Assuming I have an experimental powder XRD spectra and I have a number of computational structures without any knowledge of which is correct, how can I simulate the powder XRD spectra? Once I have determined which structure is most likely, can I use the powder XRD to further refine the lattice constants of the structure?
You have two different question in the same post.
how can I simulate the powder XRD spectra?
To just simulate the XRD spectra you can use this answer https://mattermodeling.stackexchange.com/a/146/24.
can I use the powder XRD to further refine the lattice constants of the structure?
The short answer: yes.
The long answer: For the fitting process, or refinement, you don't need a set of starting structures. One of the successful method to obtain the crystal parameters from a powder diffraction pattern is the Rietveld method.
In this method, you only need one starting structure. Then, the software generate a theoretical pattern (as in your first question) and then the method uses a least squares approach to refine/fit the theoretical pattern to the experimental one. Here several parameters are changed. This process is repeated several times until attain convergence. In case the figure of merit (see below) be bad, you can then use another structure from your set as starting one.
The parameters that are fitted are several and can be used further to describe your structure:
- Background parameters: usually 1 to 12 parameters.
- Sample displacement: sample transparency, and zero shift corrections. (move peak position)
- Multiple peak shape parameters.
- FWHM parameters: i.e. Caglioti parameters (see section 3.1.2)
- Asymmetry parameters (FCJ parameters)
- Unit cell dimensions
- one to six parameters (a, b, c, α, β, γ), depending on the crystal family/system, for each present phase.
- Preferred orientation, and sometimes absorption, porosity, and extinction coefficients, which can be independent for each phase.
- Scale factors (for each phase)
- Positional parameters of all independent atoms in the crystal model (generally 0 to 3 per atom).
- Population parameters
- Occupation of site positions by atoms.
- Atomic displacement parameters
- Isotropic and anisotropic (temperature) parameters.
The parameters that measure the quality of the fit (figures of merit) indicating the quality of the fit are:
- Profile Residual (reliability factor)
- Weighted profile residual
- Bragg residual
- Expected profile residua
- Goodness of fit