Single-crystal Neutron Diffraction Research Articles

Chiral magnetic structure of spin-ladder multiferroic BaFe2Se3

A complex magnetic structure with specific symmetry properties is a relevant ingredient for magnetoelectric multiferroicity. In BaFe${}_{2}$Se${}_{3}$, a member of the Fe spin-ladder family of pressure-induced superconductors, the authors reveal this complexity. Using polarized and unpolarized single-crystal neutron diffraction, they evidence a chiral, umbrella-like magnetic order made of blocks of quasiferromagnetic spins, antiferromagnetically ordered along the ladder. Such accurate information on the magnetism will help to understand the magnetoelectric coupling in this family and its puzzling phase diagram.

Inelastic neutron studies and diffraction in magnetic fields of TbFeO3 and YbFeO3

Investigations of the orthoferrites TbFeO3 and YbFeO3 were performed by neutron inelastic scattering and neutron single crystal diffraction in magnetic fields. The low temperature evolution of energy gaps was explored for both compounds and considered from the point of view of changes of rare earth ion anisotropy. Exchange parameters between nearest neighbors for Fe3+ in TbFeO3 were obtained. The magnetic phase diagram for YbFeO3 was obtained and discussed as a result of the energy balance between Heisenberg exchange interactions, Dzyaloshinsky-Moriya interaction, anisotropy and external magnetic field.

Reactivity of a β-diketiminate ytterbium(II) hydride with cyclopentadiene derivatives

Cyclopentadienyl ligands are ubiquitous in lanthanide chemistry and have been used extensively to stabilize low-valent lanthanide(II) complexes. We now report the reactions of the β-diketiminate ytterbium(II) hydride [(BDIDipp)Yb(H)]2 (BDIDipp = HC{C(Me)NDipp}2, Dipp = 2,6- i Pr2C6H3) with tetramethylcyclopentadiene, pentamethylcyclopentadiene, indene, and fluorene; to provide ready access to the heteroleptic ytterbium(II) η5-C5R5 cyclopentadienides with extrusion of hydrogen gas. The tetramethylcyclopentadienyl, pentamethylcyclopentadienyl, indenyl, and fluorenyl compounds were characterized by 1H and 13C{1H} NMR, elemental analysis, and single crystal X-ray diffraction. These data indicated that each heteroleptic cyclopentadienide exists as a mononuclear species in both solution and the solid state, formulated with retention of the oxidation state on the ytterbium(II) center. Additionally, the pentamethylcyclopentadienyl derivative was characterized by single crystal neutron diffraction, a study that explored the nature of an agostic interaction between a methyl group and the ytterbium(II) center.

Possible stripe phases in the multiple magnetization plateaus in TbB4 from single-crystal neutron diffraction under pulsed high magnetic fields

We present a single-crystal neutron diffraction study on the Shastry-Sutherland lattice system TbB$_4$ at zero magnetic field and under pulsed high magnetic fields up to 35 T applied along the crystallographic $c$ axis. While our results confirm the magnetic structures at zero-field as well as those at the half- and full-magnetization plateaus, they offer new insight into the $\frac{2}{9}$- and $\frac{1}{3}$-magnetization plateaus observed in this system. A stripe model of polarized 4-spin-plaquettes whose stripe density proportionally increases with the macroscopic magnetization is in full agreement with the neutron diffraction data. Equally well suited alternative models exist which explain the observed Bragg peaks which are inherently limited in a pulsed high magnetic field experiment. We discuss the different intensity distribution in $Q$ space which can be used to distinguish these models in future experiments.

Structure of high-pressure ices revealed from single-crystal and powder neutron diffraction

Structure of high-pressure ices revealed from single-crystal and powder neutron diffraction

Weak itinerant magnetic phases of La2Ni7

La2Ni7 is an intermetallic compound that is thought to have itinerant magnetism with a small moment ordering below 65 K. A recent study of single crystal samples by Ribeiro et. al. [Phys. Rev. B 105, 014412 (2022)] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T1 ~ 61.0 K, T2 ~ 56.5K, and T3 ~ 42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. We find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations.

