Ferroelectricity Induced by Cations of Nonequivalent Spins Disordered in the Weakly Ferromagnetic Perovskites,YCr_1–<i>x</i>M_<i>x</i>O₃ (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. arXiv2012, 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, YCr0.5Fe0.5O3, where the Cr3+ (S = 3/2) and Fe3+ (S = 5/2) ions at the B-site are disordered, exhibits ferroelectricity at TN = 260 K. Ferroelectricity is observed not only when the relative proportions of Fe3+ and Cr3+ ions as in YCr1–xFexO3 are varied but also when Y3+ ions are replaced by other nonmagnetic R-ion and the transition metal ions changed as in YCr1–xMnxO3. 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.