Preface: Special Topic on Advances in Modern Neutron Diffraction at Oak Ridge National Laboratory

The Oak Ridge National Laboratory (ORNL) celebrated its 75th anniversary this year, and it was at this laboratory that the neutron diffraction technique was pioneered. Originally founded as a part of the Manhattan project, the scientific scope of the laboratory’s work force quickly evolved, including the work of Shull and Wollan developing the neutron diffraction technique.¹ This work ultimately won Shull the Nobel Prize in Physics in 1994. Although advanced for the time, the equipment used on these seminal first experiments was very basic by modern standards, as can been seen in Fig. 1 but produced the key results and information that birthed a whole new area of science. Over the subsequent years, many neutron sources have been constructed, upgraded, and indeed decommissioned. However, the High Flux Isotope Reactor (HFIR) at ORNL remains one of the brightest steady state neutron sources in the world and the Spallation Neutron Source (SNS), also at ORNL, is the brightest neutron source of its type. In this Special Topic of Review of Scientific Instruments, a series of articles describe the current capabilities and new developments that are keeping ORNL at the forefront of neutron diffraction science, in line with this laboratory’s rich heritage.

The Special Topic Advances in Modern Neutron Diffraction at Oak Ridge National Laboratory in Review of Scientific Instruments highlights the technical capabilities, achievements, and future directions of the neutron powder and single crystal diffraction instrument suites, novel sample environments, and analysis tools at ORNL. Capabilities at the HFIR and SNS broadly support scientific exploration in biology, chemistry, physics, materials science, geoscience, medicine, and more. The Special Topic connects technical advancements with the scientific discoveries they enable, providing a platform for increased visibility and stimulating further developments. The topic is divided in four parts: (1) Diffraction Suite Reviews; (2) State-of-the Art Neutron Instruments at ORNL; (3) Novel Neutron Sample Environments; and (4) Advances in Neutron Diffraction Measurements.

As mentioned above, ORNL has a rich history in neutron scattering dating back to the graphite-moderated X-10 pile which began operations at Clinton Laboratories (codenamed site X) under the supervision of Enrico Fermi in November 1943.² By May 1944, Wollan and Borst had started to attempt to record Bragg scattering from crystals using a recently installed neutron single crystal spectrometer at X-10. While the first attempts were unsuccessful, using improved equipment, they were successful in obtaining Bragg scattering from crystals of salt and gypsum [Fig. 2(a)] in December 1944. During the 1960s, ORNL constructed the High Flux Isotope Reactor (HFIR) which contributed to the growth of the ORNL’s neutron scattering program. In the early 2000s, a cold source was added to HFIR to produce slower neutrons with longer wavelengths to enable more types of neutron experiments to be conducted, while in 2006, the construction of the Spallation Neutron Source (SNS) was completed.

Over the decades, suites of instrumentation have been developed across HFIR and SNS which allow single crystal experiments to be conducted on small molecules to study nuclear and magnetic structures all the way up to large proteins consisting of thousands of atoms [Fig. 2(b)]. This single crystal suite and its capabilities are highlighted in the article by Coates et al. A highly capable powder diffraction suite consisting of six instruments has also been constructed across the HFIR and SNS facilities at ORNL and is reviewed in the article by Calder et al. The powder instruments at the SNS excel at full refinements over large Q ranges and total scattering methods. The high time averaged neutron flux at the HFIR gives unmatched performance for examining a limited range of Q while performing fast measurements of nuclear and magnetic peaks under changing conditions.

In particular, the recently upgraded Wide Angle Neutron powder/single crystal diffractometer (WAND²) instrument is described by Frontzek et al. This instrument can be utilized for both powder and single crystal experiments. The wide selection of sample environments available allows the realization of many experimental conditions which make WAND² an instrument well suited for non-standard experiments. Finally, the Neutron Residual Stress Facility (NRSF2) instrument is described by Cornwell et al. This instrument uses a monochromatic neutron beam and takes advantage of the high penetrating power of thermal neutrons to map residual strains in materials. This allows users to acquire a much more complete picture of residual stresses and their gradients within engineering components.

A diffractometer alone is, however, often not sufficient for the study of the majority of problems in the scientific fields that require neutron scattering. Instead most require the study of their samples under in situ or in operando conditions such as during chemical reactions, under the application of extreme conditions of temperature, pressure, magnetic or electric fields, in the presence of inert or reactive gases, and many more sample environments. The powerful capabilities of the various SNS and HFIR diffractometers are thus being complimented by ever more advanced and sophisticated sample environments. Several of the most recent developments in this area are also presented in this focus issue here, particularly sample environments focused on gas handling and the application of extreme conditions of compression and temperature.

Two different gas flow systems optimized for operation under elevated temperatures have been integrated into SNS’s NOMAD and POWGEN beamlines, respectively. Olds et al. described a high temperature gas flow system on NOMAD capable of reaching 800 °C at a moderate maximum rate of ∼0.08 °C/s. Inert gases such as argon but also reactive gases such as CO₂ can be used to study, for example, failure mechanisms in energy materials or carbon capture by in situ total scattering. Similarly, the Automated Gas Environment System (AGES) for the POWGEN beamline detailed in the study by Kirkham et al. enables heating up to 850 °C in a vacuum furnace while handling inert gases such as nitrogen or helium and reactive gases such as oxygen, methane, hydrogen, or CO₂ as well as the mixing of all these. This enables the answer of many materials science problems such as through the study of oxygen storage or of oxygen transport pathways, for example.