Canted antiferromagnetic phases in the candidate layered Weyl material EuMnSb2

${\mathrm{EuMnSb}}_{2}$ is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. Here we present single-crystal neutron diffraction, magnetization, and heat-capacity data as well as polycrystalline $^{151}\mathrm{Eu}$ M\"ossbauer data which show that three AFM phases exist as a function of temperature, and we present a detailed analysis of the magnetic structure in each phase. The Mn magnetic sublattice orders into a C-type AFM structure below ${T}_{{\text{N}}_{\text{Mn}}}=323(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Mn magnetic moment ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below ${T}_{{\text{N}}_{\text{Eu1}}}=23(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with the ordered Eu magnetic moment ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ canted away from the layer normal and ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ retaining its higher temperature order. ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}{a}^{\ensuremath{'}}$ with the chemical and magnetic unit cells having the same dimensions. Cooling below ${T}_{{\text{N}}_{\text{Eu2}}}=9(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ reveals a third AFM phase where ${\mathbf{\ensuremath{\mu}}}_{\mathbit{\text{Mn}}}$ remains unchanged but ${\mathbit{\ensuremath{\mu}}}_{\text{Eu}}$ develops an additional substantial in-plane canting. This phase has MSG $P11\frac{{2}_{1}}{{a}^{\ensuremath{'}}}$. We also find some evidence of short-range magnetic correlations associated with the Eu between $12\phantom{\rule{4pt}{0ex}}\text{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}30\phantom{\rule{4pt}{0ex}}\text{K}$. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg model. We then discuss implications of the various AFM states in ${\mathrm{EuMnSb}}_{2}$ and their potential for tuning topological properties.

Accurate crystal structure of ice VI from X-ray diffraction with Hirshfeld atom refinement.

Water is an essential chemical compound for living organisms, and twenty of its different crystal solid forms (ices) are known. Still, there are many fundamental problems with these structures such as establishing the correct positions and thermal motions of hydrogen atoms. The list of ice structures is not yet complete as DFT calculations have suggested the existence of additional and - to date - unknown phases. In many ice structures, neither neutron diffraction nor DFT calculations nor X-ray diffraction methods can easily solve the problem of hydrogen atom disorder or accurately determine their anisotropic displacement parameters (ADPs). Here, accurate crystal structures of H2O, D2O and mixed (50%H2O/50%D2O) ice VI obtained by Hirshfeld atom refinement (HAR) of high-pressure single-crystal synchrotron and laboratory X-ray diffraction data are presented. It was possible to obtain O-H/D bond lengths and ADPs for disordered hydrogen atoms which are in good agreement with the corresponding single-crystal neutron diffraction data. These results show that HAR combined with X-ray diffraction can compete with neutron diffraction in detailed studies of polymorphic forms of ice and crystals of other hydrogen-rich compounds. As neutron diffraction is relatively expensive, requires larger crystals which can be difficult to obtain and access to neutron facilities is restricted, cheaper and more accessible X-ray measurements combined with HAR can facilitate the verification of the existing ice polymorphs and the quest for new ones.

Spin Waves and Magnetic Exchange Hamiltonian in CrSBr.

CrSBr is an air‐stable two‐dimensional (2D) van der Waals semiconducting magnet with great technological promise, but its atomic‐scale magnetic interactions—crucial information for high‐frequency switching—are poorly understood. An experimental study is presented to determine the CrSBr magnetic exchange Hamiltonian and bulk magnon spectrum. The A‐type antiferromagnetic order using single crystal neutron diffraction is confirmed here. The magnon dispersions are also measured using inelastic neutron scattering and rigorously fit the excitation modes to a spin wave model. The magnon spectrum is well described by an intra‐plane ferromagnetic Heisenberg exchange model with seven nearest in‐plane exchanges. This fitted exchange Hamiltonian enables theoretical predictions of CrSBr behavior: as one example, the fitted Hamiltonian is used to predict the presence of chiral magnon edge modes with a spin‐orbit enhanced CrSBr heterostructure.

Atomic View of Aqueous Cyclosporine A: Unpacking a Decades-Old Mystery.

An atomic view of a main aqueous conformation of cyclosporine A (CycA), an important 11-amino-acid macrocyclic immunosuppressant, is reported. For decades, it has been a grand challenge to determine the conformation of free CycA in an aqueous-like solution given its poor water solubility. Using a combination of X-ray and single-crystal neutron diffraction, we unambiguously resolve a unique conformer (A1) with a novel cis-amide between residues 11 and 1 and two water ligands that stabilize hydrogen bond networks. NMR spectroscopy and titration experiments indicate that the novel conformer is as abundant as the closed conformer in 90/10 (v/v) methanol/water and is the main conformer at 10/90 methanol/water. Five other conformers were also detected in 90/10 methanol/water, one in slow exchange with A1, another one in slow exchange with the closed form and three minor ones, one of which contains two cis amides Abu2-Sar3 and MeBmt1-MeVal11. These conformers help better understand the wide spectrum of membrane permeability observed for CycA analogues and, to some extent, the binding of CycA to protein targets.

