A suite-level review of the neutron single-crystal diffraction instruments at Oak Ridge National Laboratory

The Oak Ridge National Laboratory (ORNL) neutron scattering facility hosts seven single-crystal diffractometers. At the Spallation Neutron Source (SNS), there are a high-resolution neutron time-of-flight (TOF) Laue diffractometer (TOPAZ) for small molecule and chemical crystallography, a macromolecular diffractometer (MaNDi), a diffuse scattering spectrometer focusing on quantum materials (CORELLI), and a high-pressure dual-purpose—powder and single crystal—diffractometer [Spallation Neutrons at Pressure (SNAP)]. At the High Flux Isotope Reactor (HFIR), there are Wide-Angle Neutron Diffractometer squared (WAND²), a dual-purpose powder/single-crystal diffractometer, and a four-circle diffractometer (FCD) for the determination of magnetic and crystal structures from small single crystals. IMAGINE in the cold-guide hall at HFIR is focused on the neutron structure determination of macromolecules. ORNL has both a spallation neutron source (SNS) and a reactor neutron source (HFIR) co-located at the same site which has enabled the construction of a complete suite of single crystal diffractometers. Most of these ORNL instruments, except the HFIR FCD, are still ramping up in productivity (TOPAZ, MaNDi, and IMAGINE) or are either in or just coming out of commissioning (CORELLI, SNAP for single-crystal studies, and WAND²). New sample environments are either under active development or in commissioning for these instruments and limited polarization options have been available (e.g., FCD), but for each instrument, improvements are also ongoing for data acquisition and analysis software, and/or detectors and detector coverage.

The Anger camera detectors¹ on several of these instruments (TOPAZ, MaNDi, SNAP, and FCD) are state of the art and designed, and originally built, in-house. A next-generation Anger camera has already been designed, built, and tested that has a higher resolution, is insensitive to stray magnetic fields, and costs half as much to construct compared to the first generation. A third-generation design is possible with even smaller pixel size. Many of these instruments are pushing the limits of minimum crystal size needed for neutron single-crystal diffraction, which traditionally has been a disadvantage of this method. The elastic diffuse scattering instrument, CORELLI, is unique in being able to reconstruct the elastic-only scattering and offers a powerful new tool for studying atomic/spin disorder and short-range order in crystals through diffuse scattering. The progression in the unit cell size that can be efficiently studied by TOPAZ (10⁵ ų) < IMAGINE (10⁶ ų) < MaNDi (10⁷ ų), combined with in-house protein crystal growth and deuteration facilities, is enabling a broad range of macromolecular neutron single-crystal studies of complex biological systems. In a structural model, the positions of most hydrogen atoms can be reliably deduced from the chemical groups to which they are bound. This is, however, not the case for the small fraction of hydrogen atoms that are exchangeable and more labile. For these functionally and often extremely important hydrogen atoms, neutron diffraction is the only technique that allows the experimental determination of their positions. It is fitting that ORNL is poised to become a world leader in single-crystal neutron diffraction studies given that the first such experiments and first single-crystal diffractometer automation were done at ORNL more than half a century ago.

Magnetic structural studies are accomplished best using neutron diffraction and typically require a combination of low and high temperatures (with respect to magnetic phase transition) at minimum and, for more advanced studies, applied magnetic fields, polarization analysis, and high pressure. Sometimes magnetic structures cannot be unequivocally solved using powder diffraction data alone, and single-crystal diffraction data are vital. Magnetic structural studies continue to expand to even more complexity (modulated structures, large cells, reduced dimensionalities manifesting diffuse scattering, geometrically frustrated systems that do not order, etc.). Future directions in instrumentation capabilities will include incident beam polarization options at more beamlines, higher applied magnetic fields, higher pressure sample cells combined with lower temperatures, and expanded detector coverage. Nuclear structure studies on small unit cell crystals will continue to focus on light elements (e.g., H, D, and Li), which are the key to understanding a diverse array of technologically important materials, such as batteries, metal-organic frameworks (MOFs), catalysts, and chemical sensors, to name but a few. Parametric studies of energy-related materials, such as thermoelectrics, metal hydride complexes, hybrid organic inorganic perovskites, and multiferroic materials, will continue to be well suited to neutron diffraction studies, owing to the ease of complex sample environments, such as applied electric fields and controlled atmosphere and gas exchange. Neutron single crystal diffraction on TOPAZ addresses structural problems in multiple research areas: energy materials, catalysis, hydrogen bonding, structural and magnetic properties of functional materials, minerals, and earth and environmental sciences.

