Dear Xiaoliang Zhang ,

in

https://en.wikipedia.org/wiki/Pyrite

it is stated that FeS2 starts to decompose to FeS at temperatures above 540°C. So S goes off and the relative Fe amount increases.

You know that Cu K-alpha based XRD pattern suffer from background due to Fe-K fluorescence. Increasing Fe amount will thus increase Fe-fluorescence background.

The increase of the background at higher 2theta values might be due to a temperature gradient*) in your sample during annealing, which will cause preferred decomposition of FeS2 at deeper regions in your sample (XRD information depth increases with 2theta).

It is just food for thoughts.

But you may share how do you anneal/heat up your sample.

In addition you may share your pattern as graph. I am not able to read your .raw file.

By the way; does another phase show up at your 600°C sample pattern via small peaks in addition to the pyrite peaks? (e.g. that of FeS).

Are there any estimations made on how much the sample shifts in height during heating up due to thermal expansion of the sample support?

*) e.g. by heating the back of the sample/sample support

Additional remark:

if the thermal conductivity of your sample is quite high, a significant temperature gradient near the surface will not be established.

However it depends on the experimental conditions on how your sample may be (partly) decompose.

You state, that your are doing the experiment in situ; I think this means that you heat up your sample during the XRD measurement.

If yes, then in the course of time your sample may sightly decompose (at above 540°C), but at that same course of time you perform the 2theta scan. So in consequence at the beginning (low 2theta values) only almost no decomposition has taken place; but up to the end of the scan (which might take a lot of minutes) a significant part of you sample may have been decomposed. So you propably do not have got steady sample conditions and your fluorescence background goes up in time, as the 2theta scan goes forward.

It is just an idea....

Gerhard Martens Yes, you are right! The diffraction peak of pyrrhotite (Fe1-xS) can be found in the XRD pattern at temperature above 550°C. Interestingly, the baseline begins to shift from down to top as angle increases at the same temperatuer.

The heating rate is 10°C/min at the temperature range of 0~500°C and 700~900°C, while it is reduced to 5°C/min for the stage of 500~700°C. The soaking time is 2 hours for each temperature point.

Dear Xiaoliang Zhang ,

thanks for sharing more details.

So just to avoid the fluorescence background you have to use a Co based x-ray tube. Co K-alpha is not able excite Fe K fluorescence....

In addition, the heated sample exactly undergo thermal expansion and Test surface slightly raised. Gerhard Martens

Gerhard Martens Although Cu K(alpha) radiation was used for the in-situ high temperature XRD measurement, baseline was still straight at the temperature below 550°C. Why it began to shift from down to top as angle increased at relatively higher temperatures.