Pseudospinodal mode of decomposition in films and formation of chessboard-like nanostructure.

A new decomposition mode, a coherent pseudospinodal decomposition under geometrically confined conditions, is discovered. This mode is associated with a coupling of the decomposition with the displacive crystal lattice rearrangement and results in a gradual separation of compositions of two product phases. We consider a particular case of decomposition in an epitaxial thin film. The 3D phase field microelasticity modeling demonstrates that the confined pseudospinodal decomposition dramatically affects the thermodynamics, kinetics and morphology of the system producing a vertically aligned and highly correlated regular chessboard pattern of two product phases. The study reveals the physical origin of the chessboard structure. The computer modeling predicts its geometry in striking agreement with the existing observations. The modeling shows that the transformation develops through a so-called tweed precursor state observed in many martensitic systems.