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 M_3–<i>x</i>(NH₄)_<i>x</i>Cr­(O₂)₄ family clarify this mechanism. At 295 K, (NH₄)₃Cr­(O₂)₄ is tetragonal (<i>I</i>4̅2<i>m</i>), with the NH₄⁺ ions occupying two distinctly different sites, N1 and N2. A ferroelectric transition at <i>T</i>_c1 = 250 K is revealed by λ-type anomalies in <i>C</i>_p and dielectric constant, and lowering of symmetry to C<i>mc</i>2­(1). Below <i>T</i>_c1, 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 <i>T</i>_c1 revealed that the [Cr­(O₂)₄]^<b>3–</b> ions reorient by about 10°. A minor peak at <i>T</i>_c2 ≈ 207 K is attributed to a short-range ordering that culminates in a long-range, structural order at <i>T</i>_c3 ≈ 137 K. At <i>T</i>_c3, the symmetry is lowered to <i>P</i>1 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 <i>T</i>_c2 and <i>T</i>_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 NH₄⁺ ions and is tunable by their controlled substitutions. Relevance to other classes of possible multiferroics is pointed out

Topotactic Transformations of Metal–Organic Frameworks to Highly Porous and Stable Inorganic Sorbents for Efficient Radionuclide Sequestration

Innovative solid-phase sorbent technologies are needed to extract radionuclides from harsh media for environmental remediation and in order to close the nuclear fuel cycle. Highly porous inorganic materials with remarkable sorptive properties have been prepared by topotactic transformations of metal–organic frameworks (MOFs) using both basic and acidic solutions. Treatment of Ti and Zr nanoMOFs with NaOH, Na₃PO₄, and H₃PO₄ yields Ti and Zr oxides, oxyphosphates, and phosphates via sacrificial removal of the organic ligands. This controlled ligand extraction process results in porous inorganic materials, which preserve the original MOF morphologies and impart useful surface functionalities, but are devoid of organic linkers. Structural investigation by X-ray absorption spectroscopy reveals preservation of the coordination environment of the scattering metal. Changing the MOF template introduces different metal and structural possibilities, while application of different digest solutions allows preparation of metal oxides, metal oxyphosphates, and metal phosphates. The high stability and porosity of these novel materials makes them ideally suited as nanosorbents in severe environments. Their potential for several radionuclide separations is demonstrated, including decontamination of high level nuclear waste, extraction of lanthanides, and remediation of radionuclide-contaminated seawater