Given the diversity of diffractometers we have with large, highly pixelated, 2D detectors, diffuse scattering, due to nuclear/spin disorder and/or short-range correlation, is growing into a major research focus. The CORELLI instrument will become a leader in this research arena, owing to its abilities of hosting extreme sample environments, rapid data collection rate, and reconstructing the elastic only portion from the total scattering. Historically, ORNL has had a strong research program in diffuse scattering from single-crystal alloy systems using X-ray diffraction, and the burgeoning neutron instrument suite will provide a powerful complementarity, particularly in those cases where the atomic contrast is better for neutrons and where magnetic diffuse scattering is the primary interest. It is now evident that diffuse scattering can be manifested in any kind of crystal, and the study of diffuse scattering is generally an untapped reservoir of information that can provide new insights into structure property relationships and inspire new ways to tune material properties.

Wide-Angle Neutron Diffractometer WAND²Main Science Area: Single-crystal/powder diffractometer probing materials in extreme environmentsOperational Status: Available to usersInstrument Publication: Ref. 3.

The HFIR WAND² instrument is a dual-purpose powder/single-crystal diffractometer that is supported by the U.S.–Japan Cooperative Program on Neutron Scattering. The instrument has just completed its second and final upgrade phase, which amounts to a complete replacement of the whole instrument after the monochromator. The instrument now supports a diverse array of sample environments, including ultralow temperature (50 mK), high pressure (up to 5 GPa), and high magnetic field (6 T). A major part of the upgrade was the deployment of a large 2D position sensitive detector with nearly 2 × 10⁶ pixels which expands the instrument beyond simply mapping reciprocal space² (Fig. 1) to providing quantitative 3D intensities.³ Magnetic structural studies make up a major part of their scientific portfolio for single-crystal studies, but their versatility enables single crystal studies in other areas such as lipid bilayer diffraction⁴ and oxygen diffusion in Ruddlesden-Popper phases. These upgrades have made this instrument state of the art and a uniquely versatile tool in the single crystal diffraction suite on the North American continent.

Main Science Area: Nuclear and magnetic structures as a function of temperature, pressure, magnetic field, and electric field.Operational Status: Available to usersInstrument Publication: Ref. 29.

The HFIR Four-circle Diffractometer (FCD) continues to play the principal role of determining the magnetic and crystal structures of the small unit cell systems (cell volume < 10⁵ ų) as a function of temperature, pressure, magnetic field, and electric field.¹⁰ The FCD is used to determine precisely magnetic orders and complex magnetic structures at various environmental conditions including temperature (4-800 K), fixed magnetic field (0-1 Tesla with a good Q-range access), pressure (0-5 GPa), and electric field (0-20 kV/cm), and combinations thereof. Most recently, neutron diamond anvil cells, reported in further detail elsewhere in this issue, have been successfully used at 4.5 GPa on the FCD and soon will become standard sample environment equipment for the general user program. Currently, FCD runs with a 5 × 5 cm² highly pixelated Anger camera (Fig. 2), which covers 7° in the vertical and horizontal dimensions. By aligning the detector/goniometer to each predicted reflection position, a 3D peak can be measured altogether, which provides faster speed to track structural distortions and dimensional magnetic behaviors. The bent perfect Si monochromator can switch the beam from a high-flux mode [10⁷ n/(cm²/s)] to a high-resolution mode (δq/q ∼ 0.1%), which allows optimization of the signal and q-resolution depending on the goal of the experiment. The instrument has the highest accessible q of 12.3 Å⁻¹ and unique capability for psi-scans to reduce multiple scattering, check mosaicity, and make absorption corrections. Based on the current instrument configuration and the science productivity, new superconducting materials, multiferroic materials, magnetic materials, thermoelectric materials, and battery/solar cell materials fall in the scientific portfolio of FCD. Ongoing upgrade plans include expanding the detector coverage by adding 9 new generation (magnetic field insensitive) detector modules (3× 3 Anger cameras, 35 × 35 cm² in total) and recent implementation of a polarized neutron beam option to measure weak ferromagnetic signals and complex magnetic systems, which are important for studying many exotic magnetic phases. Besides increasing the data acquisition rate, expanded detector coverage will also provide an opportunity to allow new sample environment equipment, such as ultralow temperature (50 mK) and high magnetic field (7 T) to be used effectively. An increased scientific impact and productivity will be expected for an enhanced FCD.

Main Science Area: Proteins and macromoleculesOperational Status: Available to usersInstrument Publication: Ref. 5.

