ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
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 → ε transition, whereas the commensurate-normal ε → 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/Tc)2β are used to describe the birefringence where appropriate. By analyzing systematic extinctions amongst main and satellite reflexions, the ε 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 PAbmas_{{\bar 1}}1. With the rationality of the modulation vector this leads to the three-dimensional space group Pbma.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0567739482000242
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