Single-crystal Neutron Diffraction Research Articles

Crystal and magnetic structure of the (trimim)[FeBr4] molten salt: A temperature dependence study

A new imidazolium-based halometallate complex with formula (trimim)[FeBr4] (trimim: 1,2,3-trimethylimidazolium) has been synthesized and characterized as a function of the temperature. The structural phase transitions, magnetic properties and magnetic structure of this compound have been thoroughly studied. The crystal structure evolution from 1.8 to 400 K has been investigated by a combination of synchrotron X-ray and neutron powder diffraction, as well as single crystal X-ray and neutron diffraction. Four different phases were found: phase I, from 400 to 350 K, with the orthorhombic Cmc21 space group; phase II, from 350 to 160 K, which was modelled with the monoclinic P21/m space group; and phase III, stable down to 13 K and showing the monoclinic Cm space group. The transition from phase III to II involves the appearance of disorder in the organic cations. Magnetometry measurements indicate a long-range magnetic order below 13 K, which was confirmed by single crystal and powder neutron X-ray diffraction analyses. The occurrence of new reflections that were not possible to index with the phase III unit cell suggests a symmetry breaking, giving rise to the phase IV. A joint refinement (single-crystal and powder data) revealed that the latter phase exhibits the magnetic Shubnikov Ccc space group. This model consists of a strictly antiferromagnetic structure with ferromagnetic interactions within the unit cell, which is compatible with the magnetometry results.

Neutron scattering investigation of proposed Kosterlitz-Thouless transitions in the triangular-lattice Ising antiferromagnet TmMgGaO4

The transverse-field Ising model on the triangular lattice is expected to host an intermediate finite-temperature Kosterlitz-Thouless (KT) phase through a mapping of the spins on each triangular unit to a complex order parameter. TmMgGaO$_4$ is a candidate material to realize such physics due to the non-Kramers nature of Tm$^{3+}$ ion and the resulting two-singlet single-ion ground state. Using inelastic neutron scattering, we confirm this picture by determining the leading parameters of the low-energy effective Hamiltonian of TmMgGaO$_4$. Subsequently, we track the predicted KT phase and related transitions by inspecting the field and temperature dependence of the ac susceptibility. We further probe the spin correlations in both reciprocal space and real space via single crystal neutron diffraction and magnetic total scattering techniques, respectively. Magnetic pair distribution function analysis provides evidence for the formation of vortex-antivortex pairs that characterize the proposed KT phase around 5~K. Although structural disorder influences the field-induced behavior of TmMgGaO$_4$, the magnetism in zero field appears relatively free from these effects. These results position TmMgGaO$_4$ as a strong candidate for a solid-state realization of KT physics in a dense spin system.

Operando single crystal neutron diffraction reveals insight into the field response mechanisms in the hydrogen-bonded KH2PO4 ferroelectric

The mechanism that facilitates polarization reorientation in KH2PO4 (KDP) was investigated using operando single-crystal neutron diffraction. Diffraction data were measured from a KDP single crystal during the application of alternating electric fields and were then binned into 40 increments to enable field-dependent single-crystal structure refinements. The field-dependent structures are compared with an as-grown crystal to determine how the lattice and atomic sites evolve in response to the applied electric fields. These analyses provide evidence that the reorientation of the macroscopic polarization is facilitated through a cooperative change in hydrogen bonding, which results in the reversal of the spontaneous dipole. In addition, a decrease in secondary extinction near the coercive field indicates that the inversion of the macroscopic polarization is achieved through the nucleation and subsequent growth of new domains.

High-field spin-flop state in green dioptase

The high-field magnetic properties and magnetic order of the gem mineral green dioptase Cu$_6[$Si$_6$O$_{18}]\cdot 6$H$_2$O have been studied by means of single-crystal neutron diffraction in magnetic fields up to $21~$T and magnetization measurements up to $30~$T. In zero field, the Cu$^{2+}$-moments in the antiferromagnetic chains are oriented along the $c$-axis with a small off-axis tilt. For a field applied parallel to the $c$-axis, the magnetization shows a spin-flop-like transition at $B^*=12.2~$T at $1.5~$K. Neutron diffraction experiments show a smooth behavior in the intensities of the magnetic reflections without any change in the periodicity of the magnetic structure. Bulk and microscopic observations are well described by a model of ferromagnetically coupled antiferromagnetic $XXZ$ spin-$\frac{1}{2}$ chains, taking into account a change of the local easy-axis direction. We demonstrate that the magnetic structure evolves smoothly from a deformed N\'eel state at low fields to a deformed spin-flop state in a high field via a strong crossover around $B^*$. The results are generalized for different values of interchain coupling and spin anisotropy.

