Why the lifetime of acoustic phonons is larger than that of optical phonons? for example, in GaAs, the LA phonon lifetime can be ~850 ns, while LO phonon lifetime is only about 5 ps. Why such huge difference occurs? Thanks!
 A: As user87596 has noted, optical phonons have higher energy than acoustic phonons. This means there are more relaxation mechanisms available to them. Since the net scattering rate goes as the sum of the component scattering rates
$ \frac{1}{T} = \frac{1}{T_1} + \frac{1}{T_2} + \frac{1}{T_3} $
where 1, 2, and 3 represent different scattering mechanisms.
At lower energy, where perhaps less of the mechanisms are available, the scattering rate, $(\frac{1}{T})$, will be lower, so that the net lifetime $T$ will be correspondingly larger.
A: For very basic understanding,  because optical phonon has larger energy than accoustic phonon. If you have larger energy, you will have more ways to decay, like into two lower energy accoustic phonon. If you have low energy like an accoustic phonon, there are fewer decay paths and hence is more stable.
A: Because of the way they vibrate? Acoustic phonons are basicaly longer wavelenghts and neighbouring sites are in phase while for optical ones you have shorter wavelenght and oposite phase in neighbouring sites of lattice. 
