Comment on "Structural, dielectric, and magnetic characteristics of Bi(Ni0.25Ti0.25Fe0.50)O3 ceramics" [J. Mater. Sci.: Mater. Electron. 27, 1209 (2016)]; "Structural and electrical characteristics of (Co, Ti)-modified BiFeO3" [J. Mater. Sci.: Mater. Electron. 27, 7115 (2016)]; "Structural, electrical, and magnetic characteristics of Ni/Ti-modified BiFeO3 lead-free multiferroic material" [J. Mater. Sci.: Mater. Electron. 28, 6673 (2017)]

This comment on the above three papers argues that the crystals should be of sillenite type and the correct formula should be Bi₂₅FeO₃₉:MTi (M = Co or Ni) instead of BiFeO₃. Due to the doping, a new type of unit cell, previously unknown, was proposed for such crystals. There is also about 15% of a secondary phase, BiFeO₃, in the sample. This part of the sample should be responsible for the magnetic properties recorded. [ABSTRACT FROM AUTHOR]