Phenomenological analysis of α′_L-to-β martensitic transformation in phosphorus-bearing dicalcium silicate

Crystals of (Ca_1.95□_0.05) (Si_0.9P_0.1)O₄, where □ denotes a vacancy, composed of both the α′_L and β phases, were prepared and examined by the precession method. The β phase was exclusively twinned on (100)_β, and the relative volumes of the twin-related variants were almost identical with each other. On the basis of the lattice correspondence between the two phases and their cell parameters, the phenomenological crystallographic theory was applied to determine the habit planes and the shape deformations upon α′_L-to-β martensitic transformation. The habit planes, which define the coherent interphase boundaries between α′_L and β, were nearly parallel to either (100)_α′L or (010)_α′L·. The alternate shape deformations that produce the former habit planes resulted in the actual (100) twin structure of the β phase. The total displacement was along [100]_α′L with the magnitude of 0.008. Because the transformation involved a very small volumetric shrinkage of 0.6%, the strain accommodation would be almost completed. The coherency at the interface boundaries between the two phases and the effective strain accommodation probably caused the thermoelasticity of the Ca₂SiO₄ solid solutions.