Comment on “Structural,electrical, and multiferroic characteristics of lead-free multiferroic: Bi(Co0.5Ti0.5)O3–BiFeO3 solid solution” by N. Kumar,A. Shukla,N. Kumar,R. Choudhary and A. Kumar,RSC Adv., 2018, 8, 36939”

My comments concern the significant errors in the crystallographic part of the commented paper. It was found that the studied crystal is of the sillenite type and the correct formula should be Bi₂₅FeO₃₉ : Co,Ti instead of BiFeO₃. Moreover, the type of unit cell is not correct. Due to the double doping the new type of unit cell, previously unknown, was proposed for such crystals. Furthermore, the authors did not find that the studied sample contains two slightly different phases, both of the sillenite type. The actual chemical composition of the studied crystals is not known.

The significant errors in the crystallographic part of the commented paper are described. The new chemical formula of the studied crystal is proposed: Bi₂₅FeO₃₉ : Co,Ti. The coexistence of two sillenite type phases is evidenced.