Hello, I found large Chi2 (> 300), knowing that I made edit PCR and I put initial parameters from ICSD sheets (JCPDS) and found the position of the peaks is the same except the intensities are diferent. So, I would like to know how and where I can change the parameters to decrease the Chi 2 and the Rp, Rexp, Rwp factors
Upon trying to interpret your email, I guessed that you are perfoming Rietveld refinement (likely with Fullprof) on a powder diffraction pattern recorded from a material with known crystal structure. You have retrieved crystallographic data (unit cell, atomic coordinates) from the ICSD database (which is not identical to JCPDS). Coinciding peak positions indicate that the retrieved unit cell is reasonable but the calculated reflection intensities indicate problems. Check the following:
1. Are you refining scale and are the Lorentz etc factors influencing 2theta dependence the calculated intensity adequate for the settings of the diffraction experiment?
2. You might have preferred orientation or poor crystallite statistics in your specimen of the diffracting material.
3. You have made an error upon importing the atomic coordinates (automated cif import or inserting the atomic sites "by hand"). Apart from accidental errors be sure about proper definition of the space group and its setting (e.g. sequence of axes). This may require some knowledge of space groups including different types of settings and choices of axes. Moreover, different programs have different ways to define the occupancy of the sites. Also the import can lead to funny atomic displacement parameters. A good test is checking the crystal structure under consideration by a structure drawing program.
Hii,
Can you please describe in more detail your algorithm of refinement. Only then I can help you out. Please specify your software also.
Just as @Andreas has opined that you are conducting Rietveld refinement and probably using Fullprof, an easy approach will be to import the closest cif file from the COD. As to how you can achieve reduced chi 2 and other parameters, there is no clear cut way to achieve this. You need to make it be a one step at a time process. Start by refining the Lattice parameters and move on to refining the other parameters one step at a time. As you do this, the values will keep decreasing systematically. You should pay close attention and continue the process until you achieve your goal.
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