A particle may be made up of several different crystallites

Crystallite size often matches grain size,

Though the shape of crystallites is usually irregular, we can often approximate them as:sphere, cube, tetrahedra, or octahedra parallelepipeds such as needles or plates ,prisms or cylinders

Most applications of Scherrer analysis assume spherical crystallite shapes

If we know the average crystallite shape from another analysis, we can select the proper value for the Scherrer constant K

A particle may be made up of several different crystallites

Crystallite size often matches grain size,

Though the shape of crystallites is usually irregular, we can often approximate them as:sphere, cube, tetrahedra, or octahedra parallelepipeds such as needles or plates ,prisms or cylinders

Most applications of Scherrer analysis assume spherical crystallite shapes

If we know the average crystallite shape from another analysis, we can select the proper value for the Scherrer constant K

Crystallite size is nothing but average coherence length of particular lattice plane and Grain (Particle) size is composed of many such crystallite.

The Grain size and particle size is often used interchangeably. But the Crystallite size is the size of the single crystal, and a grain may contain one or more than one crystallites.

just search on RG... the same question has already been placed several times! You can check e.g.:

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

"grains can be crystalline or amorphous. Coherently scattering domains (I don't like to call them crystallites as there is no universally accepted definition for this term) are the crystalline regions of material that scatter coherently (and in general they show no coherence with the neighboring ones). They are single crystal in nature (but they are in a powder as defined in the diffraction sense). A grain can contain multiple domains (consider e.g. two domains highly misoriented each other). The grain and the domain size are seldom the same in a polycrystalline aggregate but can be the same in a proper nanosized powder""

I think you can have a very good explanation about crystallite size, grain size and so on using Elements of X-ray diffraction - B.D. Cullity.

Dear colleague Sood,

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