Commensurate-incommensurate phase transition in the presence of crystal deformation

A simple three-dimensional orthorhombic model of particles with displacive degrees of freedom interacting with the strain variables has been studied by the molecular-dynamics technique. The modulated degrees of freedom is coupled linearly to the shear and bilinearly to the bulk deformation component. The computer simulation has confirmed that the commensurate-incommensurate phase transition mechanism relies on nucleation and growth of stripples. Different configuration patterns of the initial commensurate phase revealed that the elastic domain walls between shear domains can be used to nucleate new discommensuration planes, and stripples are able to recover planar folding defects of the lattice, defects which are the result of heavy crystal deformation. It is shown that in the incommensurate phase the shear component forms a modulated field which follows the modulation wave with the same wave vector, but the bulk deformation component doubles the modulation wave vector.