Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0 GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.