Static and dynamic properties of the order-disorder phase transition in KSCN and related crystals

A review of experiments and theoretical descriptions for the order-disorder phase transition in MSCN (M = K, Rb, NH4, TI) is given. These crystals which have an appealing structure exhibit a phase transition from an orthorhombic low-temperature phase to a tetragonal high-temperature phase at elevated temperatures. Emphasis is placed on KSCN which was investigated intensively during the last few years. We present a summary of quite complementary measurements, of birefringence, elastic constants, diffuse neutron scattering and nuclear magnetic resonance and relate these results to earlier data in the literature. Distinct precursor effects which show up in diffuse neutron scattering preface this phase transition in both phases. These order parameter fluctuations are however limited; as a result of the first-order nature of the phase transition in KSCN. The phase transitions in RbSCN and TISCN are very close to a tricritical point. The dynamical behaviour of the phase transition in KSCN is rather slow. NMR-measurements have shown a breaking of the tetragonal symmetry on the time scale 10(-5) s < T < 10(-2) s. This implies, that the high-temperature phase of KSCN consists of small orthorhombic microdomains (clusters) which have a lifetime longer than 10(-5) s and shorter than 10(-2) s. Since the SCN- -reorientations couple strongly to the lattice strains, this slow dynamics of the order parameter fluctuations can be studied by low-frequency elastic measurements. Concerning the thermodynamic properties of the phase transition we show that they can be quite consistently described on the basis of a Mean-Field Compressible Pseudospin Model. In this model the first-order nature of the phase transition turns out to be due to the strong coupling between the SCN--reorientations and the lattice strain. This order parameter-strain coupling which is also responsible for the fast saturation of the order parameter with respect to temperature leads to the characteristic anomalies which have been observed in the elastic constants, thermal expansion, birefringence, specific heat and diffuse neutron scattering around T,.