Crystallite size is the smallest - most likely single crystal in powder form. The crystallite size commonly determined by XRD.

Grain is either a single crystalline or polycrystalline material, and is present either in bulk or thin film form. During the processing, smaller crystallites come closer and grow to become larger due to kinetics. Therefore, in the most likely scenario, the grain is larger than a crystallite. And, the grain morphology is commonly determined by SEM (but not XRD).

Particle may be present as a single crystal or an agglomeration of several crystals. Therefore, particle is under no circumstances smaller than crystallite size. In the ultrafine nano regime, particle size and crystallite size may be the same. XRD and TEM are commonly employed to ensure that there is any difference between the crystallite size and particle size.

Regarding the size comparison between grains and particles, it is however difficult to say whether the grain is bigger or particle is bigger. Sometimes, particles are evolved from controlled agglomeration of small grains, and alternately grains may also be grown from smaller particles by annealing at a higher temperature.

Further, since the crystallite size and grain size depend on nucleation and growth kinetics during processing, they may be different for different materials because the temperature sensitivity and melting points are also different for different materials.

Please, have a look at this previous post:

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

Or, to make it short: No!

A grain may consist of several crystallites. Depends on the material.

Summarizing:

Crystallite size: Size of a single crystal.

Grain size: Size of a particle. In the specific phase of a particle formed by only one crystal, both terms are the same. 

By other hand, when you calculate the crystallite size by WH-Plot or Scherrer , you are determined the "diffraction coherent domain", not the grain size. 

To estimate the grain size, you have to do Particle size distribution by LASER, TEM, SEM, etc.

Crystallite size is the smallest - most likely single crystal in powder form. The crystallite size commonly determined by XRD.

Grain is either a single crystalline or polycrystalline material, and is present either in bulk or thin film form. During the processing, smaller crystallites come closer and grow to become larger due to kinetics. Therefore, in the most likely scenario, the grain is larger than a crystallite. And, the grain morphology is commonly determined by SEM (but not XRD).

Particle may be present as a single crystal or an agglomeration of several crystals. Therefore, particle is under no circumstances smaller than crystallite size. In the ultrafine nano regime, particle size and crystallite size may be the same. XRD and TEM are commonly employed to ensure that there is any difference between the crystallite size and particle size.

Regarding the size comparison between grains and particles, it is however difficult to say whether the grain is bigger or particle is bigger. Sometimes, particles are evolved from controlled agglomeration of small grains, and alternately grains may also be grown from smaller particles by annealing at a higher temperature.

Further, since the crystallite size and grain size depend on nucleation and growth kinetics during processing, they may be different for different materials because the temperature sensitivity and melting points are also different for different materials.

In addition, note that the direct application of the Debye-Sherrer formula gives the crystallite size, not the grain size

I would also like to direct you to one of the previous posting here in research gate posted by somebody else. Hope this will add more insights into particle size vs crystallite size to resolve your questions.

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

Dear colleague Ahmad,

Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size)

Crystallite size often matches grain size, but there are exceptions

Crystallites are coherent diffraction domains in X-ray diffraction.

Particles are chunks/pieces (usually very small, below 1 mm) of solid matter, ensembles of atoms. Particles can be as small as two atoms (the nitrogen particle for example, N2)

Grains are volumes, inside crystalline materials, with a specific orientation.

Particles can be polycrystalline, single crystal or amorphous. A 100 nanometer particle of gold, for instance, can be made of:

a single gold crystal,

many grains with a grain size

i am quite agree with Claus Moseke

Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size) Particle or grain which consist of several small crystallite may have different orientation as determined by XRD diffractrogram peks present.

The simple answer can be as follow:

Particle (agglomeration of some grains) => Grain (ensemble of some crystallites and sometimes consists of a single crystalline material) => Crystallite (is the smallest and can be mono- or poly-crystalline)

One can consider crystallite and grain size to be approximately the same. It is important to note that a grain consists of a single material, but maybe crystalline, or polycrystalline. 

For a complete characterization, you may need to use a TEM with additional analytical tools. But you can distinguish single-crystal grains from polycrystalline grains by switch the mode to diffraction: a single crystal grain will form a sharp diffraction pattern associated with that material, the transform of the crystal structure, while the polycrystalline grains will have multiple copies of the pattern, perhaps even rings. When you get to the particle size, it is always the largest, and may consist of several grains, and perhaps even grains of different materials. You will spend some time with your local microscopists in order to complete the characterization. 

The average crystallite size can be determined using the Scherrer equation, the grain morphology is commonly determined by SEM, and the particle size can be estimated from TEM image.

Also please note that Sherrer's formula is an approximation, and works best for nearly spherical particles. You will obtain the best results from TEM or AFM, especially for very small, or irregular shapes.

Crystallite, grain, a particle the most generic explanation for RG visitors.

kind regards

Santanu

XRD gives crystallite size. Scanning electron Microscopy gives grain size. A grain may consist of single-crystalline or polycrystalline or agglomerated units

Crystallite size is the smallest - most likely single crystal in powder form. The crystallite size commonly determined by XRD. Grain is either a single crystalline or polycrystalline material, and is present either in bulk or thin film form. Therefore, particle is under no circumstances smaller than crystallite size.

I think Mr. B. Parvatheeswara Rao explained your answer very well

This comparison is different for different samples.

I have recently watched a video on this topic. It is very effective and easy to understand. You may find this useful -

https://www.youtube.com/watch?v=hjUp2pkONRM

Nice video Sachin Kumar .

Many thanks for that link.

Gerhard Martens My Pleasure. In fact, I am the creator of this video and that YouTube channel belongs to me only. Hoping to bring more research oriented stuff in the future. Have a good day :-)

Thanks Sachin Kumar ,

for feedback.

Now I am able to pass the link of your RG homepage together with the link of your video when answering a similar question as above.

Many thanks again and best regards

G.M.

Gerhard Martens thank you very much :-)

Please note Crystal size and Crystallite size are two different things.

Crystal: Also called a Grain, where all the unit cells are arranged in a particular orientation. But there is a twist here. The unit cells are not PERFECTLY arranged in a particular orientation. There is an extremely slight misorientation even within a single grain/crystal. This gives rise to many tiny 'domains' within each crystal/grain. These domains cannot be observed under a low resolution microscope because the degree of misorientation is extremely small.

Crystallite: These smaller domains within each crystal are called crystallites. XRD is more powerful than a microscope in identifying these crystallites or rather calculate the average size of these crystallites. So, applying WH or Scherrer relation will actually give the average size of the CRYSTALLITES, not crystals/grains.

Now, some important relationships between a particle, crystal and crystallite:

A (powder) particle may have one or more grains/crystals.

Each grain/crystal will in turn have a number of crystallites.

The size of a crystallite is always equal to/smaller than that of a crystal.

The size of a crystal is always equal to/smaller than that of a particle.

a great thanks for all these clarifications