Nonlinear theory of localized and periodic waves in solids undergoing major rearrangements of their crystalline structure

This article analyses the propagation of nonlinear periodic and localized waves. It examines crystals whose lattice consists of two periodic sub-lattices. Arbitrary large displacements of sub-lattices u are assumed. This theory takes into account the additional element of translational symmetry. The relative displacement in a sub-lattice for one period (and even for a whole number of periods) does not alter the structure of the whole complex lattice. This means that its energy does not vary under such a relatively rigid translation of sub-lattices and should represent the periodic function of micro-displacement. The energy also depends on the gradients of macroscopic displacement describing alterations in the elementary cells of a crystal. The variational equations of macro- and micro-displacements are shown to be a nonlinear generalization of the well-known linear equations of acoustic and optical modes of Karman, Born, and Huang Kun. Exact solutions to these equations are obtained in the one-dimensional case—localized and periodic. Criteria are established for their mutual transmutations.