The HFIR IMAGINE,⁵ quasi-Laue diffractometer (Fig. 3) has the potential to have broad scientific impact and support a diverse user community for the analysis of light atom positions in materials that are of interest across the fields of structural biology, pharmacology, chemistry, condensed matter physics, nano-structured materials, and geological sciences. The primary science mission of IMAGINE is focused on the neutron structure determination of proteins with unit cells up to 150 Å.⁶ The IMAGINE neutron image plate diffractometer is designed for rapid collection of high resolution (D_min up to 1.1 Å) Laue or quasi-Laue data from small single crystals (>0.1 mm³) of moderately large unit cell size, primarily for structural biology. The instrument uses variable short and long wavelength cutoff optics to provide multiple wavelength configurations and a pair of elliptical focusing mirrors that deliver 3 × 10⁷ n s⁻¹ cm⁻² into a 3.5 × 2.0 mm² focal spot at the sample position (dλ/λ ∼ 25%). In 2016, the instrument was equipped with a custom closed-cycle refrigerator (CCR) allowing experiments to be conducted between 4.5 and 400 K, enabling parametric studies of magnetic and supramolecular structures, incommensurate systems, and phase transitions in soft condensed matter materials. It also provides cryo-cooling capabilities for macromolecular crystallography, which will enable data collection on enzyme samples with trapped intermediates whose room-temperature lifetime is too short to analyze structurally.⁷ 

Main Science Area: Proteins and macromoleculesOperational Status: Available to usersInstrument Publication: Ref. 8.

The SNS Macromolecular Diffractometer⁸ (MaNDi) (Fig. 4) is a powerful tool for determining the position of hydrogen atoms and water molecule orientations and for identifying different chemical species in small molecules, macromolecules, and challenging protein structures.^9–11 MaNDi provides neutron crystallography resources for large unit cell samples with unit cell edges up to 300 Ź² using TOF Laue techniques,¹³ servicing applications in bioenergy, medicine, and general biological sciences. The wavelength bandwidth is Δλ = 2.15 or 4.3 Å (when operating at 30 Hz), which can be selected anywhere between 1 and 10 Å. The beam size at the sample position is variable from 0.5 × 0.5 to 7 × 7 mm along with the divergence of the neutron beam at the sample position which can be altered from 0.12 to 0.80°. Data can be collected on single crystal samples of 0.1 mm³ or larger with unit cells in the range of 10-300 Å on edge. An experimental temperature range of 80 to 300 K is provided by an inbuilt Oxford diffraction cryostream.¹⁴ Completion of the detectors in the MaNDi data array frame which surround the sample position (this was increased from 30 to 40 in the summer of 2015) has improved the speed at which the instrument can collect full and complete datasets (Fig. 5).

The switch from H₂O to D₂O in the moderator cooling system during the early 2018 shutdown at the SNS has increased the incident flux on the sample at MaNDi by up to 30% lowering the exposure time needed for each orientation. These upgrades coupled with the SNS running at 1.4 MW will double the number of experiments on MaNDi each year. It will also allow MaNDi (Fig. 5) to collect data from smaller, more complex samples, such as membrane proteins and large enzyme complexes.

Main Science Area: Studying materials under extreme pressures and temperaturesOperational Status: Available to usersInstrument Publication: Forthcoming

The Spallation Neutrons at Pressure (SNAP) instrument is capable of both powder and single crystal diffraction experiments although most of the current experiments conducted on SNAP rely on powder diffraction data. However, with the recent upgrade of the SNAP Anger camera detectors, single crystal high pressure diffraction is poised to become a more important and fruitful part of SNAP’s capabilities. In this description, we focus primarily on single crystal diffraction parameters that enable unique high-pressure experiments.¹⁵ A more detailed exploration of available pressure devices or scientific examples will be presented elsewhere in this special issue. The SNAP instrument is a medium resolution high flux time-of-flight (TOF) diffractometer. A schematic of the beamline is shown in Fig. 6(a), while the sample area of the instrument is shown in Fig. 6(b).

