  Very sensitive responses to external forces are found near phase transitions.
However, phase transition dynamics and pre-equilibrium phenomena are difficult
to detect and control. We have directly observed that the equilibrium domain
structure following a phase transition in BaTiO3, a ferroelectric and
ferroelastic material, is attained by halving of the domain periodicity,
sequentially and multiple times. The process is reversible, displaying
periodicity doubling as temperature is increased. This observation is backed
theoretically and can explain the fingerprints of domain period multiplicity
observed in other systems, strongly suggesting this as a general model for
pattern formation during phase transitions in ferroelastic materials.
