Temperature-uniaxial pressure phase diagram of KH2PO4

In this work we present the uniaxial pressure-temperature phase diagram of KH2PO4 (KDP), for temperatures near the ferroelectric phase transition at 122 K. Using Raman measurements we show that under uniaxial pressure the KDP undergoes two metastable transitions: (i) from tetragonal D-2d(12) phase to orthorhombic C-2v(j not equal 19) phase at 126 K and sigma(6)=1.12 kbar, and (ii) from orthorhombic C-2v(19) phase to the same orthorhombic C-2v(j not equal 19) phase as before, at 110 K and sigma(6) = 0.3 kbar. We discuss the reversibility paths of these metastable phases, and provide an explanation to the appearance of the C-2v(j not equal 19) phase, based on the Gibbs free energy density of the system. [S0163-1829(99)10005-5].