Elucidating the Mechanism of Multiferroicity in (NH₄)₃Cr(O₂)₄ and Its Tailoring by Alkali Metal Substitution

The antiferromagnetic Cr­(V) peroxychromates, M3Cr­(O2)4, M = K, Rb, and Cs, become ferroelectric
when
mixed with NH4+, but the underlying mechanism
is not understood. Our dielectric relaxation, Raman scattering, and
high-frequency EPR measurements on the M3–x(NH4)xCr­(O2)4 family clarify this mechanism. At 295 K, (NH4)3Cr­(O2)4 is tetragonal (I4̅2m), with the NH4+ ions occupying two distinctly different sites, N1 and N2.
A ferroelectric transition at Tc1 = 250
K is revealed by λ-type anomalies in Cp and dielectric constant, and lowering of symmetry to Cmc2­(1). Below Tc1, the N1 sites
lose their tetrahedral symmetry and thus polarization develops. Raman
detection of translational modes involving the NH4+ ions around 193 cm–1 supports this model.
EPR around Tc1 revealed that the [Cr­(O2)4]3– ions reorient
by about 10°. A minor peak at Tc2 ≈ 207 K is attributed to a short-range ordering that culminates
in a long-range, structural order at Tc3 ≈ 137 K. At Tc3, 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 Tc1 is
related to the torsional motion of the N1 site, while Tc2 and Tc3 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 NH4+ ions
and is tunable by their controlled substitutions. Relevance to other
classes of possible multiferroics is pointed out.