L-Histidine with nitric acid: A comparison of crystal structures and Hirshfeld surfaces analysis

L-histidine crystallizes in three different crystal structure with nitric acid, i.e. L-histidinium nitrate, L-histidinium nitrate monohydrate and L-Histidinium dinitrate. Also there is DL-histidinium dinitrate. Though the structural aspects of histidinium nitrate monohydrate, L-Histidinium dinitrate and DL-histidinium nitrate are well studied, the structural studies on L-histidinium nitrate is not dealt in detail. Here we have tried to discuss in detail the crystal structure of L-histidinium nitrate using both single crystal X-ray and neutron diffraction. The crystal structure consists one L-histidine molecule which has the amino and imino groups protonated, but the carboxylic acid group is deprotonated. The nitrate ion is in anionic state ie NO3−. The conformation of histidine (N1-C2-C3-C4=64.99°) is gauche (-). Also a comparison of the structure of L-histidinium nitrate with other three available crystal structures retrieved using CSD have been carried out. It was found that L-histidine in L-histidinium nitrate monohydrate adopts gauche (+) conformation and in other three structure it is gauche (-), and L-histidinium nitrate has the lowest volume. Hirshfeld surface and interaction energies analysis of all the above structures were carried out using the x-ray data. L-histidinium nitrate monohydrate shows maximum globuarity. Interaction energy obtained from energy framework showed that L-histidinium nitrate monohydrate has maximum total energy of -41.5(kJ/Mole). The asphericity of our structure L-histidinium nitrate is 0.252 indicating prolate nature unlike the other three complexes. The percentage of H…H contacts is maximum in L-histidinium nitrate which is 27.3%. A comparison of various contacts of histidine between the x-ray and neutron structure of ours shows that the maximum change is reflected in the C…H and H…H contacts with C…H having a higher percentage and H…H having a lower percentage in the neutron structure indicating that the neutron diffraction maps shows the hydrogen bond interaction more accurately.The three complexes, except DL Histidinium dinitrate crystallizes in a non-centrosymmetric space group and hence could exhibit second-order optical non-linear properties, attempts were made to calculate heats of formation, dipole moments (D), polarizabilities (α) and first hyperpolarizabilities (β) using the semi empirical method at the PM6 level using MOPAC2009 and the packing energy as implemented in MERCURY. It was found that the average β of L-histidinium nitrate monohydrate was the highest amongst the three showing that it is more suitable for NLO material.

Reduced crystal symmetry as the origin of the ferroelectric polarization within the incommensurate magnetic phase of TbMn2O5

The precise crystal symmetry, and hence the emergence of the electric polarization, still remains an open question in multiferroic $R$Mn${}_{2}$O${}_{5}$ ($R$=rare earth, Bi, Y). While previous diffraction studies have indicated the centrosymmetric space group $P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m$, an atomic displacement allowing for electric polarization would require a noncentrosymmetric symmetry. Here, single-crystal neutron diffraction experiments on TbMn${}_{2}$O${}_{5}$ provide direct evidence of a reduced crystallographic symmetry with the polar space group $P1211$ already above the multiferroic phase transition, indicating that a symmetric ${\mathbf{S}}_{\mathbf{i}}\ensuremath{\cdot}{\mathbf{S}}_{\mathbf{j}}$ spin coupling, i.e., magnetostriction is the dominant mechanism in the commensurate magnetic phase.

