Why does PXRD peaks intensity decrease with the increase of the 2 theta degree?
There are different reasons one of these is that the x-rays are diffracted along a cone which opening is 2theta. So the part of cone that you can collect by the detector proportionally decreases with the angle.
@ Stefano: the effect you are describing is the geometry factor and - as you wrote - it causes an increase of the intensity with increasing Bragg angle (since more and more of the reflection cone is detected by the slit detector). S. Kadheminia however asked for the opposite effect that intensity is decreasing with increasing Bragg angle. However, also this is not really true since the LPG factor (Lorenz-polarization-geometry) first descreases and then increases the intensities. You can easily visualize this effect in PowderCell if you calculate the diffractogramm of a certain phase for different radiations and compare the intensities of different reflections.
I guess, that the question is not related to the intensity but to the structure factor, and for this the described effect really exists (but it is only one single term which affects the reflection intensity!). For the structure factor the reason is that with increasing Bragg angle the d-value becomes smaller and smaller so that the variation of the electron density perpendicular to the diffracting plane becomes smaller and smaller which is the major factor of the intensity formation.
Well, as I wrote, this is my guess what S. Kadheminia actually was ascing for. Maybe he will specify his question again?
Hello, dear Gert. I am working on metal oxides XRD data. I found the decreasing the intensity with increasing 2 theta. Please suggest a paper or book for more information.
Thank you
I guess you can use any monography about XRD since there the LPG factors and all other parameters are described as well as the intensity - structure factor relationship. I am also sure that you will find extensive descriptions in Wikipedia.
The generally accepted interpretation you can read in International Tables for Crystallography, vol. B (the reciprocal lattice) and C (physics behind) which is often better than any paper.
My recommendations already explain that these effects are practically known since 100 years so that nobody is discussing anymore these parameters in newer papers, although sometimes I have the impression that it would be very beneficial. It is simply fundamental knowledge which one needs to know in order to do responsible XRD interpretation.
@ Gert: Perhaps did you mean " it causes a DECREASE of the intensity with increasing Bragg angle"
https://chemistry.osu.edu/~woodward/ch754/lect2003/xrd_peakintensities.pdf
@ Stefano: I was talking about the LP (or sometimes also LPG or PLG) factor, cf. slide 9.
The geometry influence is only coming from the cone you mentioned and it is a monotonic function. The fixed detector size (slit height) collects more and more intensity with increasing Bragg angle.
Unfortunately the geometry term is sometimes not specified. G=cos(theta)/sin(2theta). the term 1/sin(2theta) considers the part of the cone covered. However, with decreasing angle the number of crystals illuminated increases which is described by the cos(theta) term. Using a variable slit (primary) this term is a constant (1).
Not the number of the crystals but the area. At lower angle the illuminated area increases but a thinner volume is involved due to the x-ray sample absorption.
The thermal vibrations of atoms has effect on the diffraction pattern . The amplitude of atomic vibration is another factor . It is also worth noting that the elastic constants of the crystal , besides decreasing the intensity of diffraction patterns it causes some generasl coherent scattering .
There are some factors affecting the relative intensity of the diffraction lines -Polarization factor , structure factor . multipilicity factor . Lorentz factor , absorption factor and temperature factor .
The peaks intensity will decease at higher angle due to the form factor.We can plot the graph I(q)q^4 vs q, the intensity of xray decrease with higher q value. so q=4ΠSin(θ)/λ ...
Ref:P.Krishnamurthy indian.J.Phys., 5 (1930) 473-500.
Sorry this sound more elementary but, why do we make plots of intensity or energy versus 2θ?
I do understand the 2θ from geometry when deriving Bragg's law but...please help
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