This web page could help you, http://www.matter.org.uk/diffraction/x-ray/indexing_powder_pattern.htm. In your case you have to use the value of theta (the position of the peak, if the data is 2theta, you have to divide it by two)

If you are looking for free software, you can download them from, http://www.ccp14.ac.uk/solution/indexing/

For this porpuse I use the X'pert HighScore ProPlus from Philips (under license), and FullProf Suite (free)

If you need help, I could help you trying to index the material.

Cheers!

JM

Best step wise producer for cubic, hexagonal and tetragonal systems are given in book:

X-Ray Diffraction: A Practical Approach by C. Surynarayana

Thanks for replies. 2 theta values of the powder sample is here:15.3020, 18.1671, 23.3280; I like to index it and to know the structure of the material.

Could tell me what elements the sample includes?

In order to know the structure of a substance you will need to know the angular position of much more than three peaks as well as their intensities.

In your case, if you can obtain only three peaks, you need to guess something abutthe composition of your material. Then you can compare reference spectra of materials that can fit that composition.