The pressure-temperature phase diagram of BaTiO3:: a macroscopic description of the low-temperature behaviour

The pressure-temperature phase diagram of BaTiO3 has been investigated using a modification of the standard Landau potential to take account of quantum saturation of the order parameter at low temperatures. The calculated phase diagram agrees well with experiment for the cubic-tetragonal and tetragonal-orthorhombic transitions, but underestimates the orthorhombic-rhombohedral transition temperature somewhat. The saturation temperature (theta(S) = 160 K) is sufficiently high that the expected critical point is not observed experimentally. Instead, each phase boundary bends sharply down, so each of the four crystalline structures of BaTiO3 has a stability field with increasing pressure at 0 K.