Magnetophoresis of paramagnetic metal ions in porous media.

We report a numerical investigation of the magnetophoresis of solutions containing paramagnetic metal ions. Using a simulated magnetic field of a superconducting magnet and the convection-diffusion model, we study the transport of transition metal salts through a porous medium domain. In particular, through a detailed comparison of the numerical results of magnetophoretic velocity and ion concentration profiles with prior published experiments, we validate the model. Subsequent to model validation, we perform a systematic analysis of the model parameters on the magnetophoresis of metal ions. Magnetophoresis is quantified with a magnetic Péclet number Pem. Under a non-uniform magnetic field, Pem initially rises, exhibiting a local maximum, and subsequently declines towards a quasi-steady value. Our results show that both the initial and maximum Pem values increase with increasing magnetic susceptibility, initial concentration of metal solutes, and ion cluster size. Conversely, Pem decreases as the porosity of the medium increases. Finally, the adsorption of metal salts onto the porous media surface is modeled through a dimensionless Damkohler number Daad. Our results suggest that the adsorption significantly slows the magnetophoresis and self-diffusion of the paramagnetic metal salts, with a net magnetophoresis velocity dependent on the kinetics and equilibrium adsorption properties of the metal salts. The latter result underscores the crucial role of adsorption in future magnetophoresis research.

Cool carriers: triplet diffusion dominates upconversion yield.

Perovskites have gained popularity both as the active material in photovoltaics and as bulk triplet sensitizers for solid-state triplet-triplet annihilation upconversion (TTA-UC). Prior to widespread implementation into commercial photovoltaics, an in-depth understanding of the environmental influences on device performance is required. To this point, the temperature-dependent structure-function properties of TTA-UC within methylammonium formamidinium lead triiodide (MAFA)/rubrene UC devices are explored. A strong temperature dependence of the underlying UC dynamics is observed, where the maximum UC efficiency is achieved at 170 K, reflecting the competition between triplet diffusion length, diffusion rate, and triplet-triplet encounter events. A combination of spectroscopic and structural methods and theoretical modelling illustrates that despite the significantly increased carrier lifetime of the perovskite at low temperatures, the TTA-UC dynamics are not governed by the underlying sensitizer properties but rather limited by the underlying triplet diffusion.

The curious case of the structural phase transition in SnSe insights from neutron total scattering.

At elevated temperatures SnSe is reported to undergo a structural transition from the low symmetry orthorhombic GeS-type to a higher symmetry orthorhombic TlI-type. Although increasing symmetry should likewise increase lattice thermal conductivity, many experiments on single crystals and polycrystalline materials indicate that this is not the case. Here we present temperature dependent analysis of time-of-flight (TOF) neutron total scattering data in combination with theoretical modeling to probe the local to long-range evolution of the structure. We report that while SnSe is well characterized on average within the high symmetry space group above the transition, over length scales of a few unit cells SnSe remains better characterized in the low symmetry GeS-type space group. Our finding from robust modeling provides further insight into the curious case of a dynamic order-disorder phase transition in SnSe, a model consistent with the soft-phonon picture of the high thermoelectric power above the phase transition.

Investigation of the monopole magneto-chemical potential in spin ices using capacitive torque magnetometry.

The single-ion anisotropy and magnetic interactions in spin-ice systems give rise to unusual non-collinear spin textures, such as Pauling states and magnetic monopoles. The effective spin correlation strength (Jeff) determines the relative energies of the different spin-ice states. With this work, we display the capability of capacitive torque magnetometry in characterizing the magneto-chemical potential associated with monopole formation. We build a magnetic phase diagram of Ho2Ti2O7, and show that the magneto-chemical potential depends on the spin sublattice (α or ß), i.e., the Pauling state, involved in the transition. Monte Carlo simulations using the dipolar-spin-ice Hamiltonian support our findings of a sublattice-dependent magneto-chemical potential, but the model underestimates the Jeff for the ß-sublattice. Additional simulations, including next-nearest neighbor interactions (J2), show that long-range exchange terms in the Hamiltonian are needed to describe the measurements. This demonstrates that torque magnetometry provides a sensitive test for Jeff and the spin-spin interactions that contribute to it.

