  The coupling between localized spins and phonons can lead to shifts in the
dielectric constant of insulating materials at magnetic ordering transitions.
Studies on isostructural SeCuO3 (ferromagnetic) and TeCuO3 (antiferromagnetic)
illustrate how the q-dependent spin-spin correlation function couples to phonon
frequencies leading to a shift in the dielectric constant. A model is discussed
for this spin-phonon coupling. The magnetodielectric coupling in multiferroic
materials can be very large at a ferroelectric transition temperature. This
coupling is investigated in the recently identified multiferroic Ni3V2O8.
