Types of Unit cells in 3D Bravais lattice | Solids | Chemistry | Khan Academy

In this video, we dive into three dimensional lattices and discover the different types possible - the 14 Bravais lattices - based on translational symmetry!
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Created by Shahzad Karim

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[@matinshaikh9678]

Thanks you So much. Sir my doubt is primitive Hexagonal Lattice have atoms at face center, so it should not be primitive any more.

[@KhanAcademyIndiaEnglish]

Great observation salehabanu ! Well you are right, hexagonal lattices are indeed hexagonal end-centred rather than primitive hexagonal! However If we analyse deeper, we will realise that - a lattice made of primitive hexagonal unit cell ( with no extra centred points ) does not have translational symmetry ( try moving the lattice from one corner to an adjacent corner !) and so it can't be a Bravais lattice. Thus, there actually can never be a primitive hexagonal unit cell! -also a lattice of hexagonal end centred unit cells is itself actually made of a primitive parallelopiped (a special kind of rhombic prism to be precise!), as outlined in the figures. However, we use the hexagon as the unit cell over the primitive parallelopiped, as it is more symmetric and thus more accurately reflects the overall symmetry of the lattice. _Highly recommend you to go through the video on 2D lattices to better understand the analogous case in 2D with rhombous-60 )_ so crystallographers generally call end-centred hexagonal unit cells as simply(&confusingly!) hexagonal !

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