On the crystal-chemistry of meyerhofferite, CaB3O3(OH)5·H2O

The crystal structure and crystal chemistry of meyerhofferite, ideally CaB3O3(OH)5·H2O, was investigated by a multi-methodological approach based on titrimetric determination of boron, gravimetric determination of calcium, determination of fluorine by ion selective electrode, determination of water content by heating, other minor elements by inductively coupled plasma atomic emission spectroscopy, along with single-crystal synchrotron X-ray and neutron diffraction. The concentration of more than 50 chemical elements was measured. The combination of these techniques proves that the composition of meyerhofferite approaches the ideal one (i.e., (Ca1.012Mg0.003) (B2.984Si0.001)O3(OH)5·1.018H2O), with only a modest fraction of Mg (with MgO ≈ 0.03 wt%) replacing Ca, and with Si the only potential substituent of tetrahedral B (with SiO2 ≈ 0.02 wt%). The content of REE and other minor elements is, overall, not significant, including that of fluorine as a potential OH− substituent (i.e., < 0.01 wt%). These findings have some relevant geochemical and technical implications, here discussed. The X-ray and neutron structure model obtained in this study prove that the building units of the structure of meyerhofferite consist of: two BO2(OH)2 tetrahedra and one BO2(OH) triangle, linked by corner-sharing to form [B3O3(OH)5]2− rings, and distorted Ca-polyhedra (with CN = 8, CaO3(OH)4(OH2)), linked by edge-sharing to form infinite chains along [001]. The B3O3(OH)5 rings are connected to the Ca-polyhedra chains by corner- and edge-sharing, on two sides of the chains. These heteropolyhedral chains, made by Ca-polyhedra and B3O3(OH)5 rings, are mutually connected through hydrogen bonding only, giving rise to the tri-dimensional edifice of meyerhofferite. The neutron structure refinement showed no evidence of static or dynamic disorder pertaining to the H sites; their libration regime was found to be significantly anisotropic. At least seven of the nine oxygen sites of the structure are involved in H-bonding, as donors or as acceptors. The role played by the H-bonding scheme on the physical properties of meyerhofferite is discussed.

Pressure dependence of the magnetic ground state in CePtSi2

CePtSi2 was reported to exhibit an antiferromagnetic order below T*=1.8 K at ambient pressure, a valence state change at ~1.2 GPa, and superconductivity in the range between 1.4 and 2.1 GPa with the maximum transition temperature of 0.14 K [T. Nakano et al., Phys. Rev. B 79, 172507 (2009)]. We have performed polycrystalline and single crystal neutron diffraction experiments to determine the magnetic structure under ambient and high pressures. We found that incommensurate magnetic peaks with the magnetic propagation vector of (0.32, 0, 0.11) at ambient pressure below T_{SDW}~1.25 K. Those magnetic peaks which originate from a spin-density-wave order with the easy axis along the c axis and an averaged ordered moment of 0.45(5) mu_B, suggesting that there may be an intermediate phase between T* and T_{SDW}. Applying pressures, the magnetic propagation vector shows no change and the magnetic order disappears around 1.0 GPa, which is much lower than the critical pressure for the superconducting phase. The results suggest that other than magnetic fluctuations may play a primary role in the superconducting pairing mechanism.

Anisotropy-driven response of the fractional antiferromagnetic skyrmion lattice in MnSc2S4 to applied magnetic fields

We theoretically and experimentally study the stability of the unconventional fractional antiferromagnetic skyrmion lattice (AF-SkL) in ${\mathrm{MnSc}}_{2}{\mathrm{S}}_{4}$ spinel under magnetic fields applied along the [1-10] crystal direction. By performing numerical Monte Carlo simulations for the minimal effective spin model that we proposed in S. Gao et al., Nature 586, 37 (2020), we show that the lattice is aligned within the equivalent and symmetric [1-11] or [1-1-1] planes, which are equally inclined to the applied magnetic-field $H$. We attribute this behavior to the magnetic anisotropy of the host material. Neutron single-crystal diffraction presents a very good agreement with the predictions of the effective model. It reveals that the topological spin texture gets destabilized at low temperatures and moderate magnetic fields and is replaced by a conical phase for $H$// [1-10]. The present study elucidates the central role of the magnetic anisotropy in the stabilization of AF-Sk states.

Evidence for pressure induced unconventional quantum criticality in the coupled spin ladder antiferromagnet C9H18N2CuBr4

Quantum phase transitions in quantum matter occur at zero temperature between distinct ground states by tuning a nonthermal control parameter. Often, they can be accurately described within the Landau theory of phase transitions, similarly to conventional thermal phase transitions. However, this picture can break down under certain circumstances. Here, we present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics of the spin-1/2 ladder compound C9H18N2CuBr4. Single-crystal heat capacity and neutron diffraction measurements reveal that the Néel-ordered phase breaks down beyond a critical pressure of Pc ∼ 1.0 GPa through a continuous quantum phase transition. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. The inelastic neutron scattering spectra at 1.3 GPa are characterized by two well-separated gapped modes, including one continuum-like and another resolution-limited excitation in distinct scattering channels, which further indicates an exotic quantum-disordered phase above Pc.

Structural &amp; Chemical Study of Molecular Crystals Based on Precise Positions of Hydrogen Atoms by Single-Crystal Neutron Diffraction

Structural &amp; Chemical Study of Molecular Crystals Based on Precise Positions of Hydrogen Atoms by Single-Crystal Neutron Diffraction

The spontaneous self-assembly of a molecular water pipe in 3D space.

