Wombat is the high-intensity neutron diffractometer in operation at the Australian Centre for Neutron Scattering. While Wombat is primarily used as a high-speed powder diffractometer, the high-performance area detector allows both texture characterization and single-crystal measurements. The instrument can be configured over a large range of operational parameters, which are characterized in this contribution to aid experimental planning. Wombat is particularly optimized for the study of materials in situ and in operando using the wide range of sample environments available at the centre. Over 17 years of operation, Wombat has been used to explore a broad range of materials, including novel hydrogen-storage materials, negative-thermal-expansion materials, cryogenic minerals, piezoelectrics, high-performance battery anodes and cathodes, high-strength alloys, multiferroics, superconductors, and novel magnetic materials. This paper will highlight the capacity of the instrument, recent comprehensive characterization measurements and how the instrument has been utilized by our user community to date.

Earth-abundant metal oxides are excellent candidates for photocatalytic applications due to their low cost and high stability in aqueous solutions. Materials that contain a combination of metal cations with an s2 electron lone pair and a d0 electronic configuration, such as BiVO4, possess favorable band gaps. BiVO4 has also been reported to possess noncentrosymmetric polar properties, such as flexoelectricity, piezo-photocatalysis, and an anomalous photovoltaic effect, despite its centrosymmetric crystal structure. Here, it is shown how centrosymmetric materials possessing s2 and d0 cations can display "hidden" local-scale features, often ignored by conventional crystallography, that influence their physical properties. Anomalous peak shapes are observed in the high-resolution synchrotron X-ray powder diffraction of BiVO4, and temperature-dependent local-scale distortions are revealed using neutron total scattering methods. Together, these suggest the polar properties of BiVO4 are related to local-scale distortions induced by the Bi3+ 6s2 electron lone pairs. This demonstrates the possibility of engineering specific interatomic distances between lone pair-bearing cations and the anion sublattice, creating new opportunities for photocatalytic and polar materials from compounds with long-range centrosymmetric structures.

Many programs for science communication are targeted towards secondary-school ages (11 +) and for good reason, as this is when students make choices on subjects to study further. It is vital that these students are supported in their continuing science education. But are we missing out on inspiring them in the first place? Can we help students to see the bigger picture of science, beyond grades and textbooks? What if we run programs that target younger students, as well as their families? For younger students, it is vital to have strong visual and hands-on components to science communication activities. Crystallography lends itself extremely well to visual science communication – we have a great history of leveraging that. But do we have enough hands-activities that are suitable for under 11s, can be undertaken cheaply, and are linked to big crystallographic science ideas? The Australian and New Zealand crystallographic community leveraged hosting the IUCr2023 meeting to launch a program of events and initiatives to communicate crystallography to those under 11 years old and their families. We undertook a range of events and activities, from pattern competitions, to a crystallographic science festival, [1] to even attempting to break a world record. For this we used existing ideas on 3D printing structures [2] (but upsized it), modified established hands-on activities [3] [4] [5], and developed more. In this contribution I’ll review what we carried out, what did (and didn't) work and how we are planning to continue the momentum into the future which could be applied at IUCr2026.

Long-term storage of CO2 in nesquehonite (MgCO33H2O) relies on its transformation to highly stable magnesite (MgCO3) in a naturally occurring, centuries-long process. Here, we pair in situ X-ray diffraction (XRD) and thermogravimetric analysis to investigate the thermal transformation (30-650 degrees C, 5 degrees C/min) of nesquehonite to magnesite, under both open and closed experimental conditions in a supplied atmosphere of CO2 or N2, and the presence or absence of self-generated gases (i.e., CO2, water vapor). We found that following the structural collapse of nesquehonite, magnesite only forms in the presence of gaseous CO2, whether that be externally supplied or self-generated. This is consistent with a dehydration-crystallization mechanism, with increased local accumulation of CO2 (and in a closed system, H2O vapor) shifting thermal events to higher temperatures, allowing for the crystallization of magnesite. Approximately 20 wt% more magnesite formed when nesquehonite was flushed with CO2 gas during heating in an open system, rather than held within a closed, static CO2 atmosphere. We hypothesize that this difference is due to complete dehydration being more difficult to achieve in a closed system, delaying the crystallization of magnesite. Additionally, the distribution of passivating reaction products on unreacted mineral cores may occur in closed systems, where self-generated humidity is retained and the dissolution-precipitation of reaction products may occur at mineral surfaces. We also found that amorphousmaterials are dominant intermediate stores for CO2, which is significant given they are not typically considered during carbon accounting in natural landscapes or engineered settings. We proposed a novel method to accurately quantify amorphous solids from XRD data during in situ studies where significant gas loss occurs. Our findings further our mechanistic understanding of how magnesite forms from crystalline and amorphous precursors under a range of environmental and industrial conditions, which is key to optimizing stable CO2 storage in Mg-carbonate minerals. In particular, it highlights the importance of considering the role of amorphous phases, atmospheric composition, and self-generated gas retention during magnesite formation.

The behavior of a coformer mixture of acetonitrile and trimethylacetonitrile has been examined up to 5.90 GPa using in situ high-pressure single crystal X-ray diffraction and periodic Density Functional Theory. A single-component phase of trimethylacetonitrile is formed at 0.29 GPa, which is isostructural to its previously reported high-pressure phase. Between 1.73 and 5.90 GPa, a 1:1 cocrystal of trimethylacetonitrile and acetonitrile formed in space group P21/m, dominated by C-HN and C-Hpi interactions. The cocrystal was grown in situ by heat annealing the sample above 1.1 GPa. This is the first ever reported cocrystal of trimethylacetonitrile and acetonitrile. The structure is resistant to compression, with a bulk modulus of 20(1) GPa, due to H-bonding networks and short NH contacts. The arrangement of trimethylacetonitrile molecules within the cocrystal mirrors their organization in the high-pressure phase of pure trimethylacetonitrile, revealing a supramolecular synthon and highlighting common crystalline packing motifs of small nitrile-containing species.