Zero-field magnetic ground state of EuMg2Bi2

Layered trigonal EuMg$_2$Bi$_2$ is reported to be a topological semimetal that hosts multiple Dirac points that may be gapped or split by the onset of magnetic order. Here, we report zero-field single-crystal neutron-diffraction and bulk magnetic susceptibility measurements versus temperature $\chi(T)$ of EuMg$_2$Bi$_2$ that show the intraplane ordering is ferromagnetic (Eu$^{2+},\, S= 7/2$) with the moments aligned in the $ab$-plane while adjacent layers are aligned antiferromagnetically (i.e., A-type antiferromagnetism) below the N\'eel temperature.

Correlated Li-ion migration in the superionic conductor Li10GeP2S12

A combination of single-crystal neutron diffraction experiments at low temperature and first-principles calculations revealed that a correlated migration of the densely packed Li ions governs the overall Li-ion conduction in Li10GeP2S12.

Structural, electronic, and magnetic properties of nearly ideal Jeff=12 iridium halides

Heavy transition metal magnets with $J_{\rm eff}$ $=$ 1/2 electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal $J_{\rm eff}$ $=$ 1/2 states are scarce due to the importance of non-cubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e. K$_2$IrCl$_6$, K$_2$IrBr$_6$, (NH$_4$)$_2$IrCl$_6$, and Na$_2$IrCl$_6 \cdotp $ 6(H$_2$O)] with Ir$^{4+}$ electronic ground states in extremely close proximity to the ideal $J_{\rm eff}$ $=$ 1/2 limit, despite a variation in the low-temperature global crystal structures. We also find ordered magnetic ground states for the three anhydrous systems, with single crystal neutron diffraction on K$_2$IrBr$_6$ revealing Type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet.

Noncollinear magnetic structure and magnetoelectric coupling in buckled honeycomb Co4Nb2O9 : A single-crystal neutron diffraction study

Through analysis of single crystal neutron diffraction data, we present the magnetic structures of magnetoelectric Co4Nb2O9 under various magnetic fields. In zero-field, neutron diffraction experiments below TN=27 K reveal that the Co2+ moments order primarily along the a* direction without any spin canting along the c axis, manifested by the magnetic symmetry C2/c'. The moments of nearest neighbor Co atoms order ferromagnetically with a small cant away from the next nearest neighbor Co moments along the c axis. In the applied magnetic field H//a, three magnetic domains were aligned with their major magnetic moments perpendicular to the magnetic field with no indication of magnetic phase transitions. The influences of magnetic fields on the magnetic structures associated with the observed magnetoelectric coupling are discussed.

Experimental Evidence of the Coexistence of Proper Magnetic and Structural Incommensurability on the [CH3NH3][Ni(COOH)3] Compound.

The present work is dedicated to characterization of the structural phase transition and incommensurate magnetic structure of the [CH3NH3][Ni(COOH)3] (1) perovskite-like metal-organic compound. The structural and magnetic characterization has been performed through variable-temperature single-crystal and powder neutron diffraction. Compound 1 crystallizes in the orthorhombic Pnma space group at room temperature. Below 84 K, a new phase has been observed. The occurrence of new reflections, which can be indexed with a wavevector along the c axis [q = 0.1426(2)c*], suggests the occurrence of an incommensurately modulated crystal structure. The structure was determined using the superspace group formalism on the Pnma(00γ)0s0 space group. This incommensurate phase remains unchanged with a decrease of the temperature up to the base temperature (ca. 2 K). Moreover, the magnetic susceptibility data, collected under zero-field-cooled and field-cooled conditions at different applied magnetic fields, show that compound 1 exhibits antiferromagnetic behavior below 34 K. In the current paper, we have confirmed that compound 1 presents the coexistence of nuclear and proper magnetic incommensurability below TN.

Magnetic structures, spin-flop transition, and coupling of Eu and Mn magnetism in the Dirac semimetal EuMnBi2

We report here a comprehensive study of the AFM structures of the Eu and Mn magnetic sublattices as well as the interplay between Eu and Mn magnetism in this compound by using both polarized and non-polarized single-crystal neutron diffraction. Magnetic susceptibility, specific heat capacity measurements and the temperature dependence of magnetic diffractions suggest that the AFM ordering temperature of the Eu and Mn moments is at 22 and 337 K, respectively. The magnetic moments of both Eu and Mn ions are oriented along the crystallographic $c$ axis, and the respective magnetic propagation vector is $\textbf{k}_{Eu} = (0,0,1)$ and $\textbf{k}_{Mn}=(0,0,0)$. With proper neutron absorption correction, the ordered moments are refined at 3 K as 7.7(1) $\mu_B$ and 4.1(1) $\mu_B$ for the Eu and Mn ions, respectively. In addition, a spin-flop (SF) phase transition of the Eu moments in an applied magnetic field along the $c$ axis was confirmed to take place at a critical field of B$_c$ $\sim$ 5.3 T. The evolution of the Eu magnetic moment direction as a function of the applied magnetic field in the SF phase was also determined. Clear kinks in both field and temperature dependence of the magnetic reflections ($\pm1$, 0, 1) of Mn were observed at the onset of the SF phase transition and the AFM order of the Eu moments, respectively. This unambiguously indicates the existence of a strong coupling between Eu and Mn magnetism. The interplay between two magnetic sublattices could bring new possibilities to tune Dirac fermions via changing magnetic structures by applied fields in this class of magnetic topological semimetals.

