III. Non-radiative relaxation at metal-ion point defects in solids

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Abstract
This paper discusses the application of theories of non-radiative relaxation to metal-ions, both rare-earths and transition-metals, in a wide range of lattices varying from continuous ionic or covalent solids to discrete complex ions. The nearest neighbour cluster model, developed for spin-lattice relaxation calculations, is used to derive a set of symmetry based selection rules. Particular emphasis is placed upon the circumstances under which an exclusion rule may arise leading to especially slow radiationless transitions. The effect on rate of the electron—lattice coupling and of the nature of the phonon spectrum is treated. Evidence is assembled to show that in transition-metal complex ions high frequency ligand modes involving hydrogen atoms can be especially efficient in deactivating excited states.