SNAP views a poisoned decoupled hydrogen moderator, providing the hottest beam (biased toward short λ) and sharpest neutron pulse available at the SNS. The beam is transported to the sample position through a straight, 15 m long, flight tube (under vacuum for background reduction), with an option for a parabolic focusing guide in the last 3 m. The wavelength bandwidth used in the experiment is defined by three bandwidth disk choppers. These run typically synchronously with the SNS facility pulse at 60 Hz but can be set in either of two frames (or wavelength ranges): 0.5–3.5 Å or 3.7–6.7 Å. The direct view of the moderator allows access to the highest Q (and consequent finest real space resolution) available in the SNS’s single crystal suite. The scattered beam is detected via Anger cameras, of the same style as are deployed on the TOPAZ and MaNDi instruments. Each camera’s 15 × 15 cm area contains 256 × 256 pixels, resulting in a nominal pixel size of 0.58 mm but an effective resolution of about 1.2 mm. The SNAP detectors are positioned in two flat banks of 3 × 3 Anger cameras (∼450 mm each side) and located nominally at 500 mm from the sample position. These sit on circular rails, allowing independent radial placement about the sample position, with the center of each detector located anywhere from 50° to 115°, from the beam direction, as required for each experiment. While for a single setting the solid angle covered by the setup is only 1.354 sr (0.677 sr per bank), the available coverage reaches almost 3 times that. For single crystal scattering, a number of improvements are being pursued. Chief among these is a new focusing guide that will improve significantly Bragg peak shapes and flux at higher Q. Another important requirement is the incorporation of sophisticated corrections to account for pressure cell absorption for each Bragg peak.

Main Science Area: Elastic scattering of material structures and responses under controlled environmental conditionsOperational Status: Available to usersInstrument Publication: Ref. 17.

The SNS TOPAZ diffractometer is a high-resolution single crystal diffractometer for small molecule and chemical crystallography. The smallest crystals studied on TOPAZ so far are 0.065 mm³ single crystals of a nanospheric polyhydrido copper cluster with 57% hydrogen content in the crystal structure.³¹ TOPAZ uses the neutron wavelength-resolved Laue technique for data collection up to 0.25 Å in d_min or 25 Å⁻¹ in Q_max, the highest in Q coverage among the suite of dedicated neutron single-crystal diffraction instruments at HFIR and SNS. Figure 7 shows the layout of the TOPAZ instrument.

The size of the single crystal that can be used for TOPAZ is sample dependent. We have successfully collected data from single crystals as small as ∼0.05 mm³ in volume for the location of hydrides in transition metal clusters. The instrument has recently been upgraded with installation of a focusing high-flux neutron guide end-section which enables TOPAZ to perform routine measurements of submillimeter size single crystal samples. Hydrogen atom positions can be well resolved from high resolution data collected on TOPAZ. No deuteration is needed for most hydrogenated single crystal samples. TOPAZ is capable of continuous 3D Q-space mapping of a specific region of the reciprocal space volume from a stationary single-crystal sample, much desired for the parametric study of phase transitions and diffuse scattering with either nuclear or magnetic origin. TOPAZ has the most mature data acquisition, visualization, and reduction tools of the single-crystal diffractometer suite^17,18 and has exported these tools to other instruments (MaNDi and CORELLI). However, TOPAZ has yet to realize its full potential for its science capabilities because the accessible sample temperature is limited currently to the range 90 K–500 K with the use of a nitrogen cryostream; however, commissioning of a low-temperature (5 K) goniometer is anticipated in 2018. Future plans include detector-buildout and an option to polarize the incident neutron beam using an in situ ³He neutron spin filter system. An overview of the TOPAZ detector array tank and sample goniometer is given in Fig. 8.

Main Science Area: Studies complex disorder in crystalline materials through diffuse scattering of single-crystal samplesOperational Status: Available to usersInstrument Publication: Ref. 30.

The SNS instrument CORELLI is an elastic diffuse scattering spectrometer optimized for studying the complex disorder and/or short-range correlation of both magnetic and structural origins in single-crystal materials. Although single crystal neutron diffuse scattering has been studied for decades, technical challenges that have prevented it from becoming a widely used tool remain. Accurate modeling of defect structures requires measurements over large volumes of three-dimensional (3D) reciprocal space, with sufficient momentum resolution to distinguish diffuse components from Bragg scattering. These can now be measured with the suitable design of high-resolution white-beam TOF Laue neutron diffractometers.¹⁹ However, one key obstacle in using TOF instruments is the lack of capability to discriminate elastic diffuse scattering from vibrational and other inelastic scattering, which are often comparable in intensity. The implementation of the cross correlation technique (whereby the incident beam is modulated in time in a pseudorandom way) has been previously well studied using steady state sources and spallation neutron sources.^20–23 CORELLI is the latest instrument that combines the efficiency of the white-beam TOF Laue diffractometer with the energy discrimination provided by the cross correlation method,²⁴ with a gain of up to two orders of magnitude in efficiency for elastic diffuse scattering experiments as compared to the standard TOF spectrometer. Figure 9 shows the schematic layout of the major components of the instrument. The instrument uses ambient decoupled water as a moderator. Straight focusing neutron guides are employed to maximize the neutron flux at the sample position with ∼2 × 10⁹ n/cm²/eV/s/MW for E_i = 50 meV (1.28 Å). In typical operation, an incident neutron energy band from 10 to 200 meV is selected using bandwidth control choppers that enable measurement over a large momentum transfer range (0.5 < Q < 16 Å) and appropriate energy resolution [for instance, ΔE (FWHM) ∼ 0.89 meV at E_i = 25 meV]. By operating the correlation chopper asynchronously to the pulsed neutron source, all incident wavelengths are measured in a single run, saving the total TOF as well as the current chopper phase for each detected neutron. This allows the reconstruction of elastic signals using the cross correlation method. The detector tank is capable of hosting 91 modules (each module has sixteen ³He tubes with 12.5 cm diameter and 87 cm length), and 75 modules have been installed. The current configuration has a solid angle coverage of approximately 2 steradian with in-plane detector coverage from −18° to 148° and out-of-plane coverage from −26.5° to +29.5°.

