-cluster f-factor, also resolved the problem of the apparent deficiency in the total f-factor at 1.25 K when compared to bulk gold. 

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