Neutron Diffraction Refinement Research Articles

Efficient Exploratory Synthesis of Quaternary Cesium Chlorides Guided by In Silico Predictions.

Exploratory synthesis of solids is essential for the advancement of materials science but is also highly time- and resource-intensive. Here, we demonstrate an efficient strategy to explore solid-state synthesis of quaternary cesium chlorides in the search space of CsnAIBCl6 (n = 2 or 3, A = Li, Na or K, and B = d or p-block metal), where the target compositions are selected from a pool of candidates based on computationally predicted stabilities and availability of viable precursor powders. Synthesizability of the targets is assessed by observing the evolution of starting phases upon heating under in situ synchrotron X-ray diffraction. Laboratory synthesis is attempted for promising targets, and resulting materials are characterized by powder X-ray and neutron diffraction and subsequent Rietveld refinement. We focus on how computational predictions can be bridged to experimental characterizations in exploratory synthesis and report on successful and failed synthesis attempts for compounds of type Cs2AIBIIICl6, revealing underexplored variants including new polymorphs of Cs2LiCrCl6 and Cs2LiRuCl6, and a new compound Cs2LiIrCl6.

Unusually High Magnetic Moments Reported for the High-Spin Co2+ Ions at Axially Elongated CoO4X2 (X = Cl, Br, S, Se) Octahedral Sites: Overestimated Magnetic Moments Caused by Underestimated Magnetic Form Factors

An irreconcilable discrepancy between theory and experiment concerning the magnetic moments of the high-spin Co2+ (d7, S = 3/2) ions at the axially elongated CoO4X2 (X = Cl, Br, S, Se) octahedral sites was pointed out in our recent study [ Inorg. Chem. 2020, 59, 18319−18324]. The magnetic moments μobs of the Co2+ ions in materials containing the CoO4X2 units, determined from refinements of powder neutron diffraction patterns collected in their magnetically ordered states, were found to be notably larger than the spin-only moment of 3 μB (namely, 3.22–4.45 μB). Characteristically, the moments increase almost linearly with the bond-length ratio R = rCo–X/rCo–O, despite the fact that the orbital moments of the Co2+ ions are essentially quenched according to theoretical analyses and DFT + U + SOC computations. In this work, we probe the probable cause for the overestimation of magnetic moments in the ordered magnetic structures. We argue that the overestimated magnetic moments of the Co2+ ions and their increase with the bond-length ratio R originate from the underestimated magnetic form factors of the Co2+ ions at the CoO4X2 octahedral sites. The magnetic form factor F(q) of the Co2+ ion in each CoO4X2 octahedron is the Fourier transform of the magnetic moment distribution ρ(r) around the Co2+ ion, which is the electron density distribution associated with the magnetic orbitals (i.e., the singly occupied d-states) of the CoO4X2 octahedron. The conventional implementation of the magnetic form factor in refinement codes employed an approximation with a q-dependence calculated from the atomic wave function of the isolated ion. Since the spin moment distribution of ρ(r) at the axially elongated CoO4X2 (X = Cl, Br, S, Se) octahedral sites is delocalized from the Co2+ sites to its surrounding ligands, the moment distribution becomes anisotropic in shape. Use of the truncated and spherical approximation underestimates the form factor of the Co2+ ion, and its use in neutron diffraction refinements overestimates the magnetic moments. This effect becomes more pronounced as the moment density distribution is more delocalized along the Co–X direction.

H-Atom Assignment and Sb-O Bonding of [Mes3SbOH][O3SPh] Confirmed by Neutron Diffraction, Multipole Modeling, and Hirshfeld Atom Refinement.

Neutron wavelength-resolved Laue diffraction experiments permit accurate refinement of the H-atom positions and anisotropic displacement parameters of [Mes3SbOH][O3SPh]. A multipole-based charge density refinement and a topological analysis of the refined electron density were also performed. Hirshfeld atom refinement (HAR) recovers the neutron-determined H-atom parameters, and the quantum-mechanical electron density used in HAR recovers the electron density topology from the refined multipole model. These results confirm that [Mes3SbOH][O3SPh] does indeed feature a hydroxystibonium cation with a nominal Sb-O single bond and not a stibine oxide with an Sb=O/Sb+-O- bond.