Metal Halide Scaffolded Assemblies of Organic Molecules with Enhanced Emission and Room Temperature Phosphorescence.

Ionically bonded organic metal halide hybrids have emerged as versatile multicomponent material systems exhibiting unique and useful properties. The unlimited combinations of organic cations and metal halides lead to the tremendous structural diversity of this class of materials, which could unlock many undiscovered properties of both organic cations and metal halides. Here we report the synthesis and characterization of a series benzoquinolinium (BZQ) metal halides with a general formula (BZQ)Pb2X5 (X = Cl, Br), in which metal halides form a unique two-dimensional (2D) structure. These BZQ metal halides are found to exhibit enhanced photoluminescence and stability as compared to the pristine BZQ halides, due to the scaffolding effects of 2D metal halides. Optical characterizations and theoretical calculations reveal that BZQ+ cations are responsible for the emissions in these hybrid materials. Changing the halide from Cl to Br introduces heavy atom effects, resulting in yellow room temperature phosphorescence (RTP) from BZQ+ cations.

Layer- and gate-tunable spin-orbit coupling in a high-mobility few-layer semiconductor.

Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving in an electric field experiences an effective magnetic field in its rest frame. In crystals without inversion symmetry, it lifts the spin degeneracy and leads to many magnetic, spintronic, and topological phenomena and applications. In bulk materials, SOC strength is a constant. Here, we demonstrate SOC and intrinsic spin splitting in atomically thin InSe, which can be modified over a broad range. From quantum oscillations, we establish that the SOC parameter α is thickness dependent; it can be continuously modulated by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Unexpectedly, α could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

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.

Order-disorder transition in the S = ½ kagome antiferromagnets claringbullite and barlowite.

The transition in the quantum magnets barlowite, Cu4(OH)6FBr, and claringbullite, Cu4(OH)6FCl is of an order-disorder type, where at ambient temperature interlayer Cu2+ ions are dynamically disordered over three equivalent positions. The disorder becomes static as the temperature is decreased, resulting in a lowering of symmetry. Ab initio density functional theory calculations explain this structural phase transition and provide insights regarding the differences between these two materials.

Enhanced thermoelectric performance of heavy-fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) at low temperatures.

Thermoelectricity allows direct conversion between heat and electricity, providing alternatives for green energy technologies. Despite these advantages, for most materials the energy conversion efficiency is limited by the tendency for the electrical and thermal conductivity to be proportional to each other and the Seebeck coefficient to be small. Here we report counter examples, where the heavy fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) exhibit enhanced thermoelectric performance including a large power factor (PF = 74 µW/cm-K2; TM = Ir) and a high figure of merit (ZT = 0.07; TM = Ir) at 35 K. The combination of the strongly hybridized electronic state originating from the Yb f-electrons and the novel structural features (large unit cell and possible soft phonon modes) leads to high power factors and small thermal conductivity values. This demonstrates that with further optimization these systems could provide a platform for the next generation of low temperature thermoelectric materials.

Orbital Fingerprint of Topological Fermi Arcs in the Weyl Semimetal TaP.

The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accommodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and first-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projected Weyl nodes, and show how they facilitate a topological classification of the surface band structure. Our findings establish a critical role of the orbital degrees of freedom in mediating the surface-bulk connectivity in Weyl semimetals.

Ba3CrN3H: A New Nitride-Hydride with Trigonal Planar Cr4.

The nitride-hydride Ba3CrN3H was obtained in single crystalline form using flux growth techniques based on alkaline earth metals. Ba3CrN3H crystallizes in the hexagonal space group P63/ m (Nr 176), with the lattice parameters a = 8.0270(2) Å, c = 5.6240(1) Å, and Z = 2. The structure comprises [CrN3]5- trigonal planar units and [HBa6]11+ octahedral units. The presence of anionic hydrogen in the structure has been verified by 1H NMR experiments. DFT calculations show that the addition of hydrogen increases the stability of the phase versus Ba3CrN3. The two d-electrons of Cr4+ are located in the nonbonding d z2 orbital, rendering Ba3CrN3H nonmagnetic and insulating.

Modification of spin-ice physics in Ho2Ti2O7 thin films.

