I must repeat the first sentence of Ian's answer:

This is an interesting result....

Looking at the  weight contributions in your sample of about 20% CuO and about 80% SiO2 and in addition looking at the densities of CuO (6,4g/cm³) and SiO2 (2,6g/cm³)  you will end up with a spacial dilution of the CuO particles of at  least 1 : 10 or even 1: 12. Thus  you will have only much less than 1/10 of your CuO material in your net x-ray beam cross-section for the case of the blended sample compared to the pure CuO sample.

Here I think orientational artifacts of the CuO particles  easily show up. This issue is already picked up by Ian and Maria. 

Instrinsc x-ray attenuation in the CuO sample is reduced due to the dilution by SiO2. Therefore the peak heights should not go down to less than 1/10 but should be (very) roughly estimated to be around 1/5 compared to the primary CuO sample.

As Ian suggested; please repeate the measurements (at least at different sections of your sample; including increased counting statistics). So you will see how reliable your diffractograms are. Afterwards we can have a further look on the new results.

Good luck.

Do you may show us your spectra and how much is the peak shift?

May be due to (decreasing or increasing) background slope caused by to the large amount of amorphous material you added. But looking at the spectra will help to find a reason for this.

Your observations seems to relate to changes in intensity, but your question is related to peak shifting.  These are two very different effects and have different causes. I suspect this is a matter of semantics. There should be little reason to have peaks shifting (meaning a diffraction peak from a lattice plane changing its position on the 2-theta axis, indicating a change in d-spacing) upon a simple blend of materials, however it is easy to invoke a change in crystal preferred orientation from a raw sample to a blended sample - this could account for your observations. As Gerhard says we need to see your diffractograms. Tell us what the materials are and how you have prepared them for XRD in detail.

@Gerhard Martens and Ian J Slipper, Pls see attached image for xrd diffractograms of SiO2-CuOC

This is an interesting result...

The correct intensities for S.G. C2/c should be:

110 = 13%

002 = 37%

111 = 100% + 200= 21%, both occur together so the line is slightly broader

20-2=30%

So the CuO trace (a) has the correct intensity ratio between 110, 111 and 20-2 but the 002 is overrepresented. This could be due to a preferred orientation effect on the basal spacing, which is quite common.

However, I am struggling to interpret the SiO2+CuO trace (c) in terms of the relative intensities. Certainly there is no peak shifting, the peaks are in the same positions so the lattice parameters have not changed. However the relative intensities are incorrect for your starting material. I would want to re-prepare the material and re-gather the data (with longer counting times) to make sure this is a real result. If it is repeatable then its time to think again...

I seem to say this every time I answer a question but it will help you in the future if you can gather better data. the signal is very noisy. Also please note XRD is not a spectroscopic technique, it is diffraction, so technically these are not spectra but diffractograms.

Each reflection peak corresponds to a specific family of grains and its intensity is proportional to the contribution of the number of grains of the family in the irradiated volume. Therefore, you could look for the effect of  the variations among the individual planes and the specific grain population orientations.

I must repeat the first sentence of Ian's answer:

This is an interesting result....

Looking at the  weight contributions in your sample of about 20% CuO and about 80% SiO2 and in addition looking at the densities of CuO (6,4g/cm³) and SiO2 (2,6g/cm³)  you will end up with a spacial dilution of the CuO particles of at  least 1 : 10 or even 1: 12. Thus  you will have only much less than 1/10 of your CuO material in your net x-ray beam cross-section for the case of the blended sample compared to the pure CuO sample.

Here I think orientational artifacts of the CuO particles  easily show up. This issue is already picked up by Ian and Maria. 

Instrinsc x-ray attenuation in the CuO sample is reduced due to the dilution by SiO2. Therefore the peak heights should not go down to less than 1/10 but should be (very) roughly estimated to be around 1/5 compared to the primary CuO sample.

As Ian suggested; please repeate the measurements (at least at different sections of your sample; including increased counting statistics). So you will see how reliable your diffractograms are. Afterwards we can have a further look on the new results.

Good luck.

Really, It is a very good explanation from both Gerhard Martens and Ian

My regards

An addition to all the latter. An increase of intensity in a given reflection means preferred orientation, this means that the crystal phase is growing more on that particular Miller indices direction. This would be possible if in a composite material  one layer, the base, is influencing the orientation of the growing one, just as an example. Best wishes with your results and analysis. Hiram

Dear S. Akilu

In most cases the shift in XRD peaks is due to the different in atomic radius between matrix and dopant materials.  

@Essam, this is not the case here. S Akilu has data that shows there is no peak shift in the generally accepted sense, but a change in intensity - see my post above. Also there is no dopant in this case; he clearly states this is a blend of materials.