The correlation between the covalent bonds and magnetocaloric properties of the Mn2−xFexPyGe1−yMz compounds

In recent years, MnFePGe magnetocaloric materials have been widely studied as promising candidates for magnetic refrigeration materials. The Curie temperature of MnFePGe can be adjusted to around room temperature by changing the element ratio or doping with other elements. Due to its first-order magnetic and structural transition, it engenders a large entropy change but unfortunately also exhibits a large thermal hysteresis during the phase transition, which leads to energy loss and lower refrigeration capability. In this paper, we establish a correlation between the in-plane covalent bonding and Curie temperature (TC), thermal hysteresis (ΔThys), two-phase coexistence zone (ΔTcoex), and entropy change (ΔSDSC) using 54 Mn2−xFexPyGe1−yMz (where M is a metallic or nonmetallic doped element) samples with different components. Neutron diffraction and XRD diffraction data and refinements have been employed to allow a detailed electron density reconstruction of six typical samples with the maximum entropy method. We find that the length of the in-plane bonding is closely correlated with the TC and ΔThys, while the TC, ΔThys, ΔTcoex, and ΔSDSC have no significant correlation with the length of the interlayer covalent bond. Moreover, we find that the ΔThys correlates most strongly with the change in the bond length when undergoing the paramagnetic-to-ferromagnetic phase transition rather than the absolute value of the bond length. These results provide an understanding of how to control the properties, enabling effective ways to tune the composition of magnetic refrigeration materials to tailor magnetocaloric properties for optimal performance.

Orbital Magnetic Moments of the High-Spin Co2+ Ions at Axially-Elongated Octahedral Sites: Unquenched as Reported from Experiment or Quenched as Predicted by Theory?

Neutron diffraction studies on magnetic solids composed of axially elongated CoO4X2 (X = Cl, Br, S, Se) octahedra show that the ordered magnetic moments of their high-spin Co2+ (d7, S = 3/2) ions are greater than 3 μB, i.e., the spin moment expected for S = 3/2 ions, and increase almost linearly from 3.22 to 4.45 μB as the bond-length ratio rCo-X/rCo-O increases from 1.347 to 1.659 where rCo-X and rCo-O are the Co-X and Co-O bond lengths, respectively. These observations imply that the orbital moments of the Co2+ ions increase linearly from 0.22 to 1.45 μB with increasing the rCo-X/rCo-O ratio from 1.347 to 1.659. We probed this implication by examining the condition for unquenched orbital moment and also by evaluating the magnetic moments of the Co2+ ions based on DFT+U+SOC calculations for those systems of the CoO4X2 octahedra. Our work shows that the orbital moments of the Co2+ ions are essentially quenched and, hence, that the observations of the neutron diffraction studies are not explained by the current theory of magnetic moments. This discrepancy between experiment and theory urges one to check the foundations of the current theory of magnetic moments as well as the current method of neutron diffraction refinements for ordered magnetic structures.

Canted Eu magnetic structure in EuMnSb2

Magnetic ordering breaks the time-reversal symmetry, greatly impacting material topological properties. We report the investigation of the magnetic properties of the layered $\mathrm{EuMnS}{\mathrm{b}}_{2}$, which has two sets of magnetic sublattices. Both the magnetization and electrical resistivity reveal two phase transitions with one at ${T}_{\mathrm{N},\mathrm{Eu}}\ensuremath{\sim}21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the other at ${T}_{\mathrm{N},\mathrm{Mn}}\ensuremath{\sim}346\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Single crystal neutron diffraction refinement indicates that both transitions are originated from magnetic ordering. Below ${T}_{\mathrm{N},\mathrm{Mn}},$ the Mn sublattice forms the C-type antiferromagnetic (AFM) structure with moments [$(4.5\ifmmode\pm\else\textpm\fi{}0.6){\ensuremath{\mu}}_{B}$ at 7 K] pointing along the $a$ axis. Below ${T}_{\mathrm{N},\mathrm{Eu}}$, the Eu sublattice forms the canted A-type AFM structure with moments [$(5.9\ifmmode\pm\else\textpm\fi{}0.8){\ensuremath{\mu}}_{\mathrm{B}}$ at 7 K] lying in the $ac$ plane but pointing (41 \ifmmode\pm\else\textpm\fi{} 1)\ifmmode^\circ\else\textdegree\fi{} away from the $a$ axis. Quantitative analysis indicates that the spin-spin correlation length, while anisotropic, has long-range characteristic in all directions for both the Eu and Mn sublattices.

