# Optical and acoustic branch

Does a diatomic crystal posses both optical and acoustic branch simultaneously; Or, whether the vibration is either optical or acoustic?

It's not clear to me what exactly you're asking, but for a crystal with $$N$$ atoms in the unit cell, you will get $$3N$$ phonon branches. The first 3 will be acoustic, and the remaining will be optical. So yes, for a diatomic crystal, we have 3 acoustic modes and 3 optical modes.
• If diatomic crystal means that the crystalline structure is described with a Bravais lattice with a basis of two atoms, there are 3 acoustic and 3 optic branches, also in according with the total $3N$ you report. – GiorgioP Apr 4 '19 at 6:56