Phenomenological Theory of an Isomorphous Transition in Polar Crystals-Application to the II-III Transition in Ferroelectric Ca_2Pb(C_2H_5COO)_6

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A phenomenological theory is presented for a possible isomorphous phase transition in polar crystals. The elastic Gibbs function $G$ is expanded in a power series of a deviation of the polarization $P$ from a fixed value $P_{0}$ as; $G{=}G_{0}+a_{0}(T-T_{0}-Kp)\varDelta P+b_{0}(p-p_{0})(\varDelta P)^{2}+c(\varDelta P)^{3}+d(\varDelta P)^{4}$, where $T$ and $p$ are temperature and hydrostatic pressure, respectively, and $\varDelta P{\equiv}P-P_{0}$. A first order isomorphous phase transition takes place at pressures less than a critical value of $p_{\text{crit}}{=}p_{0}+3c^{2}/(8b_{0}d)$, and above the critical pressure no phase transition exists. The critical behavior of the order parameter, dielectric constant, and specific heat is discussed. By choosing proper values of parameters, a qualitative agreement is obtained betweent the calculated and the observed pressure and temperature dependence of low frequency dielectric constant of Ca2Pb(C2H5COO)6 in the vicinity of the critical point.
一般社団法人日本物理学会の論文
1976-02-15