Muhammad - the answer depends on the nature of the material you're looking at. Assuming that you have a powder with randomly oriented crystals, the integrated peak intensity tell you about the atomic positions in the crystal lattice. If you are growing nanocrystals of some solid solution in which order/disorder relations are possible, that could be the reason for the intensity difference. However, if the nanocrystals have a strongly anisotropic morphology, the difference could be due to preferred orientation. Crystals prepared at one set of conditions may have a somewhat difference morphology than those prepared at another set of conditions, leading to certain crystal planes being oriented parallel to the X-ray beam.

With a perfect source and a perfect crystalline lattice of only component, the diffraction pattern would be delta function lines. The first broadening occurs because the source is not monochromatic. The second broadening occurs because the lattice is not a perfect crystal. The third change occurs when you add more than one component.

Since the source is the same, your result is either because the nature of the crystalline order has changed or the chemistry (composition) has changed.

It is possible that you have XRD amorphous material present at lower temperatures. At higher temperatures all (or more) of the material you are analysing is crystalline.and hence you will measure a larger peak area as X Ray diffraction only gives meaningful information on crystalline phase.

Alternatively you are not correctly deconvoluting peaks from sample background or overlapping reflections. In poorer crystalline materials (broader reflections) this is more likely an explanation than in highly crystalline materials

                With increasing the crystalline phase in the material the XRD peaks become more sharp means FWHM (Full Width Half Maximum) small and vice versa.

            By deconvoluting the XRD peaks one can get grain or particle size, strain, defects density,  lattice parameters, etc. from peak position, intensity, and FWHM.

Integrated peak area measurements can give a good idea about quantity of a phase. With different temperature treatment the quantity of the particular crystalline  phase may be changing. 

Kesava yellareddy thanks but!

I confused I think the amorphous materials shows more sharp peak intensity pattern.

Can you give me reference paper for grain size and lattice parameters calculation or how we calculate hkl values?

Dear Dr Umer

Amorphous materials are noncrystalline hence they do not show any sharp diffraction peaks except a broad halo. In fect, absence of sharp diffraction peaks in considered a test for a material to be amorphous.

When a sample is crystalline, the diffraction produced by each plane are added and produce well-defined peaks in the diffraction patterns. If your sample is amorphous, because you do not have a symmetry in the structure, these signs of diffraction counteract and the diffraction peaks are, usually, nonexistent.