  It is observed that low-lying transitions of an interstitial hydrogen adatom
on a metallic lattice correspond classically to dipoles oscillating at
frequencies where band electrons typically have a low electromagnetic
absorption. Such transitions hence give rise to an essentially unscreened
time-dependent perturbation of the lattice potential experienced by
neighbouring hydrogen states. It is shown that above a certain hydrogen
concentration, coherently phased oscillations at adjacent sites may result in a
dipole-dipole interaction larger than the excitation energy. This would give
rise to a new ground state for the ensemble of coupled oscillators and a
reduction in lattice symmetry below some phase transition temperature. An
explicit calculation is carried out for PdH and PdD, and indirect evidence for
the existence of the postulated phase is presented.