The wide wavelength band, high neutron flux, and large detector coverage enable CORELLI to perform elastic single crystal neutron diffuse scattering experiments at an unprecedented data collection rate. Recent installation of automated sample slits located at 0.5 m from the sample position has enabled users to quickly align samples in the vertical direction.

Figure 10 shows a reciprocal space map of molecular material benzil, C₁₄D₁₀O₂, where rich mapping of diffuse scattering allows detailed modeling of the origin of the disorder in the system, i.e., in this case, the diffuse scattering is primarily due to phonons.²⁵ 

Benefitting from a large sample scattering vessel, a versatile set of sample environments has been commissioned, including ultra-low temperature (50 mK dilution refrigerator insert and 300 mK ³He insert), high magnetic field (5 T vertical field for diffuse scattering and 8 T for Bragg diffraction), high voltage (10 kV), and high hydrostatic pressure (up to 2 GPa using a clamp cell made of copper-beryllium alloy and up to 4.5 GPa using a diamond anvil cell¹⁵). These extreme sample environments are now strongly demanded by the user community of hard condensed matter and materials science.

For biological samples, laboratory infrastructure to support deuteration is an essential component of the suite, with the main instrument beneficiaries being protein crystallography studies using MaNDi and IMAGINE. Deuteration is performed at the Center for Structural Molecular Biology (CSMB) user facility, which is co-located in the SNS lab space. Perdeuteration reduces the incoherent scattering coming from the sample by a factor of forty, greatly enhancing the data signal-to-noise ratio and enabling much quicker and more accurate data collection from crystals 3-4 times smaller than hydrogenous samples.^26,11 Laboratory infrastructure to assist users in growing large crystals is also available because despite progress in instrumentation and optics, the crystal size required for neutron crystallography remains significantly larger than that required for X-ray crystallography.²⁷ 

The SNS is currently undergoing a power upgrade project (PUP) to raise the operating power of the first target station to 2 MW with 800 kW available to power a second target station (STS). A key factor in producing high intensity peak neutron flux at the STS is the use of a compact target. In particular, the STS design incorporates a compact, stationary, water cooled tungsten target. Surrounding the target will be two parahydrogen cold moderators and a single cold parahydrogen/water moderator surrounded by a water-cooled beryllium reflector. The high brightness of the cold neutrons currently being designed for the second target station (STS) of the SNS at the Oak Ridge National Laboratory (ORNL) will be ideally suited for experiments that require focusing optics to enable measurements on smaller samples than is currently possible using the SNS. Macromolecular neutron single crystal diffraction utilizes cold neutrons; thus, we have designed a macromolecular single crystal diffractometer for the STS. For example, the Extended Wide Angle Laue Diffractometer (Ewald)²⁸ has a simulated increase in neutron flux of ×60 compared to MaNDi at the first target station, which will reduce the minimum crystal size required and enable new types of proteins to become amenable to neutron protein crystallography.

The suite of neutron single crystal diffractometers at ORNL enables researchers to study almost any crystalline material from small molecules with less than ten atoms up to large proteins composed of over 30 000 atoms in the unit cell. Continuous improvement across the instrument suite with several key upgrades such as the deployment of incident neutron beam polarization for TOPAZ, FCD, and WAND² will expand their capability for particular kinds of magnetic structural studies. Key upgrades at SNS, such as the power upgrade project (PUP) to raise the beam power to 2 MW, will be particularly beneficial for collecting data from the weakly scattering protein crystal samples used on MaNDi. Finally, the construction of the STS at SNS promises the ability to collect data for crystals an order of magnitude smaller than that currently possible at SNS, enabling neutrons to be used to study even more complex systems.

Research at ORNL’s HFIR and Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The Office of Biological and Environmental Research supported research at Oak Ridge National Laboratory’s Center for Structural Molecular Biology (CSMB), using facilities supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.