# Determine Crystal structure of non-cubic crystal in XRD plot

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1. Mar 1, 2015

### ralden

Given only 2theta and intensity, how to know the crystal structure (except cubic) of non cubic crystals, and also determine their lattice parametera,b,and c, if there's a link or website discussing techniques to determine crystal structure of all crystals, please don't hesitate to put it here. thanks.

2. Mar 2, 2015

3. Mar 3, 2015

### ralden

actually this is an exercise to make a program that can determine the crystal structure of non-cubic, i'm having a problem in finding a pattern or techniques to determine non-crystal structure unlike cubic.

4. Mar 3, 2015

### M Quack

One technique seems to be to calculate Q^2 for all observed reflections, then assume the lowest reflections are (100), (110), (111), (200), ... and see what matches.

If your lattice angles are all 90 deg, then Q^2 = H^2 a*^2 + K^2 b*^2 + L^2 c*^2. For hexagonal this also seems to work quite well. For monoclinic and triclinic this is more of a mess.

I guess in a computer programme one could one by one go through the 14 Bravais lattices, add a few assumptions to each one (like a>c or a<c for tetragonal, as this could invert the order of peaks), and calculate the lattice parameters from the first few reflections.

Note that the space group may lead to systematic absence of certain peaks, so a better critirium for fit/no fit is if there are peaks in the data that have no corresponding HKL.