Magnetic properties and magnetic structure of the frustrated quasi-one-dimensional antiferromagnet SrCuTe2O6

Magnetization measurements on single-crystal cubic SrCuTe$_2$O$_6$ with an applied magnetic field of along three inequivalent high symmetry directions $[100]$, $[110]$, and $[111]$ reveal weak magnetic anisotropy. The fits of the magnetic susceptibility to the result from a quantum Monte Carlo simulation on the Heisenberg spin-chain model, where the chain is formed via the dominant third-nearest-neighbor exchange interaction $J_3$, yield the intra-chain interaction $(J_3/k_B)$ between 50.12(7) K for the applied field along $[110]$ and 52.5(2) K along $[100]$ with about the same $g$-factor of 2.2. Single-crystal neutron diffraction unveils the transition to the magnetic ordered state as evidenced by the onset of the magnetic Bragg intensity at $T_\textrm{N1}=5.25(9)$ K with no anomaly of the second transition at $T_\textrm{N2}$ reported previously. Based on irreducible representation theory and magnetic space group analysis of powder and single-crystal neutron diffraction data, the magnetic structure in the Shubnikov space group $P4_132$, where the Cu$^{2+}$~$S=1/2$ spins antiferromagnetically align in the direction perpendicular to the spin chain, is proposed. The measured ordered moment of $0.52(6)~\mu_B$, which represents 48% reduction from the expected value of $1~\mu_B$, suggests the remaining influence of frustration resulting from the $J_1$ and $J_2$ bonds.

Understanding the Correlation between Oxide Ion Mobility and Site Distributions in Ba3NbWO8.5.

A correlation between oxygen site distributions and ionic conductivity has been established in the recently discovered family of oxide-ion conductors Ba3M2O8.5±δ (M = Nb, V, Mo, W). We rationalize this observation on the basis of structural insights gained from the first single-crystal neutron diffraction data collected for a member of this family, Ba3NbWO8.5, and theoretical considerations of bonding and O site energies.

Stacking Faults and Short‐Range Magnetic Correlations in Single Crystal Y5Ru2O12: A Structure with Ru+4.5 One‐Dimensional Chains

The synthesis, structure, and physical properties of Y5Ru2O12, a new member of the Ln5B2O12 class (Ln = lanthanide, B = Ru, Re, Mo) are reported. Single crystals of Y5Ru2O12 are synthesized using a high‐temperature, high‐pressure (700 °C, 200 MPa) hydrothermal method. The structure features edge‐sharing RuORu 1D chains of distorted RuO6 octahedra. The YOn (n = 6 and 7) polyhedral groups bridge RuORu chains across the unit cell by forming a complex 3D structure. The RuORu chains contain alternating shorter and longer RuRu distances along the chain. Single crystal neutron diffraction shows streak‐like diffuse scattering along the H‐direction, due to stacking faults. Temperature‐dependent anisotropic magnetic measurements reveal a broad feature over a wide temperature range, with a maximum at ≈70 K indicating potential short‐range ordering, with canted antiferromagnetic behavior below 30 K. The dominant magnetic interactions are antiferromagnetic with a large negative Weiss temperature θ ≈ −200 K. The compound has an effective magnetic moment that is anomalously large for this class of compounds (≈4 μB Ru−1), and a phenomenological spin Hamiltonian describing the unusual anisotropic magnetization of Y5Ru2O12 is proposed.

Pseudospin-lattice coupling and electric control of the square-lattice iridate Sr2IrO4

$\rm Sr_2IrO_4$ is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted novel states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a newly observed microscopic rotation of the IrO$_6$ octahedra below 50~K measured by single crystal neutron diffraction. This sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.

