Dielectric dispersion and absorption and proton spin-lattice relaxation at the phase transition in the layered crystal SnCl2·2H2O

Measurements of the real and imaginary parts of the dielectric constant at audio frequencies and of proton spin-lattice relaxation in the laboratory and in the rotating frames in a single crystal around the ferroelectric-type phase transition are reported. The experimental results are related to the order-disorder critical dynamics of the protons in the two-dimensional hydrogen-bonded network and discussed, mainly in the light of the dynamical theories for planar lattices of interacting Ising spins. The occurrence of a nonclassical critical behavior, connected to the short range of the interactions and to the restricted dimensionality, has been put in evidence. The static uniform susceptibility diverges with a critical exponent close to 1.75. From $T_{1\rho }$ the relaxation time of the polarization fluctuations is evaluated ${\tau }_p\simeq 1.2\times {10}^{-5\mathrm{}}$ sec at $T\simeq T_c+50\mathrm{}$°K, while from the dielectric dispersion ${\tau }_p\simeq 1.8\times {10}^{-2\mathrm{}}$ sec at $T\simeq T_c+2\mathrm{}$°K, in agreement with a divergence with critical exponent around 2. For $T\lesssim T_c+1.5\mathrm{}$°K, however, ${\tau }_p$ decreases on approaching the transition, probably due to crossover towards three-dimensional antiferroelectrically correlated fluctuations. Finally, a polydispersive nature of the dynamical susceptibility near $T_c$ is evidenced.