A theory of void-lattice formation

A theory of void-lattice formation in crystals under irradiation is presented that is based on the so-called two-interstitial model and makes use of no additional assumptions. It is shown that the formation of void lattices having the same crystallographic type and orientation as the host crystal lattices, as found by experiment, is intimately related to the existence of a one-dimensionally diffusing metastable excited self-interstitial state (crowdion). The transition from a random assembly of voids to a regular void lattice takes place if the void number density is so high that in their excited state self-interstitials can transverse the mean void distance before returning to their ground state. If the void density is lower and does not increase in the course of irradiation, the random void arrangement reaches a steady state. From the view-point of thermodynamics, void-lattice formation is a non-equilibrium phase transition in an open system. It is an example of self-organization of an ordered structure in a dissipative system in the sense of synergetics. (orig.)