Achieving Canted-Spin Weak Ferromagnetism and Negative Thermal Expansion in A- and B-Site-Substituted Bismuth Ferrite

BiFeO₃ is the most intensively investigated multiferroic material. It has a cycloidal spin modulation superimposed on G-type antiferromagnetic ordering of Fe³⁺ (S = 5/2), which prohibits the appearance of a net magnetization. We found that the simultaneous substitution of Ru⁴⁺ or Ir⁴⁺ for Fe³⁺ and Ca²⁺ for Bi³⁺ suppressed the cycloidal modulation and induced canted weak ferromagnetism at room temperature, while preserving the polar rhombohedral crystal structure. Moreover, the A- and B-site substitutions substantially lowered the ferroelectric transition temperature and resulted in a 1.77% volumetric negative thermal expansion around room temperature. These findings open an avenue for designing new BiFeO₃-based functional materials.