Another system developed for SNS’s VULCAN beamline, but potentially also portable to others, combines high temperatures with flow of inert gases and also uniaxial tension and compression. The Resistive Heating Gas Enclosure Loadframe (RHEGAL) presented by An et al. allows for maximum loads of 4.4 kN at a maximum temperature of 1200 °C in an inert atmosphere. This is combined with a capability for rapid heating (up to 60 °C/s) making it an highly optimized system for the study of non-equilibrium phase transformations in materials such as steel. A further uniaxial load frame has been developed by White et al. for the NOMAD beamline. This system was optimized for the study of cementitious materials with total scattering techniques and allows for a load of ∼1.7 kN to achieve 0.022 GPa. The load frame with dimensions in the order of ∼30 cm height is easily portable and adaptable to other beamlines.

In addition to these uniaxial compression environments, two contributions focus on the application of hydrostatic compression. Dos Santos et al. described the existing gas pressure cell capabilities available for SNS and HFIR beamlines. Furthermore, a new radial collimator optimized for SNS SNAP’s beamline has been developed for these cells to reduce sample environment scatter. While these gas pressure cells are limited to a maximum of 0.7 GPa, Haberl et al. presented a newly optimized diamond cell for single crystal neutron diffraction to 10 GPa. The maximum of 10 GPa under hydrostatic compression is thereby locked in upon load application up to 98.1 kN. This cell can be cooled to 5 K and has been demonstrated for single crystal quantum materials and other crystals at SNS’s CORELLI and SNAP beamlines as well as HFIR’s HB-3A four circle and IMAGINE diffractometers.

These many new sample environments present an important contribution to the development of capabilities and the advancement of neutron diffraction methods, at ORNL, in addition to the diffractometer developments themselves.

Integral to the success of every modern neutron diffraction measurement is the data reduction and analysis workflow available to users.^3,4 Modern computers and algorithms are continuously ushering in new possibilities for data-informed experimental feedback during measurements and increasingly complex, data-rich, and robust analyses post experiment. The fourth section of the Special Topic describes several areas in neutron diffraction data reduction and analysis recently advanced at ORNL. The topics are meant to provide a flavoring of recent developments, rather than a complete review of current capabilities.

Most spallation neutron sources in the world, including the SNS, have data acquisition systems that provide event recording (recorded scattering events remain stored at the facility in such a way that they can be re-reduced or examined in new ways after collection). Peterson et al. introduced the main principles of event data and detailed how the method is opening up new possibilities for in situ measurements. Here, they demonstrated a model independent method of grouping data via hierarchical clustering methods and showed how this can be used to improve calibration, reduction, and data exploration, while measurements are taking place. A scientific application of data event filtering is demonstrated in the article by Fancher et al., who followed ferroelectric/ferroelastic domain wall motion during electric field cycling of single crystal BaTiO₃. Data are binned stroboscopically into subsets that provide details of structural responses to specific applied stimuli. Both papers highlight that event data can be readily applied to investigate a wide array of dynamic phenomena.

Another advantage of many modern instruments is a large array of detector pixels, often available over wide angular ranges. Coupled with the high incident fluxes available at sources like the SNS and HFIR, this opens up new opportunities for orientation-dependent data analyses. Usher-Ditzian et al. described the development of a new electric field cell for SNS’s NOMAD, including data reduction routines for orientation-specific diffraction and pair distribution function measurements. This capability allows for total scattering (local atomic structure to long range crystallographic) studies of ceramic materials such as ferroelectrics under the application of electric fields up to 10 kV. Fancher et al. proposed a new route to the crystallographic texture analysis of materials using the full 3-dimensonal set of pole spheres available at the time-of-flight Laue diffractometer TOPAZ at the SNS. This approach is compared to those available at two of ORNL’s constant wavelength diffractometers, the 2nd generation Neutron Residual Stress Mapping Facility (NRSF2) and wide-angle neutron diffractometer (WAND) at the HFIR. The new method avoids the need for pole figure inversion routines and offers the possibility of higher fidelity determination.

Finally, Liang et al. described a new method of visualizing the diffraction parameter space associated with crystallographic site scattering power through the use of f* diagrams. The improved approach is capable of detecting occupancy defects with an exceptional sensitivity of 0.1% (absolute) in the class of layered NMC Li-ion battery cathode materials. The method is broadly applicable to ternary compounds and allows the global minimum fit to be easily identified, permitting a robust determination of the number and type of occupancy defects within a structure.

This Special Topic describes the current state and new developments in neutron diffraction at ORNL. However, the innovations are continuing, with concepts being explored to complete the instrument suites at the first target station at the SNS, upgrade diffraction suite capabilities at the HFIR, and indeed the design and instrumentation for a future second target station at the SNS. We are confident these new developments will help us meet the challenges for neutron diffraction in the coming decades at ORNL.