Studies of the elastic, dielectric and thermal expansion properties of CsH2PO4

Cesium dihydrogen phosphate CsH₂PO₄ (CDP) is a ferroelectric with ordering temperature T_c = 154 K. In many respects CDP is a unique material, displaying a chain and layer-like structure and a pseudo-one-dimensional ordering of the hydrogen ions at T_c, making it an attractive candidate for the application of a pseudo-one-dimensional Ising model.

The elastic constants of a material completely describe its

elastic behaviour. As a first step in the understanding of the lattice dynamics of CDP, the room temperature elastic constants have been determined using ultrasonic velocity measurements. The constants have been used to calculate a number of elastic properties: the phase and group velocities, Young's modulus, the bulk modulus, linear compressibilities, and the elastic Debye temperature. The calculations showed a marked elastic anisotropy which has been correlated with the chain and layer-like structure of this material.

The one-dimensional nature of the ordering of the hydrogen ions can be expected to be evident in a wide range of critical phenomena. In an attempt to elucidate further the transition mechanism of CDP, the temperature dependences of the velocity and attenuation, thermal expansion, and dielectric constant were measured in the critical region. The critical point analyses of the dielectric and ultrasonic anomalies showed evidence of a temperature region above T_c in which one-dimensional, short-range forces were dominant. However, very close to T_c, the usual 3-D, long-range, dipole-dipole interaction was found to be dominant. Calculations of the Grilneisen parameters based on the expansion measurements have revealed that, from the thermodynamic point of view, the degree of "one-dimensionality" of CDP increases as the transition is approached. However, despite the one-dimensional nature of the transition, a significant anomaly was observed in the dielectric constant measured orthogonal to the ferroelectric axis. Whilst most of the analysis of this study was for T > T_c, a very long term relaxation effect in the dielectric constant was observed below T_c , which suggested that CDP has a very "soft" domain structure in the ferroelectric state. Several suggestions are made as to possible further investigations into these domain effects. The findings of this study constitute one more step in the overall understanding of the lattice dynamics of CDP. Directions for future research based on these findings are also presented.