Three-dimensional phase diagram of a ferroelectric

On the basis of the molecular-dynamic theory, by phenomenologically generalizing the expression for the energy of a ferroelectric, an equation of state is derived here for a ferroelectric in electric field intensity as well as temperature and mechanical stress. The σ(mechanical stress) — T(temperature) phase diagram is analyzed, whereupon ths spinodal, the critical point, and the supercritical line extending into the low-temperature range are plotted. A superposition of this diagram with the E (electric field intensity) — T(temperature) diagram yields a three-dimensional phase diagram in all three state variables. Its elements are the spinodal surface, the critical line, and the supercritical surface bordering on the critical line.