Real-Space Infrared Spectroscopy of Ferroelectric Domain Walls in Multiferroic h-(Lu,Sc)FeO3.

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.

Electron-Beam Induced Emergence of Mesoscopic Ordering in Layered MnPS3.

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.

High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)].

We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2[FeCl5·(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)2[FeCl5·(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.

Spectroscopic Analysis of Vibronic Relaxation Pathways in Molecular Spin Qubit [Ho(W5O18)2]9-: Sparse Spectra Are Key.

Vibrations play a prominent role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the magneto-infrared properties of the molecular spin qubit system Na9[Ho(W5O18)2]·35H2O. Our results place significant constraints on the pattern of crystal field levels and the vibrational excitations allowing us to unravel vibronic decoherence pathways in this system. We observe field-induced spectral changes near 63 and 370 cm-1 that are modeled in terms of odd-symmetry vibrations mixed with f-manifold crystal field excitations. The overall extent of vibronic coupling in Na9[Ho(W5O18)2]·35H2O is limited by a modest coupling constant (on the order of 0.25) and a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and MJ levels apart. These findings advance the understanding of vibronic coupling in a molecular magnet with atomic clock transitions and suggest strategies for designing molecular spin qubits with improved coherence lifetimes.

Excitations of Intercalated Metal Monolayers in Transition Metal Dichalcogenides.

We combine Raman scattering spectroscopy and lattice dynamics calculations to reveal the fundamental excitations of the intercalated metal monolayers in the FexTaS2 (x = 1/4, 1/3) family of materials. Both in- and out-of-plane modes are identified, each of which has trends that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. We test these trends against the response of similar systems, including Cr-intercalated NbS2 and RbFe(SO4)2, and demonstrate that the metal monolayer excitations are both coherent and tunable. We discuss the consequences of intercalated metal monolayer excitations for material properties and developing applications.

Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices.

Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ â†’  Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.

Developing the Pressure-Temperature-Magnetic Field Phase Diagram of Multiferroic [(CH3)2NH2]Mn(HCOO)3.

We combined Raman scattering and magnetic susceptibility to explore the properties of [(CH3)2NH2]Mn(HCOO)3 under compression. Analysis of the formate bending mode reveals a broad two-phase region surrounding the 4.2 GPa critical pressure that becomes increasingly sluggish below the order-disorder transition due to the extensive hydrogen-bonding network. Although the paraelectric and ferroelectric phases have different space groups at ambient-pressure conditions, they both drive toward P1 symmetry under compression. This is a direct consequence of how the order-disorder transition changes under pressure. We bring these findings together with prior magnetization work to create a pressure-temperature-magnetic field phase diagram, unveiling entanglement, competition, and a progression of symmetry-breaking effects that underlie functionality in this molecule-based multiferroic. That the high-pressure P1 phase is a subgroup of the ferroelectric Cc suggests the possibility of enhanced electric polarization as well as opportunity for strain control.

Spin-Lattice Coupling Across the Magnetic Quantum-Phase Transition in Copper-Containing Coordination Polymers.

We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu(pyz)2(2-HOpy)2](PF6)2 and [Cu(pyz)1.5(4-HOpy)2](ClO4)2, under different external stimuli in order to explore the microscopic aspects of spin-lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic → fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF6- compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO4- system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin-lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu(pyz)1.5(4-HOpy)2](ClO4)2 because of its ladderlike character. Although spin-lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO4- compound may be ferroelectric.

Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications.

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.

Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine.

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(Hsq23-)(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.

Large Positive Zero-Field Splitting in the Cluster Magnet Ba3CeRu2O9.

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 Stot = 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.

Frustration and Glasslike Character in RIn1- xMn xO3 (R = Tb, Dy, Gd).

We bring together ac susceptibility and dc magnetization to uncover the rich magnetic field-temperature behavior of a series of rare earth indium oxides, RInO3 (R = Tb, Dy, and Gd). The degree of frustration is much larger than expected, particularly in TbInO3, and the ground states are glasslike with antiferromagnetic tendencies. The activation energy for spin reorientation is low. Chemical substitution with Mn3+ ions to form TbIn1- xMn xO3 ( x ≤ 0.01) relieves much of the frustration that characterizes the parent compound and slightly enhances the short-range antiferromagnetic order. The phase diagrams developed from this work reveal the rich competition between spin orders and provide an opportunity to compare the dynamics in the RInO3 and Mn-substituted systems. These structure-property relations may be useful for understanding magnetism in other geometrically frustrated multiferroics.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni).

