Abstract

By measuring the birefringence on (001) platelets of PAMC as a function of temperature, previously determined phase transitions have been confirmed. Large effects were measured on passing through the normal-incommensurate transition and through the normal-commensurate transition, whereas the commensurate-normal and the incommensurate-normal transitions are less pronounced, in accordance with former thermoanalytic studies. A comparison is made with a similar study on EAMC. The optical anisotropy is discussed for both compounds in terms of the structural changes of the perovskite-type layers. Power laws of the form n ~ (1 -- T/Tc)2 are used to describe the birefringence where appropriate. By analyzing systematic extinctions amongst main and satellite reflexions, the phase of PAMC is proposed to be a (3 + 1)-dimensional commensurately modulated version of the phase. The superspace group is determined to be PAbmas1. With the rationality of the modulation vector this leads to the three-dimensional space group Pbma.