I have run a sample at various temperatures with out removing / rotating the sample or changing the diffractometer, it was locked in the machine the whole time. There are definitely diffrences in peak intensity that are monotonic with temperature and reversible...I'm not sure how to explain it.
Any ideas?
There are different effects related to temperature. The first one depending on your material may be a variation of the unit cell metric due to thermal expansion/shrinkage which may be observable as peak shifts. The second and third one is related to thermal "vibrations" of the "atoms" composing the unit cell in the common approach. The structure factor is modified by either isotropic or anisotropic displacement parameters or sometime called "temperature factors" i.e. Beq/Biso or Ueq/Uiso or in case of anisotropic refinement as components of a symmetric 3x3 matrix B11 B12 ... or U11 U12 and so on. As the therm "temperature factors" indicate those factors depend also but not exclusively on the measurement temperature. Commonly if the diffracting volume of your sample is constant a increase of "intensity" or respective structure factors with decreasing temperature would be expected. Another temperature related effect is the so called thermal diffuse scattering which also decreases with decreasing temperature, but is rarely taken into account in common lab scale measurements.
Regards
Dominique
http://pd.chem.ucl.ac.uk/pdnn/diff2/thermal.htm
Scattering from a crystal is a "structure based event" which means it can change from plane to plane. Heat treatment should cause a re-formation in your crystal lattice and hence you may observe differencies. Your maximum intensity should change, your diffractometer should be missed the most intense peak or already saw it but registed it in lower count. So your material should be transformed from face centered to cubic. It wont be so easy to make estimations in which peak shifting is only temperature based or it is a result of symmetry change.
As Dominique has explained in great detail the temperature effects on crystal structure, there is also another possibility. Without any more details about your experimental setup, regarding the cooling of the sample, one possible effect might be condensation of moisture and other atmospheric fluids on the surface of your sample (if unprotected) during the cooling phase, that might interfere with intensity of your diffraction peaks.
XRD peak intensity is proportional to quantity of a phase( in crystalline form) in a sample. The change in peak intensity may be due to change in quantity of available crystalline phase of the sample as a result of heat treatment/cooling. I think quantity of crystalline part is a function of temperature in your case.
Reading all the answers, to my point of view it will be a reversible phase transformation. Did you check your powder with DSC? There you should have an effect at the same temperature as in the powder diffractogramm!
The material is inorganic? It is a pure oxide, a doped one or a more complex material? With a little more information we could be more helpful. The intensities are also related to the occupancy factors of the sites. And what about the peak positions?
I agree with Dr. Marcella Bini, that more information is required about the specimen. I will also try to add additional information required for analysis.
(1) Are area under each intensity peaks are same? or they differ with temperature?
(2) Experimental profiles at different temperatures .
In my opinion these information helps in understanding the observations.
If the crystal belongs to the cubic symmetry it is impossible. If it has parameters a, b, c they can be changed by the different ways during cooling or heating. I this case the electron density of crystallographic planes will be changed. As a result, you will see different relationship between intensity of the diffraction peaks at different temperatures. If parameters a, b, c are changed by the same way during cooling or heating the relationship between intensity of the diffraction peaks is not changed (as in a cubic crystal symmetry you will see just shift of the peaks).
Trying to answer this hypothetical question is like trying to provide validity to the Biblical story of Genesis with DNA, impossible :-) But then as Napoleon Hill would say, "I've seen the impossible happen so many times that I don't think it exists" (paraphrasing)!
Not having the subject diffractogram is a huge handicap. Besides, not knowing anything about the materials system is not helpful either.
Jason! I'd suggest presenting all the relevant information along with XRD instrumentation details in order to leverage the "extreme" expertise in RG :-) You may also increase the number of topics up top to up to 15 instead of just the 4 you've selected for wider circulation and better feedback.
"There are definitely differences in peak intensity that are monotonic with temperature and reversible".
Do you mean,
Please post your data! Did you normalize the XRD data w.r.t. the total integrated intensity under individual diffractograms in order to effectively compare?
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