TOPAS is the best program.

GSAS is better than xpert. But Fullprof is also best refinement program

topas is based on the refinement of fundamental parameters to describe the X-ray optics. This is fine, but topas has to be paid even by academy.

The free programs fullprof and gsas are the most popular choice in the academic community, and they are highly developed to resolve almost all problems one may be faced with in powder diffraction for structure and microstructure refinement using Xray and/or neutron diffraction, and new features are continuously implemented. Some of the optics is described by empirical parameters, but this is sometimes preferrable because their dependence on fundamental parameters may need to decide for a model.

A less known free program using fundamental parameters is maud, also very mature and continuously being developed.

X'pert or, more correctly, the software HighScorePlus which comes with Panalytical's diffractometer, may not contain all the latest features being developed in academy, but other aspects of course may be important for your choice.

When comparing the different programs, it might be a good idea to use a standard sample which you can prepare by yourself or take it from nature without much cost. Grain size, amorphous content, absorption and preferred orientation are the main issues with QPA, there are excellent round robin studies about this, described e.g. in

Madsen et al; J. Appl. Cryst. (2001). 34, 409-426

Scarlett et al. J. Appl. Cryst. (2002), 35, 383–400.

or you may look in

http://www.iucr.org/__data/iucr/powder/QARR/intro.htm

where you can even find standard patterns and all you have to do is checking the different programs with such data.

QPA by Rietveld is very powerful but Bragg-Brentano is not the best technique as it most strongly depends on the surface of the sample. The Debye-Scherrer technique and more penetrating radiation incl neutrons should be considered.

"Wich program between GSAS and Xpert is more reliable to get the phase quantification results?"

I'm using both, the differences is because the sequence and the refinement paramaters is different.

Make sure u know every parameter that u refined as well sequences

@ Miguel: Maud and FP approach? I was not aware of that. Is this a recent feature?

@Dominique: Both FP and Maud have been doing QPA for many years. My interest in Maud is microstructure (texture, grain size etc) and capability to work with 2D intensity data (films, imaging plate).

"@ Miguel: Maud and FP approach? I was not aware of that. Is this a recent feature?" NO, up to the v2.7.

Sorry I meant that I was not aware of a fundamental parameter implementation in maud

maud still use srm to determine the instrumental broadening not fpa like topas or bgmn.

Now GSASII substituted GSAS

FullProf Programs is the most popular and below link is about structural refinement using FullProf. You can try refining your data using FullProf.

https://www.youtube.com/watch?v=mWNpIIwEcDk&t=401s

The differences in your output CIF may be due to the different sequences and also the number of refinement parameters your are perform may be completely different. May be in one case you will be relaxing more parameters then in other case.

HighScore Plus version 4.9 has included a number of features which makes it a very preferable choice amongst industry and academicians:

You can request a demo version which works for 2 weeks from website: https://www.malvernpanalytical.com/en/products/category/software/x-ray-diffraction-software/highscore-with-plus-option

Some useful webinars:

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W20200529XRD

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W20200611XRD

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W20200702XRD?

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W20200728XRD.html

TOPAS from BRUKER is a excellent program!