For example, given some quartz particles are wrapped by a kind of amorphous aluminosilicate gel, would the amorphous gel disturb the path of X-Ray beam, causing the peaks to shift?
Thank you very much.
No, an amorphous phase (as you call it "substance") will not shift the peaks of the crystalline phase. The peaks appear when the Bragg's law if met, which happens for certain theta angles only. I might imagine that the amorphous phase can slightly broaden (NOT shift) the peak, if the beam reflected from the crystalline planes is re-reflected by the amorphous phase, but - frankly speaking - this would be a negligible effect. Of course, the interatomic distances in the amorphous phase would have to be very close to the interplanar distances of the crystalline phase, if they were to interact with the reflected beam.
There may be another reason for the peak shift: uniaxial stress caused by the presence of the amorphous substance. However, if you think of a gel wrapping the quartz particles, there will be no such effect, because a gel layer will not excert forces able to squeeze quartz particles and distort their crystal lattice. Usually, the stress will not be uniaxial, so there will be areas with positive stress and negative stress, meaning shifting the peak left and right, which simply means peak broadening, not shifting.
So, to sum this up, in my opinion, there will be no shift observed.
Peaks will only shift, if the interplanar spacing of the crystals is changed due to doping or incorporation of impurities or any other kind of interactions present in different phases or constituents of the system.
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