A multi-methodological study of kernite, a mineral commodity of boron

Abstract Kernite, ideally Na2B4O6(OH)2∙3H2O, is a major constituent of borate deposits and one of the most important mineral commodities of B. The chemical composition and crystal structure of kernite from the Kramer Deposit (Kern County, California) were investigated by a suite of analytical techniques (i.e., titrimetric determination of B content, gravimetric method for Na, ion selective electrode for F, high-T mass loss for H2O content, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H contents) and single-crystal X-ray (at 293 K) and neutron (at 20 K) diffraction. The concentrations of more than 50 elements were measured. The general experimental formula of the kernite sample used in this study is Na1.99B3.99O6(OH)2∙3.01H2O. The fraction of other elements is, overall, insignificant: excluding B, kernite from the Kramer Deposit does not act as geochemical trap of other technologically relevant elements (e.g., Li, Be, or REE). The X-ray and neutron structure model obtained in this study confirms that the structure of kernite is built up by: two (crystallographically independent) triangular BO2OH groups and two tetrahedral BO4 groups, which share corner-bridging O atoms to form threefold rings, giving chains running along [010], and NaO4(OH)(OH2) and NaO2(OH)(OH2)3 polyhedra. Positional disorder of two H sites of H2O molecules was observed by the neutron structure refinement and corroborated by the maximum-entropy method calculation, which consistently provided a model based on a static disorder, rather than a dynamic one. The H-bonding network in the structure of kernite is complex, pervasive, and plays a primary role on its structural stability: the majority of the oxygen sites are involved in H-bonding, as donors or as acceptors. The potential utilizations of kernite, as a source of B (B2O3 ~50 wt%), are discussed, on the basis of the experimental findings of this study.

Incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pm{\delta}$), where ${\delta}$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

Large spin-driven dielectric response and magnetoelectric coupling in the buckled honeycomb Fe4Nb2O9

By combining single crystal x-ray and neutron diffraction, and the magnetodielectric measurements on single crystal Fe4Nb2O9, we present the magnetic structure and the symmetry-allowed magnetoelectric coupling in Fe4Nb2O9. It undergoes an antiferromagnetic transition at TN=93 K, followed by a displacive transition at TS=70 K. The temperature-dependent dielectric constant of Fe4Nb2O9 is strongly anisotropic with the first anomaly at 93 K due to the exchange striction as a result of the long range spin order, and the second one at 70 K emanating from the structural phase transition primarily driven by the O atomic displacements. Magneticfield induced magnetoelectric coupling was observed in single crystal Fe4Nb2O9 and is compatible with the solved magnetic structure that is characteristic of antiferromagnetically arranged ferromagnetic chains in the honeycomb plane. We propose that such magnetic symmetry should be immune to external magnetic fields to some extent favored by the freedom of rotation of moments in the honeycomb plane, laying out a promising system to control the magnetoelectric properties by magnetic fields.

Metamagnetic transitions and magnetoelectric responses in the chiral polar helimagnet Ni2InSbO6

Magnetic-field effect on the magnetic and electric properties in a chiral polar ordered corundum Ni$_2$InSbO$_6$ has been investigated. Single-crystal soft x-ray and neutron diffraction measurements confirm long-wavelength magnetic modulation. The modulation direction tends to align along the magnetic field applied perpendicular to the polar axis, suggesting that the nearly proper-screw type helicoid should be formed below 77\,K. The application of a high magnetic field causes a metamagnetic transition. In a magnetic field applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition takes place through the intermediate soliton lattice type state. On the other hand, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.

Flux Synthesis of a Metal Carbide Hydride Using Anthracene As a Reactant.

La15(FeC6)4H was synthesized from the reaction of iron and anthracene in La/Ni eutectic flux. Anthracene was the source of both the carbon and hydrogen in the product. The structure of this metal carbide hydride features hydride ions in tetrahedral interstitial sites surrounded by lanthanum ions, which was confirmed by single-crystal neutron diffraction studies. The trigonal planar FeC6 units in which the central iron atom is coordinated by three ethylenide groups are similar to those found in La3.67FeC6, a previously reported compound that is formed in the absence of a hydride source. Magnetic susceptibility data confirm that the iron sites do not have magnetic moments. Density of states calculations indicate that La15(FeC6)4H is metallic and is stabilized by the incorporation of hydride anions.

Cover Feature: Polyhydrido Copper Nanoclusters with a Hollow Icosahedral Core: [Cu30H18{E2P(OR)2}12] (E=S or Se; R=nPr, iPr or iBu) (Chem. Eur. J. 46/2020)

The structurally precise polyhydrido copper nanocluster (NC) [Cu30H18{S2P(OnPr)2}12] which contains an unprecedented inner hollow Cu12 icosahedral core has been fully characterized by single-crystal neutron diffraction; while Ag12 and Au12 inner hollow icosahedra encircled within Ag44 and Au32 NCs have been reported by Bigioni and Wang in 2013 and 2019, respectively. Pd doping on the Cu30 NC led to the isolation of Pd−Cu bimetallic NCs, which exhibit a linear [(PdH2)2−]-centered icosahedral core. More information can be found in the Full Paper by J.-Y. Saillard, C. W. Liu, et al. on page 10471.