Elucidating the Mechanism of Multiferroicity in (NH_4)_3Cr(O_2)_4 and Its Tailoring by Alkali Metal Substitution

摘要

The antiferromagnetic Cr(V) peroxychromates, M_3Cr(O_2)4, M = K, Rb, and Cs, become ferroelectric when mixed with NH_4~+, but the underlying mechanism is not understood. Our dielectric relaxation, Raman scattering, and high-frequency EPR measurements on the M_(3-x)(NH_4)_xCr(O_2)_4 family clarify this mechanism. At 295 K, (NH_4)_3Cr(O_2)_4 is tetragonal (I42m), with the NH_4~+ ions occupying two distinctly different sites, N1 and N2. A ferroelectric transition at T_(c1) = 250 K is revealed by λ-type anomalies in C_p and dielectric constant, and lowering of symmetry to Cmc2(1). Below T_(c1), the N1 sites lose their tetrahedral symmetry and thus polarization develops. Raman detection of translational modes involving the NH_4~+ ions around 193 cm~(-1) supports this model. EPR around T_(cl) revealed that the [Cr(O_2)_4]~(3-) ions reorient by about 10°. A minor peak at T_(c2) ≈ 207 K is attributed to a short-range ordering that culminates in a long-range, structural order at T_(c3) ≈ 137 K. At T_(c3), the symmetry is lowered to P1 with significant changes in the cell parameters. Rb~+ and Cs~+ substitutions that block the N1 and N2 sites selectively show that T_(c1) is related to the torsional motion of the N1 site, while T_(c2) and T_(c3) are governed by the motional slowing down of the N2 site. These data show that the multiferroic behavior of this family is governed by the rotational and translational dynamics of the NH_4~+ ions and is tunable by their controlled substitutions. Relevance to other classes of possible multiferroics is pointed out.