ROLE OF THE PROTON TUNNELING IN THE PHASE-TRANSITION OF K3D1-XHX(SO4)(2)

Dielectric and thermodynamic properties have been investigated for isotopically mixed crystals of K3D1-xHx(SO4)(2), which are composed of structurally isolated dimeric units SO42-...H-SO42-. The critical temperature T-c for the antiferroelectric phase transition decreases with proton concentration (x) and eventually the phase transition vanishes at x(c) similar to 0.66+/-0.04. The x-dependent behavior of T-c, the dielectric constant epsilon(T) and the specific heat C(T) are well accounted for by the mean-field theory for the transverse Ising model, indicating an important role of the proton tunneling on the phase transition and related properties. These features are compared with conventional hydrogen-bonded dielectrics with infinite networks of the hydrogen bonds.