You cannot get an accurate cell parameter just by looking at one peak. In 2014 there are computers and the Rietveld method that can help doing what you need in a fast and accurate way. Otherwise search match and if you don't see any peak from the axes of the cell you just need to solve a system with the general formula for the d spacing, the observed hkl and the observed peak positions (that I imagine you will extract somehow)

Dear Volker, I have gone through that question already but that question is regarding single phase material (ZnO hexagonal). What if we have the two different phases one of which is cubic and other is hexagonal. What will be the lattice parameter of the specimen in this case.

First you need to identify which peaks correspond to cubic and which to hexagonal phases and then it should not be a problem, as mentioned above, to calculate the cell parameters. If in doubt then read the book of B.D. Cullity, elements of X-ray Diffraction, or any other standard book on X-ray diffraction .

You cannot get an accurate cell parameter just by looking at one peak. In 2014 there are computers and the Rietveld method that can help doing what you need in a fast and accurate way. Otherwise search match and if you don't see any peak from the axes of the cell you just need to solve a system with the general formula for the d spacing, the observed hkl and the observed peak positions (that I imagine you will extract somehow)

you can get the lattice parameter by comparing XRD peak with corresponding JCPDS data to get its hkl value from which u can arrive lattice parameter values

I agree with Matteo, you should do Rietveld refinement.

If you need software you could go for Fullprof ( https://www.ill.eu/sites/fullprof/php/programs8809.html?pagina=FullProf ).

First I think you should know which peaks is cubic and which peaks is hexagonal structure, becuae usually speaking, they are has different position even in one patterm. For the coincident peak, you can ignore it, because every peak can get the lattice parameter. Secondly, you can use the XRD software (GSAS, EXPGUI) fit whole XRD patterm using two structure (cubic and hexagonal), it can decompose the coincident peak into two peak or multiple peaks, and it can also give you the lattice parameter for two structure. I hope the advice can help you.

To measure the lattice parameter, you must choose one peak and identify the plane of the peak. Then using the lattice parameter equation with the plane data and the 2theta of the peak, you can measure the lattice parameter.

I agree with Dr Zuo. First of all, you try to match your data with standard ICDD cards, and while selecting a card, you should try to match the three highest intensity peaks with your data. After matching, you can calculate lattice parameters using simple relation of d-spacing with lattice parameters.

learn x-ray crystallography. Along the way if something doesn't make sense ask about it here. It is impossible to answer your question on a discussion board as entire textbooks are written on the subject. As Fallak mentioned Cullity is probably the best introduction to the field, and will provide everything you need to know to index your 2 phases and fit the lattice parameters.