ENERGY MIGRATION AND TRANSFER IN THE 5D0 STATE OF CS2NAEUCL6

Luminescence decay curves from the 5D0 state of the cubic hexachloroelpasolite crystals Cs2NaEuCl6 have been measured over the temperature range 12-340 K. The decay curves are markedly non-exponential, especially at low temperatures and are not well described by standard kinetic treatments. Energy migration in the 5D0 state occurs at all temperatures studied. Below 75 K an exchange mechanism is important for the migration, whilst at higher temperatures a vibronic electric dipole mechanism dominates. Emission occurs from both the regular and defect sites. The defect sites have a similar energy but a much shorter lifetime than the regular sites, the fast decay being the result of an accompanying fast non-radiative process. Energy migration, energy transfer to defects and relaxation at both defect and regular sites occur at comparable rates which, together with back-transfer from the defect sites, account for the complex kinetics.