A proton-transfer reaction between squaric acid (H2sq) and 2,3-dimethylpyrazine (2,3-Me2pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me2pyzH+)(Hsq–)·H2O (1), which ...possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me2pyzH+ cations and strips featuring extensive hydrogen bonding between the Hsq– anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me2pyzH+)2(Hsq2 3–)(H5O2 +), whereas the AFE phase can be described as (2,3-Me2pyzH+)(Hsq–)(H2O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.
van der Waals materials are exceptionally responsive to external stimuli. Pressure-induced layer sliding, metallicity, and superconductivity are fascinating examples. Inspired by opportunities in ...this area, we combined high-pressure optical spectroscopies and first-principles calculations to reveal piezochromism in MnPS3. Dramatic color changes (green → yellow → red → black) take place as the charge gap shifts across the visible regime and into the near infrared, moving systematically toward closure at a rate of approximately −50 meV/GPa. This effect is quenched by the appearance of the insulator–metal transition. In addition to uncovering an intriguing and tunable functionality that is likely to appear in other complex chalcogenides, the discovery that piezochromism can be deterministically controlled at room temperature accelerates the development of technologies that take advantage of stress-activated modification of electronic structure.
Abstract
Kagomé metals are widely recognized, versatile platforms for exploring topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In the
R
V
...6
Sn
6
family of materials (
R
= Sc, Y, Lu), ScV
6
Sn
6
hosts an unusual charge density wave ground state as well as structural similarities with the
A
V
3
Sb
5
system (
A
= K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal phonon mixing processes in the charge density wave state of ScV
6
Sn
6
. In the low temperature phase, we find at least four new peaks in the vicinity of the V-containing totally symmetric mode near 240 cm
−1
suggesting that the density wave acts to mix modes of
P
6/
m
m
m
and
$$R\bar{3}m$$
R
3
¯
m
symmetry - a result that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the short-range ordered density wave state under compression and propose that both physical and chemical pressure quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system.
We present the synthesis and magnetic characterization of a polycrystalline sample of the 6H-perovskite Ba3CeRu2O9, which consists of Ru dimers based on face-sharing RuO6 octahedra. Our ...low-temperature magnetic susceptibility, magnetization, and neutron powder diffraction results reveal a nonmagnetic singlet ground state for the dimers. Inelastic neutron scattering, infrared spectroscopy, and the magnetic susceptibility over a wide temperature range are best explained by a molecular orbital model with a zero-field splitting parameter D = 85 meV for the S tot = 1 electronic ground-state multiplet. This large value is likely due to strong mixing between this ground-state multiplet and low-lying excited multiplets, arising from a sizable spin molecular orbital coupling combined with an axial distortion of the Ru2O9 units. Although the positive sign for the splitting ensures that Ba3CeRu2O9 is not a single molecule magnet, our work suggests that the search for these interesting materials should be extended beyond Ba3CeRu2O9 to other molecular magnets based on metal–metal bonding.
Ordered mesoscale structures in 2D materials induced by small misorientations have allowed for a wide variety of electronic, ferroelectric, and quantum phenomena to be explored. Until now, the only ...mechanism to induce this periodic ordering was via mechanical rotations between the layers, with the periodicity of the resulting moiré pattern being directly related to twist angle. Here we report a fundamentally distinct mechanism for emergence of mesoscopic periodic patterns in multilayer sulfur-containing metal phosphorus trichalcogenide, MnPS3, induced by the electron beam. The formation under the beam of periodic hexagonal patterns with several characteristic length scales, nucleation and transitions between the phases, and local dynamics are demonstrated. The associated mechanisms are attributed to the relative contraction of the layers caused by beam-induced sulfur vacancy formation with subsequent ordering and lattice parameter change. As a result, the plasmonic response of the system is locally altered, suggesting an element of control over plasmon resonances by electron beam patterning. We pose that harnessing this phenomenon provides both insight into fundamental physics of quantum materials and enables device applications by enabling controlled periodic potentials on the atomic scale.
We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2FeCl5·(H2O)a type II molecular ...multiferroic in which the mixing between charge, structure, and magnetism is controlled by intermolecular hydrogen and halogen bonds. Electric polarization is sensitive to the series of field-induced spin reorientations, increasing linearly with the field and reaching a maximum before collapsing to zero across the quasi-collinear to collinear-sinusoidal reorientation due to the restoration of inversion symmetry. Magnetoelectric coupling is on the order of 1.2 ps/m for the P∥c, H∥c configuration between 5 and 25 T at 1.5 K. In this range, the coupling takes place via an orbital hybridization mechanism. Other forms of mixing are active in (NH4)2FeCl5·(H2O) as well. Magneto-infrared spectroscopy reveals that all of the vibrational modes below 600 cm–1 are sensitive to the field-induced transition to the fully saturated magnetic state at 30 T. We analyze these local lattice distortions and use frequency shifts to extract spin-phonon coupling constants for the Fe–O stretch, Fe–OH2 rock, and NH4 + libration. Inspection also reveals subtle symmetry breaking of the ammonium counterions across the ferroelectric transition. The coexistence of such varied mixing processes in a platform with intermolecular hydrogen- and halogen-bonding opens the door to greater understanding of multiferroics and magnetoelectrics governed by through-space interactions.
Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, ...because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I “Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications”, which took place within the 2018 European Materials Research Society fall meeting.
van der Waals antiferromagnets with chemical formula M P X 3 ( M = V, Mn, Fe, Co, Ni, Cd; X = S, Se) are superb platforms for exploring the fundamental properties of complex chalcogenides, revealing ...their structure-property relations and unraveling the physics of confinement. Pressure is extremely effective as an external stimulus, able to tune properties and drive new states of matter. In this review, we summarize experimental and theoretical progress to date with special emphasis on the structural, magnetic, and optical properties of the M P X 3 family of materials. Under compression, these compounds host inter-layer sliding and insulator-to-metal transitions accompanied by dramatic volume reduction and spin state collapse, piezochromism, possible polar metal and orbital Mott phases, as well as superconductivity. Some responses are already providing the basis for spintronic, magneto-optic, and thermoelectric devices. We propose that strain may drive similar functionality in these materials.
We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (h) Lu0.6Sc0.4FeO3, and we compare our findings with a ...detailed symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in h-Lu0.6Sc0.4FeO3 are broad and semiconducting, a finding that we attribute to the presence of an A-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the A site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexible defect states.
We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (
) Lu
Sc
FeO
, and we compare our findings with a detailed ...symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in
-Lu
Sc
FeO
are broad and semiconducting, a finding that we attribute to the presence of an
-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the
site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexible defect states.
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