The fact that the diffraction lines at low angles tend to be very intense is common, but certainly not a regularity. It has to do with the atomic arrangement of substances. If the structure contains a motif or structural planes with corresponding spacing of atoms, then these lines exist. It also affects which atoms occupy these planes and how densely (what are the structural factors of these planes). For example, planar 2D structures show a high intensity of the first diffraction lines. Conversely, metals and intermetallics do not behave this way.

Because of the phase change. Compared with the size of atom, wavelength of X is not so small. That means if you check two lights, one passes the top of an atom, the other passes the bottom, and scattering to theta angle. A higher theta means high phase difference (d sinθ), leading to lower density. This trend can be seemed from f~sinθ/λ figure (I suppose most textbook of diffraction would have this part). That is why when you go to the high angle part, intensity decreases. You can also find that this kind of decrease is connected with the radius of atom, or ion.

Dear Sahil Aman ,

in addition to the answer of Dalibor Matýsek I would like to add that there are indeed some factors involved in the XRD peak intensity which show a slight decreasing tendency with increasing 2theta, such as the

atomic form factor f, the Lorentz factor as well as the Debye-Waller factor.

Even the XRD pattern of a sample layer of finite thickness exhibits such a dependence due to the x-ray attenuation.

See for example details of these items in the following link

https://application.wiley-vch.de/books/sample/3527310525_c01.pdf

or google for them...

Good luck and

best regards

G.M.

To clarify some of the previous answers,

Dalibor said, "It has to do with the atomic arrangement of substances." No, the reason you get peaks at all is due to the atomic arrangement, but that is not why low angle peaks usually have higher intensity that high angle. Nor is it correct to say "For example, planar 2D structures show a high intensity of the first diffraction lines. Conversely, metals and intermetallics do not behave this way." What is he talking about "2D structures" ??? You won't get any diffraction from a single layer of atoms. And metals most certainly do exhibit this trend. So you should really ignore most of that answer.

Sihan said "Because of the phase change." What? No! The trend you ask about has nothing to do with phase changes. The rest of this response is also incomprehensible or incorrect.

Do pay attention to anything Gerhard says. To focus your attention on the correct explanation, note figures 1.14 and 1.16 in the article he references. Read the text surrounding those figures. You could summarize the effects as (mostly) due to the geometry of the diffraction experiment and the absorption of x-rays in samples.

Note that the trend you mention does not mean that every higher angle peak will have lower intensity than every peak at lower angle. The structure factor and multiplicity factors are more complicated than that. But calling it a general trend is correct. Most metals have FCC or BCC structure and show this general trend. But there are exceptions: Some metal compounds have simple cubic (SC) structure and have diffraction patterns with extra peaks (compared to BCC or FCC) that occur at low angles with very low intensity.

As the 2theta diffraction angle increases, the interference (path difference) from different electrons of the atoms increases, resulting to a decrease of the atomic form factor.

Baihong Sun is right and Sihan Zhang is partly*) right.

Both colleagues describe the reason of the decrease of the atomic scattering factor with increasing theta or sin(theta)/lambda.

Please see the sketch in the attachment (taken from the book of Klug and Alexander)...

*) But Sihan Zhang , there is no direct proportionality of f and sin(theta)/lambda, as you mentioned above.

w.s.l. Boyer , the phase I mentioned is not the phase change of the materials, Gerhard Martens 's additional explanation is right. It is the phase change of the light, or the phase difference of two sin wave.

Gerhard Martens , I didn't say it is linear or something else. An example of this figure from Crystal Structure Determination by Werner Massa is attached. This kind of trend will be discussed in nearly all X-ray diffraction books. sin（theta）/lambda is used to remove the effect of X-ray wavelength. This figure is somehow the property of atoms or ions in crystal.

Thanks for feedback Sihan Zhang ,

but, sorry, you have written above:

"This trend can be seemed from f~sinθ/λ figure".

The meaning of "f~sinθ/λ " is: f is proportional to sinθ/λ.

You may replace "~" by "-" or "vs".

Then any misunderstandings will be removed...

Best regards

G.M.

Sahil Aman

This is an insightful observation and a common characteristic seen in many XRD patterns. The decrease in peak intensity with increasing 2θ angle can be attributed to several factors:

Overall, the observed trend is the result of a complex interplay of physical, structural, and instrumental effects. Understanding these helps in interpreting XRD patterns more accurately.

Wow, thanks so much Kwan Hong Tan, we would never have thought of copy/pasting from AI without understanding ourselves without your "help"!

[for non-native English speakers, that is sarcasm].

1. True - this is what Gerhard and Sihan have been explaining -why this is so.

2. No, the bunch of factors mentioned do not (usually) affect intensity. Lorentz-polarization factor accounts for decreasing intensity only up to 90° 2θ; this factor increases intensities for 2θ>90°.

3. No, the words are true but offer no explanation for decreasing intensity with increasing angle.

4. No.

Like many AI answers, it is a mixture of true, false, and irrelevant statements.