Recently, another consequence of the size of the Caions has been discovered. Iyo et al. 15 have found that a family of ordered CaAFe 4 As 4 (1144) compounds can be formed for A = K, Rb, Cs where the key to the formation is the difference in ionic size between the Ca and the A ion. This family is not a (Ca 1−x A x )Fe 2 As 2 solid-solution, where the Ca and A ions randomly occupy a single crystallographic site, 16 but rather is a distinct, quaternary, line compound in which the Ca and A sites form alternating planes along the crystallographic c-axis, separated by FeAs slabs 15 . In essence, the CaAFe 4 As 4 structure is identical to the CaFe 2 As 2 structure, just with layer by layer segregation of the Ca and A ions. The 1144 structure was also found for SrAFe 4 As 4 (A = Rb, Cs). Solid-solutions of Ca (Sr) 122 structures were found for arXiv:1605.05617v2 [cond-mat.supr-con]

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