  We consider relaxation of a rigid spin cluster in an elastic medium in the
presence of the magnetic field. Universal simple expression for spin-phonon
matrix elements due to local rotations of the lattice is derived. The
equivalence of the lattice frame and the laboratory frame approaches is
established. For spin Hamiltonians with strong uniaxial anisotropy the field
dependence of the transition rates due to rotations is analytically calculated
and its universality is demonstrated. The role of time reversal symmetry in
spin-phonon transitions has been elucidated. The theory provides lower bound on
the decoherence of any spin-based solid-state qubit.
