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 language | English |
|---|---|
| Article number | 064410 |
| Journal | Physical Review B |
| Volume | 110 |
| Issue number | 6 |
| DOIs | |
| State | Published - Aug 1 2024 |
Funding
Neutron scattering experiments were conducted at the VISION beamline at the Oak Ridge National Laboratory's Spallation Neutron Source, which is supported by the Scientific User Facilities Division, Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE). The computing resources were made available through the VirtuES and the ICE-MAN projects, funded by the Laboratory Directed Research and Development program and Compute and Data Environment for Science (CADES) at ORNL.