We present an extensive study on the effect of substrate orientation, strain, stoichiometry, and defects on spin-ice physics in Ho2Ti2O7 thin films grown onto yttria-stabilized-zirconia substrates. We find that growth in different orientations produces different strain states in the films. All films exhibit similar c-axis lattice parameters for their relaxed portions, which are consistently larger than the bulk value of 10.1 Å. Transmission electron microscopy reveals antisite disorder and growth defects to be present in the films, but evidence of stuffing is not observed. The amount of disorder depends on the growth orientation, with the (110) film showing the least. Magnetization measurements at 1.8 K show the expected magnetic anisotropy and saturation magnetization values associated with a spin ice for all orientations; shape anisotropy is apparent when comparing in- and out-of-plane directions. Significantly, only the (110)-oriented films display the hallmark spin-ice plateau state in magnetization, albeit less well defined compared to the plateau observed in a single crystal. Neutron-scattering maps on the more disordered (111)-oriented films show the Q=0 phase previously observed in bulk materials, but the Q=X phase giving the plateau state remains elusive. We conclude that the spin-ice physics in thin films is modified by defects and strain, leading to a reduction in the temperature at which correlations drive the system into the spin-ice state.

Blue Emitting Single Crystalline Assembly of Metal Halide Clusters.

The rich chemistry of organic-inorganic metal halide hybrids has enabled the development of a variety of crystalline structures with controlled morphological and molecular dimensionalities. Here we report for the first time a single crystalline assembly of metal halide clusters, (C9NH20)7(PbCl4)Pb3Cl11, in which lead chloride tetrahedrons (PbCl42-) and face-sharing lead chloride trimer clusters (Pb3Cl115-) cocrystallize with organic cations (C9NH20+) to form a periodical zero-dimensional (0D) structure at the molecular level. Blue light emission peaked at 470 nm with a photoluminescence quantum efficiency (PLQE) of around 83% was realized for this single crystalline hybrid material, which is attributed to the individual lead chloride clusters. Our discovery of single crystalline assembly of metal halide clusters paves a new path to functional cluster assemblies with highly tunable structures and remarkable properties.

Luminescent zero-dimensional organic metal halide hybrids with near-unity quantum efficiency.

Single crystalline zero-dimensional (0D) organic-inorganic hybrid materials with perfect host-guest structures have been developed as a new generation of highly efficient light emitters. Here we report a series of lead-free organic metal halide hybrids with a 0D structure, (C4N2H14X)4SnX6 (X = Br, I) and (C9NH20)2SbX5 (X = Cl), in which the individual metal halide octahedra (SnX64-) and quadrangular pyramids (SbX52-) are completely isolated from each other and surrounded by the organic ligands C4N2H14X+ and C9NH20+, respectively. The isolation of the photoactive metal halide species by the wide band gap organic ligands leads to no interaction or electronic band formation between the metal halide species, allowing the bulk materials to exhibit the intrinsic properties of the individual metal halide species. These 0D organic metal halide hybrids can also be considered as perfect host-guest systems, with the metal halide species periodically doped in the wide band gap matrix. Highly luminescent, strongly Stokes shifted broadband emissions with photoluminescence quantum efficiencies (PLQEs) of close to unity were realized, as a result of excited state structural reorganization of the individual metal halide species. Our discovery of highly luminescent single crystalline 0D organic-inorganic hybrid materials as perfect host-guest systems opens up a new paradigm in functional materials design.

Synthesis and Crystal Structure of the Layered Lanthanide Oxychlorides Ba3Ln2O5Cl2.

Single crystals of a new family of layered lanthanide oxychlorides, Ba3Ln2O5Cl2 (Ln = Gd-Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/mmm (No. 139; Z = 2) with lattice parameters a = 4.3384(1)-4.4541(1) Å and c = 24.5108(7)-24.8448(9) Å. Ba3Ln2O5Cl2 phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba2Ln2O5 formed by corner-connected LnO5 tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl2. A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. The synthesis of this family suggests a route to other potential multianion phases.

Uncovering the Origin of Divergence in the CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis.

