Pretransformation strain modulations in proper ferroelastics

Within the framework of the Landau-Ginzburg model for the 0h-D4h proper ferroelastic transformation pretransformation structural modulations may arise as (possibly defect-stabilized) pseudo-critical fluctuations that can be described by an effective elastic φ4 model. Here we show that the strain amplitude and temperature dependence of an intermediate tetragonal phase observed above and concurrently with the face-centered cubic to face-centered tetragonal transition in Fe0.7Pd0.3 can be understood semi-quantitatively in terms of the crossover behavior from the Ising to the displacive limit by applying the crossover phase diagram of Beale, Sarker and Krumhanal to the effective φ4 model. The fact that this type of pretransformation modulation has not been observed with X-ray for other ferroelastic transformations can be attributed to large values of the order parameter gradient coefficient in these cases