  Lovesey and Staub have argued [S.W. Lovesey and U. Staub, Phys. Rev. B 61,
9130 (2000)] that experimental data on the temperature dependence of the
paramagnetic relaxation of lanthanide ions doped into YBa2Cu3O6+x are in
agreement with the predictions of a model that describes the relaxation as due
to the scattering of phonons via a magnetoelastic interaction. By generalising
their model I show that the level of agreement is strongly dependent on the
number of intermediate lanthanide energy levels included in the calculation,
and that inclusion of a more complete set of levels leads to very different
results that do not necessarily support the phonon damping picture.
