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The Raman frequency of the coupled mode v À associated with the order parameter is calculated as a function of pressure from the mean field theory in the ferroelecric phase of KDP. Using the frequencies of the coupled mode, the pressure dependence of the damping constant G À is calculated by the proton spin 1 Introduction As a ferroelectric (FE) material, KDP (KH 2 PO 4 ) was discovered [1] and its order-disorder phase transition (T C ¼ 122 K) due to the proton [2] was studied many years ago. This transition is of a second order from the paraelectric (PE) to the FE phase as the temperature decreases at the atmospheric pressure. At room temperature, this FE crystal is in the PE phase with the tetragonal structure and I42d (D

Temperature dependences of the spontaneous polarization and the dielectric constant are calculated near the paraelectric-ferroelectric (cubic-tetragonal) transition in BaTiO3 using our mean field model. By expanding the free energy in terms of the spontaneous polarization (order parameter), expressions for the temperature dependence of the spontaneous polarization and the dielectric constant are derived. By considering the temperature dependence of the Raman frequencies for the lattice mode (≈ 310 cm(-1)) which is related to the spontaneous polarization, the experimental data from the literature is analyzed near the first order paraelectric-ferroelectric transition in BaTiO3. The dielectric constant is then calculated as a function of temperature for the cubic-tetragonal transition in BaTiO3. Our results show that the observed behavior of the spontaneous polarization in the ferroelectric phase (TTC) and ferroelectric phases, can be described adequately by the mean field model studied here for BaTiO3.

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