What is the mean reason for the non-oscillating of structure factor around one and the reduced structure factor about zero at high k values?
I believe you are asking for the reason why the oscillations at low k values are damped at higher k values?
The answer: it is due to static and thermal disorder of the interatomic distances that give rise to the oscillations.
The Debye-Waller factor, for example, is often used in crystallography to describe the attenuation of scattering due to thermal motion.
It is instructive to think in terms of real and reciprocal spaces and Fourier transforms:
If all bonds of a given type are identical in length at all times, then in the real-space correlation function (variously defined as the pair distribution function, PDF, or radial distribution function, RDF) they are represented by a delta function. In reciprocal (k) space this is a sinusoid, that is, an oscillatory function which continues oscillating with the same amplitude out to infinite k.
On the other hand, with static and/or thermal disorder, the real space delta function is broadened out into, for example, a Gaussian distribution, in the harmonic approximation. Using the properties of Fourier transforms, we know that a Gaussian is transformed to another Gaussian, centered at the origin, and using the convolution theorem this will multiply the sinusoid giving rise to an observed damping at high k.
I hope this helps!
It looks to me like a bad data correction and form factor calculation. You can try the code PDFGetX3, where a fitting algorithm is used.
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