Phonon quasimomentum is defined as ℏq and differs from normal momentum because it is only defined within an arbitrary reciprocal lattice vector. At higher temperatures (10 K < T < Θ), the conservation of energy ℏ ℏ ω ω 1 = ℏ ℏ ω ω 2 + ℏ ℏ ω ω 3 and quasimomentum q 1 = q 2 + q 3 + G , where q 1 is wave vector of the incident phonon and q 2, q 3 are wave vectors of the resultant phonons, may also involve a reciprocal lattice vector G complicating the energy transport process. These processes can also reverse the direction of energy transport.