[Ifeffit] Temp effect

Matt Newville newville at cars.uchicago.edu
Fri Jun 17 15:52:08 CDT 2011

Hi Hana,

On Fri, Jun 17, 2011 at 2:30 PM, Hana <hana63.new at gmail.com> wrote:
> Dear EXAFS experts,
> I am quite a beginner, and that's might be an ignorant question but really
> important to my work, so I would be happy to get a direction:
> what would be the right way to weigh in the effect of temperature in soils
> samples, since solutions like Debye models will not fit to my understanding?
> specifically, I am trying to compare my measurements(collected at 10K) to
> literature data of these metals adsorbed on iron oxides (collected at room
> temperature).
> I would appreciate any advise,
> Hana

The Debye or Einstein model describes how sigma^2 changes with
temperature for a particular bond.  That can be helpful in many cases,
of course.  But for a soil sample or ion sorbed to metal oxide
surfaces, there are a few issues with this approach.

First, metal-oxygen bonds tend to be stronger than metal-metal bonds,
and so don't show a lot of temperature dependence to begin with.

Second, the Debye (Einstein) models consider only the thermal
component of sigma^2, whereas sorbates and metals substituted in metal
oxides are likely to have significant "static" disorder, as they have
a range of bond lengths.  In addition, it would probably be reasonable
to assume that for a metal sorbed on an insulating surface that each
bond would have it's own strength and so temperature dependence of

Third, soil and sorption samples tend to be heterogeneous, and so
average over an unspecified number of local environments, further
contributing to static disorder.

It's a fine idea to measure any sample at low temperature as the
sigma^2 are smaller, and radiation damage reduced.  But I'm not sure
I've seen a work that compares sigma^2 for different temperatures in
soil samples or really even tries to interpret sigma^2 for such
samples.  I wouldn't expect it to tell you much, but I'd be happy to
be shown otherwise.



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