Dihydrogen Bonds in Aqueous NaBD4 Solution by Neutron and X-ray Diffraction.

Neutron diffraction, X-ray diffraction, and empirical potential structure refinement modeling were employed to study the structure of alkaline aqueous NaBD4 solutions at different NaBD4 concentrations and temperatures. In 1.0 mol·dm-3 NaBD4 aqueous solutions, about 5.6 ± 1.6 water molecules bond to BD4- via tetrahedral edges or tetrahedral corners without a very specific hydration geometry; that is, each hydrogen atom of BD4- bonds to 2.2 ± 1.0 water molecules through dihydrogen bonds with the D(B)···D(W) distance of 1.95 Å. The number of dihydrogen bonds decreases with increasing concentration and increases with temperature. Dihydrogen bonding is a predominantly electrostatic interaction which shows relatively lower directionality and saturability in comparison with the regular hydrogen bonds between water molecules. The water orientation around BD4- shows that the proportion of tetrahedral-edge dihydrogen bonds increases with temperature and decreases with concentration.

Significant ion conduction in Cu acceptor-substituted bismuth titanate polycrystalline ceramics

This paper reports that Cu acceptor substitution at the Ti site has significant effects on structure and electrical properties of Aurivillius-phase Bi4Ti3O12 polycrystalline ceramics. Phase purity, microstructure and defect chemistry of Bi4Ti3−xCuxO12−x (BiT–100xCu, x = 0–0.12) are characterized by XRD, SEM, EDS and EPR measurements. Neutron diffraction refinement indicates that BiT–8Cu has an orthorhombic symmetry [space group: B2cb; lattice parameters: a = 5.4116(1) A, b = 32.833(1) A, c = 5.4479(1) A and V = 967.98(5) A3]. Impedance spectra of BiT–100xCu were measured under variable oxygen partial pressure (pO2). Bi4Ti3O12 shows mixed electronic (hole) and ionic conduction owing to the existence of oxygen vacancies ($$ {\text{V}}_{\text{O}}^{ \cdot \cdot } $$) arising from the loss of Bi2O3 during sintering. Interestingly, bulk conductivity in Cu-substituted Bi4Ti3O12 ceramics is predominately ionic due to the fact that there are considerably additional oxygen vacancies introduced into the perovskite lattices. The optimum composition of BiT–8Cu shows high ionic conductivity in the bulk with a value of ~ 0.007 S/cm at 650 °C.

Structure, chromium vacancies, and magnetism in a Cr12−xTe16 compound

Binary Cr-Te compounds are known to exhibit various structures and interesting physical properties, which are very sensitive to Cr content and arrangement. We report the structure and physical properties of a compound $\mathrm{C}{\mathrm{r}}_{12\ensuremath{-}x}\mathrm{T}{\mathrm{e}}_{16}$ with $x\ensuremath{\sim}2.65$. $\mathrm{C}{\mathrm{r}}_{9.35}\mathrm{T}{\mathrm{e}}_{16}$ crystallizes in the trigonal structure with the space group $P\overline{3}m1$ (No. 164), consisting of four different Cr sites (Cr1, Cr2, Cr3, Cr4) with an ordered Cr4 vacant layer. The electrical resistivity, magnetization, specific heat, and neutron diffraction measurements indicate that there is a ferromagnetic transition at ${T}_{\mathrm{C}1}\ensuremath{\sim}160$ K with the first-order nature, and an anomaly at ${T}_{\mathrm{C}2}\ensuremath{\sim}67$ K. At 5 K, the magnetic moments of Cr2 and Cr3 are aligned along the $c$ axis with $2.78\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Cr}$. Detailed neutron diffraction refinement suggests that ordered magnetic moments on Cr1 (fully occupied) and Cr4 (partially occupied) are negligible. The different magnetic response stems from different valence states and different magnetic interactions between Cr2/Cr3 and Cr1/Cr4 as evidenced by the x-ray absorption fine-structure measurements. This creates a unique two-dimensional magnetic system with ordered moments confined in the Cr2/Cr3 layer.

