There are hundreds of books and thousands of articles or websites. PLEASE, don't bother busy people with questions which cannot be explained in some sentences. This is in fact a simple but on the other hand quite complicated problem which I could tell you in a few sentences but I have to use terms you have to ask for again. So it makes absolutely no sense!

Sorry dear Nolze for wasting your time. Thanks for your suggestions!

You are right! For example, is the highest intensity for the XRD of calcite (CaCO3); and it is the cleavage or preferred orientation direction. If you do not correct the intensities due to the preferred crystallographic orientation effect in your Rietveld refinement, you will not get the good agreement between the (i) calculated and measured XRD patterns, and (ii) the refined crystal structure parameters with single crystal, see Sitepu, O'Connor and Li (2005), JApplCryst, 38, 158-167. Also, the Crystallographic R-factors and goodness-of-fit index will not be acceptable

 Subsequently, to get the crystal structure refinement right for ND data of calcite, you need to use as the direction of preferred orientation in your March- Dollase model, see Sitepu (2009), Powder Diffraction Journal, 24(4), 315-326.

I am asking about why preferred orientation along a particular direction. What parameter decides it? H SITIPU

There may be misunderstandings in your question. With respect to the cause of preferred orientation please have a look at a similar question here on RG.

Perferred orientation is just a function of the kind of sample preparation/handling:

i) growing (e.g.of film) along preferred (crystal) directions on a substrate ,

ii) crystallization of a sample from 'liquid or gas' phase such as via drying or cooling (e.g. salt,  ice and snow flakes,); here also growing along preferred crystal directions (e.g. snow flake or frost pattern/ tracery on frosted window).

iii) rolling of samples and any kind of plastic deformation, (particular directions here are governed by the direction of the applied forces for example).

etc.

Preferred orientation is characterized in that the orientation distribution of the crystallites of the sample in the angular space is not uniform. Texture is the 2D/ 3D distribution of preferred orientation in the sample.

https://www.researchgate.net/post/What_affects_preferred_orientation_of_thin_films

https://en.wikipedia.org/wiki/Texture_(crystalline)

Dear Martens thanks, it will help me a lot. But I am for sure surface energy has a lot to do with it perhaps!

perhaps long range order in a crystal along a particular direction which is a function of minimum surface energy is the real cause behind highly intense XRD peak along that direction.....so called preffered direction.

Set up of long range order in a special direction can be a consequence of minimizing surface energy (ok.)

 BUT there is no intense XRD peak in 'that' direction. Theta (or 2theta) direction of XRD angle has nothing to do with that (e.g. growing) direction or the direction of maximum extension of a crystallite. 

Please refer to basic XRD textbooks (as Gert recommended above) in order to understand the basic principles of x-ray diffraction and how XRD peaks are related to the crystal structure and directions of the crystal axes and lattice planes.

We have to use the same terminology and  have to know what is behind it.

Forget surface energy, it all depends upon the crystal structure. If you have a well formed crystal and you are diffracting down it's C axis you will get a strong and high XRD peak. If you then turn your crystal and try to diffract down, say, the 111 direction you will not get such a high peak because this is not such an "easy path" for your x-rays to interact with. Like I said, the intensity of your peaks will depend upon your crystal structure and whether or not it was grown well. A good crystal to experiment with would be silicon but if that material was not grown correctly it will not yield a good XRD spectrum. Perfectly grown Si crystals are often used as a standard to calibrate your diffraction machines - it would be a good one to start experimenting with, it would open up a whole world of interest for you. 

I am writing this answer for an ideal case where the x-rays penetrate into the sample equally, i. e., the same incident flux reaches all volumes of the sample. Different atomic planes have different atomic densities per unit area of the plane, hence, density of scatterers (add on top of that the different form factors of atoms on these planes if your sample is a compound). Intensity is proportional to the number of scatterers per unit area of a given atomic plane and therefore the peak intensities in an XRD experiment will vary. Usually, with increasing plane indices (higher angles in the pattern), the intensity of the peak goes down. There is also the form factor, i. e., how an atom scatters an incident radiation (the "f" in the structure factor formula). You can start looking into these as a starting point.

thanks Dr. Burc Misirlioglu.

I think it might be due to two main reasons, Firstly, for some crystal structure, it has more atoms in a specific plane, e.g. (111), then it might show a higher intensity in XRD pattern due to more counts accumulate in this orientation. Secondly, it might due to the measured crystals have texture in some directions, causing the higher intensity peaks. Other reasons might be due to the scanning parameters, measurement geometry setup and intrinsic properties of crystals. Hope to hear others comments on this, thank you!

I agree with Prof. Burc Misirlioglu, more dense is the plane more will be the intensity in case of ideal growth.