Dear Weizhao,

What do you mean by an "unknown XRD pattern"?

Is it a pattern of a product of your synthesis? Then, firstly, I would check if it contains any known phase(s) - expected product and precursors, etc. You can do this using the available databases (CSD, ICSD), downloading the respective structurs from them, and simulating the powder patterns of these phases (for example using Mercury: https://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx).

If there is no known phases, or there are some unknown peaks, you can try to solve and refine a new structure (if the pattern is of sufficient quality). First you need to index the peaks, using for example FOX (http://fox.vincefn.net/).

Then, you can try to solve a structure. If it is a simple cell, it is quite often possible to find an crystal composed of sibling elements. Than it would be a starting model for your refinement. Otherwise, you can do the calculations in real space using FOX.

If you find a reasonable model for refinement, you can do it using for example JANA2006 (http://jana.fzu.cz/), or GSAS-II (https://subversion.xor.aps.anl.gov/pyGSAS/trunk/help/gsasII.html).

Of course, there is plenty of other free software.

This is only a rough picture - the exact way of solving and refinement of crystal structures from powder data depends on specific problem.

Good luck!

Dziekuje bardzo, Tomasz. The problem is I have few peaks at high pressure. It is hard for me to perform the refinement, even for the Le Bail refinement. I am trying use FOX as you suggested, hope it works.Many thanks!

Have you tried to search in a crystallographic database if your substance could be stable in another symmetry, such as cubic or tetragonal? The low number of peaks suggests an high symmetry. The determination of lattice parameters and space group could be a difficult process, particularly in this case in which the cell could be distorted due to the pressure effect. You can try with an indexing programme, such as Treor or Crysfire that contain many indexing programme. However, I would try first to search other possibilities for my substance or check also the possible decomposition of the sample and the formation of impurity phases.

Dear Weizhao,

Interpretation of high pressure powder patterns is often a difficult task. As Marcella wrote - you shoul try the databases first.

What is your sample?

Thanks for the reply. I do not think databases work. Perhaps I will try to index the peaks, try to find some possible space groups.

Dear friend,

Is your material is stable at room temperature?

If your material is stable under room temperature, don't think about the pressure conditions. Here i am also working on crystals and if it is a new crystal then we will go for cell parameter study by Bruker Kappa Apex diffraction machine. After that we will check with the already reported data. If the compound it was not reported then we will go for the data collection. Identifying the space group for the new compound is only possible by solving the data. After solving the data we can easily get the power XRD pattern using so many free software available at internet like mercury software.

Step (I) check whether it is stable at room temperature or not?

Step (ii) Go for data collection for identifying the space group.

i think you can use the Reflex model in the Material Studio software to index the diffraction peak, and then perform pawley or Le Bail refinement to acquire the space group and lattice parameters.

Thanks for the response.  How do you abstain the new structure model after you identify the new phase? I mean the model like CIF, which contains lattice parameters and atomic coordinates etc.  Thanks!

It depens on the quality of your data. You can try direct space methods for structure solution, like those implemented in FOX software:

http://fox.vincefn.net/