Hollandite-Type VO1.75(OH)0.5: Effective Sodium Storage for High-Performance Sodium Ion Batteries

Lithium-ion batteries (LIBs) have attracted significant attention as high-energy-density rechargeable power sources for electric vehicles and large-scale energy storage applications. Although LIBs have been successfully commercialized in the world battery market, the expected shortage of lithium resources resulting from the ever-increasing demand for LIBs needs to be addressed. This issue has accelerated the search for alternative power sources such as sodium-ion batteries (SIBs), which are emerging as promising candidates to replace LIBs. Sodium is one of the most plentiful resources on earth; thus, SIBs have been extensively studied because of their low cost as well as their competitiveness in terms of energy density versus LIBs. However, the ionic size of Na+ (1.02 Å) is larger than that of Li+ (0.76 Å), which affects the phase stability, transport properties, and interphase formation. The low standard redox potential of sodium (−2.71 V vs. standard hydrogen electrode (SHE)) compared with that of lithium (−3.04 V vs. SHE) is another demerit of SIBs; hence, a high capacity is necessary to compensate for the low operation voltage to approximate the energy density levels of LIBs. Thus far, various types of electrode materials have been suggested as cathode materials for SIBs, including Na2/3MnO2, Na2/3[MxMn1 −x]O2 (M: transition metals), NaCrO2, α-MnO2, and Na3V2PO3. Vanadium oxides (VxOy) is of interest because of the wide range of oxidation states available from +2 (as in VO) to +5 (as in V2O5), which affects the structural, electrochemical, and magnetic properties. This property has led to the application of vanadium oxides in catalysts, chemical sensors, and energy storage devices such as rechargeable lithium batteries. Herein, we introduce new nanosized hollandite-type VO1.75(OH)0.5 as a novel cathode material for Na-ion batteries. Structural investigation based on X-ray diffraction (XRD), neutron diffraction (ND), and Rietveld refinement suggests the presence of numerous vacant sites for Na+ intercalation and structural information on hydrogen in the VO1.75(OH)0.5. All of the possible Na+ sites and tunnel-type Na+ diffusion pathways along the c-axis are confirmed by bond-valence-sum analyses. The nanosized hollandite-type VO1.75(OH)0.5 delivers an unexpectedly high specific capacity of approximately 351 mAh g−1 at 15.5 mA g-1 in the voltage range of 1.0–3.7 V (vs. Na+/Na), which agrees well with the results predicted by first-principles calculations. In addition, combined studies using first-principles calculations and several experimental techniques including in-situ operando X-ray diffraction and ex situ X-ray absorption spectroscopy confirm that the nanosized hollandite-type VO1.75(OH)0.5 undergoes a single-phase reaction with a capacity retention of 71% over 200 cycles. Furthermore, the open structure and nanosized particles of hollandite-type VO1.75(OH)0.5 contribute to its excellent power capability with 56% of the capacity measured at 0.05C being delivered at 7C. These results indicated the outstanding cyclability of Na x VO1.75(OH)0.5 as a promising cathode material for SIBs. We believe that our novel approach will provide insight for the discovery and understanding of novel electrode materials for not only SIBs but also other rechargeable batteries. Figure 1

Amphiphilically Nanostructured Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are neoteric liquids produced by mixing a high-melting-point salt and a molecular hydrogen-bond donor. Amphiphilic (self-assembled) liquid nanostructure, which is key for many of the useful properties of the related ionic liquid class, has not previously been experimentally demonstrated in DESs. Here we show how amphiphilically nanostructured DESs can be prepared using primary ammonium cations. The bulk structure of alkylammonium bromide (alkyl = ethyl-, propyl-, and butyl) and glycerol DESs at a 1:2 mol ratio is examined using neutron diffraction and empirical potential structure refinement fitting. Analysis reveals cation alkyl chain association, which is the signature of amphiphilic liquid nanostructure, in all systems, which becomes better defined with increasing chain length. The ability to form amphiphilically nanostructured DESs will enable the translation of ionic liquid properties associated with liquid nanostructure to DESs.