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH3)2NH2] M(HCOO)3 ( M = Mn2+, Co2+, Ni2+) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH3)2NH2]Mn(HCOO)3 takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across TC reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH3)2NH2]Co(HCOO)3 reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co2+ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Charge and Bonding in CuGeO3 Nanorods.

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO3 nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides.

Magnetic field control of charge excitations in CoFe2O4.

We combine magnetic circular dichroism and photoconductivity with prior optical absorption and first principles calculations to unravel spin-charge interactions in the high Curie temperature magnet CoFe2O4. In addition to revising the bandgap hierarchy, we reveal a broad set of charge transfer excitations in the spin down channel which are sensitive to the metamagnetic transition involving the spin state on Co centers. We also show photoconductivity that depends on an applied magnetic field. These findings open the door for the creation and control of spin-polarized electronic excitations from the minority channel charge transfer in spinel ferrites and other earth-abundant materials.

Spin-Lattice Coupling in [Ni(HF2)(pyrazine)2]SbF6 Involving the HF2- Superexchange Pathway.

Magnetoelastic coupling in the quantum magnet [Ni(HF2)(pyrazine)2]SbF6 has been investigated via vibrational spectroscopy using temperature, magnetic field, and pressure as tuning parameters. While pyrazine is known to be a malleable magnetic superexchange ligand, we find that HF2- is surprisingly sensitive to external stimuli and is actively involved in both the magnetic quantum phase transition and the series of pressure-induced structural distortions. The amplified spin-lattice interactions involving the bifluoride ligand can be understood in terms of the relative importance of the intra- and interplanar magnetic energy scales.

Local Lattice Distortions in Mn[N(CN)2]2 under Pressure.

We combined synchrotron-based infrared spectroscopy, Raman scattering, and diamond anvil cell techniques with complementary lattice dynamics calculations to reveal local lattice distortions in Mn[N(CN)2]2 under compression. Strikingly, we found a series of transitions involving octahedral counter-rotations, changes in the local Mn environment, and deformations of the superexchange pathway. In addition to reinforcing magnetic property trends, these pressure-induced local lattice distortions may provide an avenue for the development of new functionalities.

Pressure-induced magnetic crossover driven by hydrogen bonding in CuF2(H2O)2(3-chloropyridine).

Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. In molecular materials, these interactions determine assembly mechanisms, control superconductivity, and even permit magnetic exchange. In spite of its long-standing importance, exquisite control of hydrogen bonding in molecule-based magnets has only been realized in limited form and remains as one of the major challenges. Here, we report the discovery that pressure can tune the dimensionality of hydrogen bonding networks in CuF2(H2O)2(3-chloropyridine) to induce magnetic switching. Specifically, we reveal how the development of O-H···Cl exchange pathways under compression combined with an enhanced ab-plane hydrogen bonding network yields a three dimensional superexchange web between copper centers that triggers a reversible magnetic crossover. Similar pressure- and strain-driven crossover mechanisms involving coordinated motion of hydrogen bond networks may play out in other quantum magnets.

Spectroscopic determination of phonon lifetimes in rhenium-doped MoS2 nanoparticles.

We investigated the infrared vibrational properties of pristine and Re-substituted MoS2 nanoparticles and analyzed the extracted phonon lifetimes in terms of multiple scattering events. Our measurements reveal both size- and doping-dependent changes that we attribute to grain boundary scattering and charge and mass effects, respectively. By contrast, Born charge is affected only by size. These findings illustrate the utility of reaching beyond traditional bulk semiconductors and quantum dots to explore how doping and confinement impact carrier-phonon interactions in low-dimensional semiconducting nanomaterials.

Observation of a Burstein-Moss shift in rhenium-doped MoS2 nanoparticles.

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.