For example the ratio of the relative intensities R (R =I(0 0 2)/I(1 0 0).
It depends on the material. For example, if it is a layered solid, the reflections in the plane have almost constant intensities (assuming that the solid does not suffer alterations in the atoms composing each layer), while the reflections due to the stacking in the c-axis may be very intense if a lot of layers are correctly ordered, or may even disappear if the solid completely delaminates. As intermediate situations, the intensity increases while ordering the stacking of the layers.
In a more general way, the intensity depends of the number of atom layers correctly ordered perpendicular to the plane considered. That is, of the size of the crystal in this direction. Mathematically, this is given by Scherrer equation.
I think you could calculate the volume fraction of each planes present in XRD pattern via this formula e.g. for 002 planes:
∑I(002)/ ∑I(002)+∑I(100)+...
which I is the intensity of (002)picks.
then you have the percentage of the 002 planes in your sample. you can continue this calculation for other picks and then calculate your desired ration.
It is quite difficult to understand what the author of the question wants to know!
@ Miguel Angel: well what you say about layered structures is almost correct. The actual situation depends on the type of stacking and the type of stacking defects: e.g. a structure showing turbostratic disorder and one showing just twins can have a completely different behavior. Moreover the shape of the actual domains can play a huge role, as in several cases a perfectly untextured specimen is hard to obtain. Layered (and disordered) systems are however not the typical type of specimen for everybody!
The stuff about Scherrer equation, on the other hand is not correct. In general in a powder pattern the relative intensity depends on the structure factor and multiplicity. If you don't change the number of atoms, there is no change in intensity by changing the shape of the object. The number of atoms scales all reflections in the same way. This is quite easy to demonstrate. Of course by intensity we always mean the integrated intensity. The maximum intensity has little meaning.
Of course this is valid for an ideal powder. Texture can change things.
A system that illustrate quite well what Matteo Leoni is saying in this answer is the semi-ordered crystal structure in mineral systems such as vermiculite. You can look up the reference Amercian Mineralogist 95 (2010) 126
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