A series of f-block chromates, CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the AmIII derivatives and their lanthanide analogs. While all compounds form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the LnIII compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm(CrO4)2, α-CsEu(CrO4)2, and α-CsAm(CrO4)2 were studied using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods. Notably, the covalent contributions to bonding by the f orbitals were found to be more than twice as large in the AmIII chromate than in the SmIII and EuIII compounds, and even larger in magnitude than the Am-5f spin-orbit splitting in this system. Our analysis indicates also that the Am-O covalency in α-CsAm(CrO4)2 is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.

Highly Efficient Broadband Yellow Phosphor Based on Zero-Dimensional Tin Mixed-Halide Perovskite.

Organic-inorganic hybrid metal halide perovskites have emerged as a highly promising class of light emitters, which can be used as phosphors for optically pumped white light-emitting diodes (WLEDs). By controlling the structural dimensionality, metal halide perovskites can exhibit tunable narrow and broadband emissions from the free-exciton and self-trapped excited states, respectively. Here, we report a highly efficient broadband yellow light emitter based on zero-dimensional tin mixed-halide perovskite (C4N2H14Br)4SnBrxI6-x (x = 3). This rare-earth-free ionically bonded crystalline material possesses a perfect host-dopant structure, in which the light-emitting metal halide species (SnBrxI6-x4-, x = 3) are completely isolated from each other and embedded in the wide band gap organic matrix composed of C4N2H14Br-. The strongly Stokes-shifted broadband yellow emission that peaked at 582 nm from this phosphor, which is a result of excited state structural reorganization, has an extremely large full width at half-maximum of 126 nm and a high photoluminescence quantum efficiency of ∼85% at room temperature. UV-pumped WLEDs fabricated using this yellow emitter together with a commercial europium-doped barium magnesium aluminate blue phosphor (BaMgAl10O17:Eu2+) can exhibit high color rendering indexes of up to 85.

Low-Dimensional Organic Tin Bromide Perovskites and Their Photoinduced Structural Transformation.

Hybrid organic-inorganic metal halide perovskites possess exceptional structural tunability, with three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures on the molecular level all possible. While remarkable progress has been realized in perovskite research in recent years, the focus has been mainly on 3D and 2D structures, with 1D and 0D structures significantly underexplored. The synthesis and characterization of a series of low-dimensional organic tin bromide perovskites with 1D and 0D structures is reported. Using the same organic and inorganic components, but at different ratios and reaction conditions, both 1D (C4 N2 H14 )SnBr4 and 0D (C4 N2 H14 Br)4 SnBr6 can be prepared in high yields. Moreover, photoinduced structural transformation from 1D to 0D was investigated experimentally and theoretically in which photodissociation of 1D metal halide chains followed by structural reorganization leads to the formation of a more thermodynamically stable 0D structure.

Interacting nanoscale magnetic superatom cluster arrays in molybdenum oxide bronzes.

In this study, we examine several reduced ternary molybdates in the family of yellow rare earth molybdenum bronzes produced by electrochemical synthesis with composition LnMo16O44. These compounds contain an array of electrically isolated but magnetically interacting multi-atom clusters with composition Mo8O36. These arrayed superatom clusters support a single hole shared among the eight molybdenum atoms in the unit, corresponding to a net spin moment of 1µB, and exhibit magnetic exchange between the units via the MoO4 tetrahedra (containing Mo6+ ions) and the LnO8 cubes (containing Ln3+ ions). The findings presented here expand on the physics of the unusual collective properties of multi-atom clusters and extend the discussion of such assemblages to the rich structural chemistry of molybdenum bronzes.

Electrochemical Doping of Halide Perovskites with Ion Intercalation.

Halide perovskites have recently been investigated for various solution-processed optoelectronic devices. The majority of studies have focused on using intrinsic halide perovskites, and the intentional incoporation of dopants has not been well explored. In this work, we discovered that small alkali ions, including lithium and sodium ions, could be electrochemically intercalated into a variety of halide and pseudohalide perovskites. The ion intercalation caused a lattice expansion of the perovskite crystals and resulted in an n-type doping of the perovskites. Such electrochemical doping improved the conductivity and changed the color of the perovskites, leading to an electrochromism with more than 40% reduction of transmittance in the 450-850 nm wavelength range. The doped perovskites exhibited improved electron injection efficiency into the pristine perovskite crystals, resulting in bright light-emitting diodes with a low turn-on voltage.