Oxygen-vacancy ordering in multiferroic Bi1−xCaxFeO3−x/2 (0.1 ⩽ x ⩽ 0.5) G. L. Cao, C. Ma, Z. Chen, H. X. Yang, H. F. Tian and J. Q. Li EPL, 102 2 (2013) 27002 Published online: 07 May 2013 DOI: 10.1209/0295-5075/102/27002