Symmetry analysis of the structural and magnetic phase transitions in 122 iron arsenides

It is evident from the literature that the 122 iron arsenides, XFe₂As₂ with X = Ca, Sr, Ba or Eu, undergo one or more phase transitions from a higher-temperature paramagnetic tetragonal structure in grey group I4/mmm1' to an antiferromagnetic structure with magnetic space group C_Amca. Symmetry analysis is used to enumerate the possibilities for the transition (or transitions) between the higher- and lower-symmetry structures, and the results are used as a basis for comment on published experimental results.