The self-assembly and self-organization of water molecules are relevant in many fields of research. When water spontaneously reacts with 2,2,6,6-tetra-methyl-piperidine (TMP) to form colourless and crystalline discrete needles, only in the exact ratio of 2:1, it is important to understand the phenomenon. Single-crystal X-ray and neutron diffraction data have unveiled that TMP self-assembles around columns of water molecules, and as such, the resulting adduct may be described as a series of molecular water pipes.

Noncollinear magnetic order in epitaxial thin films of the centrosymmetric MnPtGa hard magnet

Magnetic systems exhibiting spin-canted states have garnered much attention recently for their promising rich exotic properties driven by the real-space spin textures and competing magnetic orders. In this study, we present the structural and magnetic properties of hexagonal 60 nm MnPtGa epitaxial thin films grown by magnetron sputtering on Al2O3(0001) single-crystalline substrates. The MnPtGa film crystallizes in the centrosymmetric P63/mmc (No. 194) space group, showing perpendicular magnetic anisotropy along the c-axis, with a Curie temperature TC = 263 K. In addition, the MnPtGa film undergoes a spin reorientation transition at Tsr = 160 K. We investigated the MnPtGa magnetic ground states using single-crystal neutron diffraction. A structurally forbidden (001) magnetic Bragg reflection emerges below Tsr, indicating the existence of a spin-canted state, where the magnetic moments align ferromagnetically perpendicular to the basal plane, and a non-zero in-plane component exhibits an antiferromagnetic ordering along the c-axis. At 2 K, the refined magnetic moments of Mn are μz = 4.2(4) μB and μx = 1.5(3) μB, projected onto the c-axis and basal plane, respectively. Hence, we determined a 20° Mn spin canting angle off from the c-axis.

A neutron diffraction study of boussingaultite, (NH4)2[Mg(H2O)6](SO4)2

Abstract The crystal structure and chemical composition of boussingaultite from Pécs-Vasas, Mecsek Mountains, South Hungary, were investigated by single-crystal neutron diffraction (at 20 K) along with a series of chemical analytical techniques [i.e., gravimetric determination of sulfates, EDTA titrimetric determination of magnesium, ion selective electrode for F and Cl, indirect gravimetric determination of ammonium as (NH4,Rb,Cs,K) tetraphenylborate, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H content, high-T mass loss for H2O content]. The concentrations of more than 50 elements were measured. The experimental formula of the boussingaultite is: [(NH4)1.77K0.22)Σ1.99[(Mg0.95Mn0.06)Σ1.01(H2O)5.7](SO4)1.99. Neutron data analysis confirms that the structure of boussingaultite is built up by isolated Mg(H2O)6-octahedra, along with isolated NH4- and SO4-tetrahedra connected by a complex H-bonds network. Mg2+ is completely solvated by H2O molecules in a typical octahedral bonding configuration. All the seven independent oxygen sites in the structure are involved in H-bonds, as donors or as acceptors. The geometry of all the H2O molecules, bonded to Mg, is in line with that usually observed in crystalline compounds. The H2O molecules show moderate-strong H-bonds, with H···Oacceptor and Odonor···Oacceptor ranging between 1.72–1.87 and 2.70–2.84 Å, respectively, along with Odonor-H···Oacceptor angles between 168–178°. The four independent N-H···O bonds show H···Oacceptor and Ndonor···Oacceptor distances ranging between 1.81–2.00 and 2.84–2.98 Å, respectively, with N-H···O angles between 158–176°. All the H-bonds of the H2O molecules and of the NH4-group involve the oxygen sites of the SO4-group as acceptors: the SO4-group is, therefore, the “bridging unit” between the NH4 and the Mg(H2O)6 units, via H-bonds. Our structure refinement proved, unambiguously, that the partial K+ vs. NH4+ replacement generates a local disorder. K lies at the N site, and its bonding configuration can be described by a distorted polyhedron with CN = 8. However, the K+ vs. NH4+ replacement implies a change in the configuration of the SO4-tetrahedron, through a sort of rotation of the polyhedron. This is the first evidence of the presence of a partial picromerite component in the boussingaultite structure, which gives rise to a local disorder likely due to the significantly different bonding configurations of the two cations. Our refinement also revealed that Mn2+ replaces Mg2+ at the Mg site. No evidence of distortion of the octahedron is observed in response to such a replacement, but the fraction of Mn2+ is modest. An analysis of previous Raman and IR results is provided, and is compared with the experimental results of this study.