The layered perovskites (C₃H₇NH₃)₂MnCl₄ (PAMC) and (C₂H₅NH₃)₂MnCl₄ (EAMC): birefringence studies and the symmetry of the commensurately modulated ε phase of PAMC

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 δ → γ transition and through the normal-commensurate ζ → [epsilon] transition, whereas the commensurate-normal [epsilon] → δ and the incommensurate-normal γ → β 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 Δn ~ (1 -- T/T_c)^2β are used to describe the birefringence where appropriate. By analyzing systematic extinctions amongst main and satellite reflexions, the [epsilon] phase of PAMC is proposed to be a (3 + 1)-dimensional commensurately modulated version of the δ phase. The superspace group is determined to be P^Abma_s1. With the rationality of the modulation vector this leads to the three-dimensional space group Pbma.