Linking NH4+ motion to magnetism in molecular multiferroic (NH4)2[FeCl5(H2 O)]: A neutron vibrational spectroscopy study

W. Tian, L. L. Daemen, Y. Q. Cheng, Fei Li, Jaime A. Fernandez-Baca

Research output: Contribution to journalArticlepeer-review

Abstract

We present a neutron vibrational spectroscopy study to investigate the influence of NH4+ motion on the magnetism in [(NH4)1-xKx]2[FeCl5(H2O)]. The parent compounds, (NH4)2[FeCl5(H2O)] (x=0) and K2[FeCl5(H2O)](x=1) are isostructural at room temperature, yet displaying drastically different magnetic and multiferroic behavior. K2[FeCl5(H2O)] is nonmultiferroic with type-A collinear antiferromagnetic structure below TN≈14.06K, whereas (NH4)2[FeCl5(H2O)] is a type-II multiferroic with incommensurate cycloidal spin structure below TFE≈6.8K. A recent study of the dielectric, structure, and magnetic properties in the mixed [(NH4)1-xKx]2[FeCl5(H2O)] shows that a small amount of potassium substitution to replace NH4+ transforms the spin structure from incommensurate cycloidal (x≤ 0.06) into commensurate collinear antiferromagnetic (x≥ 0.15), indicating NH4+ is essential to the emergent phenomena observed in this molecular multiferroic compound. Our vibrational spectroscopy study reveals that NH4+ libration and torsion motion exhibit substantial temperature dependence at low temperatures. The intensity of NH4+ libration and torsion modes increases slightly at 5 K in comparison with data at 25 K behaving like a magnon, indicating that they are coupled to the magnetism in (NH4)2[FeCl5(H2O)]. Comparing data of x=0, 0.06, 0.09, and 0.15 samples further illustrates that the strength of the increased signal in NH4+ libration mode is very sensitive to potassium concentration. The signal diminishes quickly with increasing potassium concentration and vanishes in the x=0.15 sample corresponding to the magnetic structure change for x≥ 0.15. The results directly link the anomalous behavior in NH4+ libration motion to the magnetism in [(NH4)1-xKx]2[FeCl5(H2O)], providing new insights into the crucial role NH4+ plays in the coupled phenomena in (NH4)2[FeCl5(H2O)]. The unique information opens a new door to go through in searching for new multifunctional materials by incorporation of NH4 via a material-by-design approach.

Original languageEnglish
Article number064410
JournalPhysical Review B
Volume110
Issue number6
DOIs
StatePublished - Aug 1 2024

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