Critical Phenomena at the Normal-Incommensurate Phase Transition in Rb2ZnCl4

The X-ray reflection intensity and the dielectric constant are measured around the normal-incommensurate phase transition. From the temperature dependence of a satellite reflection at \((2~0~2/3+δ)\), the critical exponent 2 β is determined to be 0.69 ±0.01. This value is very close to the exponent of the three-dimensional n =2-vector model. The dielectric constant along the ferroelectric axis shows a weak anomaly at the transition point, which is discussed with the aid of the remormalization-group method.