Ferroelectricity Induced by Cations of Nonequivalent Spins Disordered in the Weakly Ferromagnetic Perovskites,YCr_1–xM_xO₃ (M = Fe or Mn)

Rare-earth orthochromites, RCrO₃ (R = rare-earth and Y) with magnetic R-ions are reported to exhibit ferroelectricity at the magnetic ordering temperature of Cr³⁺-ions (Rajeswaran, B.; Khomskii, D. I.; Zvezdin, A. K.; Sundaresan, A.; Rao, C. N. R. arXiv 2012, 1201, 0826v1 ). Interestingly, we observe ferroelectricity at the Néel temperature of orthochromites containing a nonmagnetic R-ion (Y) upon introducing cation-disorder at the Cr-site by incorporation of another magnetic ion of nonequivalent spin. For example, the weakly ferromagnetic perovskite, YCr_0.5Fe_0.5O₃, where the Cr³⁺ (S = 3/2) and Fe³⁺ (S = 5/2) ions at the B-site are disordered, exhibits ferroelectricity at T_N = 260 K. Ferroelectricity is observed not only when the relative proportions of Fe³⁺ and Cr³⁺ ions as in YCr_1–xFe_xO₃ are varied but also when Y³⁺ ions are replaced by other nonmagnetic R-ion and the transition metal ions changed as in YCr_1–xMn_xO₃. While the local polarization cancels out in the weakly ferromagnetic YMO₃ (M = Cr and Fe), a nonzero polarization in these materials seem to arise due to disordered transition metal ions of nonequivalent spins.