  Spin-phonon relaxation due to two-phonon processes of a spin cluster embedded
in an elastic medium has been studied. For the case of uniaxial anisotropy,
relaxation rates due to Raman processes and processes involving the emission of
two phonons have been obtained. For a biaxial spin Hamiltonian, the rates of
transitions between tunnel-split levels have been computed. By comparison with
the rates of corresponding direct processes, we have established temperature
ranges where the Raman mechanism dominates over the one-phonon decay mechanism.
