difference between crystallite size and grain size

could any one please explain me

What is the difference between crystallite size and grain size? some how i have a feeling of both being same.

If both are same then why two terminologies being used.

And that too if we observe these two terminologies are not being used just like that. It is being used specifically. The term crystallite size is being used specifically in XRD and TEM whereas the term grain size being used in Optical microscope. hence i have a doubt whether both are same or different?

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crystallite size is size of One crystal. whereas when we consider grain size, it may consists of several crystals.
Will grain contain several crystals?
could you pls explain me this further
The grain contains several crystals aligned in a same direction and the two grains are separated by grain boundaries because of misalignment of the crystals by differing their orientation and that differs two grains. Thus a crystal is a unit lattice in which atoms are arranged in a specific position, where as a grain is bunch of crystals (rather unit lattice) arranged in a same direction. As you mentioned, Grain and grain size is usually used in Optical Microscopy and at lower magnification.

At higher magnification we go in deep with a single grain where in you can see the particles or crystals in one grain.

And about the terminologies used in XRD is crystal size because the X ray determines the crystal structure by determining the close pack planes and distance between the two atoms.

Whereas in case of electron microscopy like SEM and TEM, we use PARTICLE SIZE (I'm in doubt between particle and crystal size) rather GRAIN SIZE as for as I know about best of my knowledge.

Please let me know if I'm wrong so that I can update in my mind!

Thanks for your reply.

in case of particle size, one particle can have many grains. generally particle is larger than grain. in TEM we wont say it as particle size we tell it as crystal size.
Thanks for correcting me Kousaalya. But did u able to get the solution for this? If the particle contains many grains then even I have a doubt in the terminologies used in both cases. I'll get back to u later. Please update in ur profile if u do get any answers for this question.
Hi Kousaalya,

i got something like this about the difference b/w grain and crystal size.



crystalline size is smaller than grain or particle size.  inside  a grain or particle we have many crystals of identical orientation. in case of crystalline size we were able specify a boundary (ie) grain boundary. but in case of grain or particle size where is the boundary? up to where the measurement will be?

Dear Kaousaalya,

I have recently joined this forum and found your post. I hope you must have found a satisfactory answer to your question. However, I would add something further on this subject. The term crystallite have been usually used by physicists and chemists, while grain size is mostly used by metallurgists. To understand the the difference in terminology, you have to look at the hierarchy of micro-structure in the materials. Crystallites are the "coherently diffracting domains" of crystals and grains may contain several of these domains.  For example, a single crystal can be termed as a single crystallite. Now if you deform this single crystal, several sub-grains having different orientation are formed. Now each and every subgrain will be considered a crystallite. On the severe plastic deformations, more and more finer subgrains will be formed, and each subgrain will be a crystallite. The same thing happens when you mechanically mill a powder or carry out severe plastic deformation of a metallic specimen. Now, if a grain is a coherently diffracting domain, it can be termed as crystallite. On the other hand, if a grain contains several subgrains, then each subgrain is a crystallite. To understand the effect of hierarchy of microstructure on the results obtained by various characterization methods, XRD, TEM, AFM; you can go through our recent publication in Journal of materials science:

S.K. Vajpai, R.K. Dube and M. Sharma; Studies on the mechanism of the structural evolution in Cu-Al-Ni elemental powder mixture during high energy ball milling; J. Materials Science, Vol. 44, 2009, pp. 4334-4341.


Hi S.K.Vajpai, 

It was very informative about mechanical alloying from your paper. I have a doubt that, from XRD the size which we determine is average crystallite size or average grain size?. Can you please explain me about "coherently diffracting domains" ?

Also how is it different from crystallite size? 

Thanks & regards,



Dear surendra....what we measure from XRD is the average crystallite size and not the grain size ...of course when I say grain size..it means i am talking about the conventional notion of grains..separated by high angle grain boundaries. ..as far as your second question is concerned...I mentioned in my last post that coherently diffracting domains are called crystallites...e.g. a single crystal is a crystallite... ..Hopefully..it answers your question..


can you tell me how can I measure the particle size of powder sample...If I have a SEM image of the powder from that what I will measure??is that particle size or  grain size??? plz help me


Particle size of the powder can be measured by SEM, but it is very loclized. So you cant conclude onky from SEM. It is better to do laser particle analyser. this gives you the particle size distribution. however the size you get from SEM is particle size.

And Particle size and grain size are not same. one particle can have several grains. unless you produce a particle which is single crystal.




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