A Spin-Canted Antiferromagnetic Ground State in CeRu2Al10

Resonant polarised soft x-ray scattering at the cerium M-edge has been used to refine the magnetic structure of CeRu2Al10. A strong resonant feature at the cerium MIV-edge was observed at the disallowed \((0,1,0)\) Bragg position, consistent with previous neutron diffraction refinement of the moment pointing along the c-axis. The magnetic peak was found to have a temperature dependence expected for the paramagnetic–antiferromagnetic transition, disappearing above around 30 K. The polarisation dependence of the scattered x-rays conclusively shows that the low-temperature antiferromagnetic structure is non-collinear in nature. Fitting the polarisation dependence of the obtained Stokes parameters was undertaken with models for canting along either the a-axis or the b-axis. The experimental data agrees better with the model involving canting towards the a-axis. However, this is inconsistent with the Cmcm space group, suggestive of a symmetry lowering to either Pmnm or Cm2m. The resulting model is then achieve...

On the structure of an aqueous propylene glycol solution.

Using a combination of neutron diffraction and empirical potential structure refinement computational modelling, the interactions in a 30 mol. % aqueous solution of propylene glycol (PG), which govern both the hydration and association of this molecule in solution, have been assessed. From this work it appears that PG is readily hydrated, where the most prevalent hydration interactions were found to be through both the PG hydroxyl groups but also alkyl groups typically considered hydrophobic. Hydration interactions of PG dominate the solution over PG self-self interactions and there is no evidence of more extensive association. This hydration behavior for PG in solutions suggests that the preference of PG to be hydrated rather than to be self-associated may translate into a preference for PG to bind to lipids rather than itself, providing a potential explanation for how PG is able to enhance the apparent solubility of drug molecules in vivo.

Structure of Aqueous Trehalose Solution by Neutron Diffraction and Structural Modeling.

The molecular structure of an aqueous solution of the disaccharide trehalose (C12H22O11) has been studied by neutron diffraction and empirical potential structure refinement modeling. Six different isotope compositions with 33 wt % trehalose (corresponding to 38 water molecules per trehalose molecule) were measured to ensure that water-water, trehalose-water, and trehalose-trehalose correlations were accurately determined. In fact, this is the first neutron diffraction study of an aqueous trehalose solution in which also the nonexchangeable hydrogen atoms in trehalose are deuterated. With this approach, it was possible to determine that (1) there is a substantial hydrogen bonding between trehalose and water (∼11 hydrogen bonds per trehalose molecule), which is in contrast to previous neutron diffraction studies, and (2) there is no tendency of clustering of trehalose, in contrast to what is generally observed by molecular dynamics simulations and experimentally found for other disaccharides. Thus, the results give the structural picture that trehalose prefers to interact with water and participate in a hydrogen-bonded network. This strong network character of the solution might be one of the key reasons for its extraordinary stabilization effect on biological materials.

Lightweight Polyanion Frameworks for Li- and Mg-Ion Batteries

Mg-ion batteries are a promising “beyond Li-ion” technology as they offer high theoretical capacities, use cheap and abundant materials, and Mg metal can be used as the anode.1 However, the search for high voltage cathodes has been hindered by electrolyte incompatibility and sluggish kinetics which can impede intercalation and charging rates.2 MgMB2O5 (M = Mn, Fe, Co) are shown to be high voltage, high capacity cathode materials for both Li- and Mg-ion batteries. The open structure facilitates ion diffusion while the light weight polyanion framework increases capacity and voltage. The materials are produced from metal oxalate and boric acid starting materials via a solid state synthesis process. The Mg:M ratio can be altered to manipulate capacity and ordering of the cations.Mg can be fully removed from MgMnB2O5 giving a capacity of up to 300 mAhg-1 while maintaining the structure. Removal of Mg is confirmed by refinement of powder neutron diffraction data. In a full Mg-ion cell a reversible discharge capacity of ~100 mAhg-1 has been achieved. However, due to the incompatibility of the cell with available electrolytes this is accompanied by irreversible side reactions on charge. Some promising resolutions to the electrolyte problem are discussed. Mg can also be removed from the structure when M = Co or Fe, with the change in transition metal leading to a change in operating voltage. The results presented show that this class of material is a promising candidate for a high capacity, high voltage Mg-ion cathode material.In a Li-ion battery 1.5 Li can be intercalated into the manganese based material after Mg has been removed, giving a specific capacity of up to 250 mAhg-1 above 1.5 V. The capacity is retained over hundreds of cycles with little drop off observed even at rates of 5C. Ex-situ XRD shows a doubling of the unit cell when the material is cycled, suggesting charge ordering is occurring. Neutron diffraction refinements show a change in volume of ~2%, associated with insertion or removal of Li ions.

Neutron diffraction analysis and electrochemical performance of spinel Ni(Mn2−xCox)O4 as anode materials for lithium ion battery

The effects of Co substitution on the structure and electrochemical performances of spinel Ni(Mn2−xCox)O4 (x=0, 0.5, and 1.0) have been investigated. With the increase of Co content the lattice parameters decrease owing to the smaller ion radius of Co than Mn. The reversible capacity and cyclability of Ni(Mn2−xCox)O4 first increase and then decrease with the increase of Co content and NiMn1.5Co0.5O4 shows the best electrochemical performance in compared to the other two samples. Neutron diffraction and Rielveld refinement are further applied to analyze the site occupancies of the elements of Mn, Co, O and Ni. A certain amount of Co ions substitution are favorable to enhance the electrochemical performance, but excessive Co ions in 8a and 16d sites reduce the stability of host structure which leads to the poor electrochemical performance.

Bulk nanostructure of the prototypical 'good' and 'poor' solvate ionic liquids [Li(G4)][TFSI] and [Li(G4)][NO3

The bulk nanostructures of a prototypical 'good' solvate ionic liquid (SIL) and 'poor' SIL have been examined using neutron diffraction and empirical potential structure refinement (EPSR) simulated fits. The good SIL formed by a 1 : 1 mixture of lithium bis(trifluoromethylsulfonyl)imide (Li[TFSI]) in tetraglyme (G4), denoted [Li(G4)][TFSI], and the poor SIL formed from a 1 : 1 mixture of lithium nitrate (Li[NO3]) in G4, denoted [Li(G4)][NO3], have been studied. In both SILs there are strong Lewis acid-base interactions between Li(+) and ligating O atoms. However, the O atoms coordinated to Li(+) depend strongly on the counter anion present. LiO coordination numbers with G4 are 2-3 times higher for [Li(G4)][TFSI] than [Li(G4)][NO3], and conversely the LiO anion coordination number is 2-3 times higher in [Li(G4)][NO3]. In both solvates the local packing of Li around G4 O atoms are identical but these interactions are less frequent in [Li(G4)][NO3]. In both SILs, Li(+) has a distribution of coordination numbers and a wide variety of different complex structures are present. For [Li(G4)][NO3], there is a significant proportion uncoordinated G4 in the bulk; ∼37% of glyme molecules have no LiO contacts and each G4 molecule coordinates to an average of 0.5 Li(+) cations. Conversely, in [Li(G4)][TFSI] only ∼5% of G4 molecules lack LiO contacts and G4 molecules coordinates to an average of 1.3 Li(+) cations. Li(+) and G4 form polynuclear complexes, of the form [Lix(G4)y](x+), in both solvates. For [Li(G4)][TFSI] ∼35% of Li(+) and G4 form 1 polynuclear complexes, while only ∼10% of Li(+) and G4 form polynuclear complexes in [Li(G4)][NO3].

Ionic liquid nanostructure enables alcohol self assembly.

Weakly structured solutions are formed from mixtures of one or more amphiphiles and a polar solvent (usually water), and often contain additional organic components. They contain solvophobic aggregates or association structures with incomplete segregation of components, which leads to a poorly defined interfacial region and significant contact between the solvent and aggregated hydrocarbon groups. The length scales, polydispersity, complexity and ill-defined structures in weakly structured solutions makes them difficult to probe experimentally, and obscures understanding of their formation and stability. In this work we probe the nanostructure of homogenous binary mixtures of the ionic liquid (IL) propylammonium nitrate (PAN) and octanol as a function of composition using neutron diffraction and atomistic empirical potential structure refinement (EPSR) fits. These experiments reveal why octanol forms weakly structured aggregates in PAN but not in water, the mechanism by which PAN stabilises the octanol assemblies, and how the aggregate morphologies evolve with octanol concentration. This new understanding provides insight into the general stabilisation mechanisms and structural features of weakly structured mixtures, and reveals new pathways for identifying molecular or ionic liquids that are likely to facilitate aggregation of non-traditional amphiphiles.

Prediction of Spin Orientations in Terms of HOMO-LUMO Interactions Using Spin-Orbit Coupling as Perturbation.

For most chemists and physicists, electron spin is merely a means needed to satisfy the Pauli principle in electronic structure description. However, the absolute orientations of spins in coordinate space can be crucial in understanding the magnetic properties of materials with unpaired electrons. At low temperature, the spins of a magnetic solid may undergo long-range magnetic ordering, which allows one to determine the directions and magnitudes of spin moments by neutron diffraction refinements. The preferred spin orientation of a magnetic ion can be predicted on the basis of density functional theory (DFT) calculations including electron correlation and spin-orbit coupling (SOC). However, most chemists and physicists are unaware of how the observed and/or calculated spin orientations are related to the local electronic structures of the magnetic ions. This is true even for most crystallographers who determine the directions and magnitudes of spin moments because, for them, they are merely the parameters needed for the diffraction refinements. The objective of this article is to provide a conceptual framework of thinking about and predicting the preferred spin orientation of a magnetic ion by examining the relationship between the spin orientation and the local electronic structure of the ion. In general, a magnetic ion M (i.e., an ion possessing unpaired spins) in a solid or a molecule is surrounded with main-group ligand atoms L to form an MLn polyhedron, where n is typically 4-6, and the d states of MLn are split because the antibonding interactions of the metal d orbitals with the p orbitals of the surrounding ligands L depend on the symmetries of the orbitals involved.1 The magnetic ion M of MLn has a certain preferred spin direction because its split d states interact among themselves under SOC.2,3 The preferred spin direction can be readily predicted on the basis of perturbation theory in which the SOC is taken as perturbation and the split d states as unperturbed states by inspecting the magnetic quantum numbers of its d orbitals present in the HOMO and LUMO of the MLn polyhedron. This is quite analogous to how chemists predict whether a chemical reaction is symmetry-allowed or symmetry-forbidden in terms of the HOMO-LUMO interactions by simply inspecting the symmetries of the frontier orbitals.4,5 Experimentally, the determination of the preferred spin orientations of magnetic ions requires a sophisticated level of experiments, for example, neutron diffraction measurements for magnetic solids with an ordered spin state at a very low temperature. Theoretically, it requires an elaborate level of electronic structure calculations, namely, DFT calculations including electron correlation and SOC. We show that the outcomes of such intricate experimental measurements and theoretical calculations can be predicted by a simple perturbation theory analysis.

Magnetic structure driven by monoclinic distortions in the double perovskite Sr2YRuO6

The monoclinic double perovskite Sr2YRuO6 has recently gained a renewed interest in order to get a deeper insight into the exotic magnetic ground states associated with geometric frustration. Striking discrepancies between the spin order derived from the neutron diffraction refinements and the macroscopic magnetic and thermal responses is a major challenge that must be addressed. In this work, detailed neutron diffraction measurements as a function of temperature yield a completely different interpretation of the patterns. We show that at low temperatures a spin structure of the K2NiF4-type is an accessible configuration for the magnetic ground state. In the neighborhood of the magnetic transition, this configuration evolves into a canted superstructure. The deduced temperature dependence of the canting angle exhibits two closely spaced peaks, which are in excellent agreement with the double peaks in the magnetic contribution to the specific heat and in the thermal expansion coefficient. We explain these features in terms of reorientation of the net ferromagnetic moment of the noncollinear spin state, due to the local breaking of the inversion symmetry promoted by the monoclinic distortions, with structural changes acting as the driving force.