With its large size, dense atmosphere, methane-based hydrological-like cycle, and diverse surface features, the Saturnian moon Titan is one of the most unique of the outer Solar System satellites. Study of the photochemically produced molecules in Titan's atmosphere is critical in order to understand the mechanics of the atmosphere and, by extension, the interactions between atmosphere, surface, and subsurface water ocean. One example is propyne vapor, a photochemically produced species in Titan's upper atmosphere expected to condense in Titan's stratosphere at lower altitudes. Propyne may also be a trace species in Titan's stratospheric co-condensed ice clouds detected by the Cassini Composite InfraRed Spectrometer. Bulk structural characterization of propyne ice is currently incomplete and is lacking in published laboratory Raman spectra and X-ray diffraction data. Here, we present a laboratory characterization of propyne ice, including the first published X-ray diffraction and Raman spectroscopy results for propyne ice.

We have made the compound 2O-BaPtO3 by high-pressure, high-temperature synthesis, determined its structure, and tested its catalytic activity. Compounds of the same stoichiometry have been reported and tentatively identified as hexagonal perovskites, and although no structural model was ever established, 2O-BaPtO3 is clearly different and, to the best of our knowledge, unique. It features continuous chains of face-sharing PtO6 octahedra, like the well-known 2H hexagonal perovskite type, but with a staggered offset between the chains that breaks hexagonal symmetry and disrupts the close-packed array of A = Ba and X = O that is a defining characteristic of ABX3 perovskites. We investigated this structure and its stability vs the conventional 2H form using X-ray and neutron diffraction, X-ray absorption spectroscopy, and ab initio calculations. Catalytic testing of 2O-BaPtO3 showed that it is active for hydrogen evolution.

Nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) are important molecules for astrochemistry and prebiotic chemistry, as their occurrence spans from interstellar molecular clouds to planetary systems. Their formation has been previously explored in gas phase experiments, but the role of solid-state chemical reactions in their formation under cryogenic conditions remains elusive. Here, we explore the formation of NPAHs through vacuum ultraviolet (VUV) irradiation of pyridine:acetylene ices in amorphous and co-crystalline phases, with the aim to simulate conditions relevant to the interstellar medium and Titan's atmosphere. Our results show that the synthesis of ethynylpyridines from VUV-irradiated pyridine:acetylene amorphous ices is achievable at 18 K. In the co-crystal phase, photolysis at 110 K leads to the formation of NPAHs such as quinolizinium+ and precursors, reflecting a dynamical system under our conditions. In contrast, irradiation at 90 K under stable conditions did not produce volatile photoproducts. These results suggest that such chemical processes can occur in Titan's atmosphere and potentially in its stratosphere, where the co-condensation of these molecules can form composite ices. Concurrently, the formation of stable co-crystals can influence the depletion rates of pyridine, which suggests that these structures can be preserved and potentially delivered to Titan's surface. Our findings provide insights into the molecular diversity and chemical evolution of organic matter on Titan, crucial for future space exploration missions, such as the Dragonfly mission, which may uncover higher-order organics derived from pyridine precursors on Titan's surface.

LiNiO2 remains a critical archetypal material for high energy density Li-ion batteries, forming the basis of Ni-rich cathodes in use today. Nevertheless, there are still uncertainties surrounding the charging mechanism at high states of charge and the potential role of oxygen redox. We show that oxidation of O2- across the 4.2 V plateau forms O2 trapped in the particles and is accompanied by the formation of 8% Ni vacancies on the transition metal sites of previously fully dense transition metal layers. Such Ni vacancy formation on charging activates O-redox by generating non-bonding O 2p orbitals and is necessary to form vacancy clusters to accommodate O2 in the particles. Ni accumulates at and near the surface of the particles on charging, forming a Ni-rich shell approximately 5 nm thick, enhanced by loss of O2 from the surface, the resulting shell composition is Ni2.3+1.75O2. The overall Ni oxidation state of the particles measured by XAS in fluorescence yield mode after charging across the plateau to 4.3 V is approximately +3.8; however, taking account of the shell thickness and the shell Ni oxidation state of +2.3, this indicates a Ni oxidation state in the core closer to +4 for compositions beyond the plateau.

Zero thermal expansion materials have a unique ability to withstand thermal shock over varied temperature ranges, enabling potential applications as components of high precision devices. The zero thermal expansion material Sc1.5Al0.5W3O12 exhibits zero thermal expansion over the largest temperature range reported to date, from 4 to 1400 K (Liu et al. Chem. Mater.2021, 33, 3823). In this work, the pressure stability of this material is investigated using high-pressure neutron and X-ray powder diffraction as well as Raman spectroscopy and supported with density functional theory (DFT) calculations. Sc1.5Al0.5W3O12 shows an orthorhombic to monoclinic transition with the application of a small amount of pressure which is similar to that reported for other compositions in this family. Additional peak splitting is noted above 3.4 GPa which may indicate an additional phase transition. A pressure induced amorphization is found between 4.3 and 5.7 GPa and an amorphous to amorphous phase transition is further evidenced at around 15 GPa. The tungsten coordination sphere evolves from WO4 to WO6 as pressure is increased, which is evidenced by both DFT calculations and Raman spectroscopy. The <3.4 GPa monoclinic phase has a bulk modulus, B-0, of 17.4(14) GPa determined with X-ray powder diffraction by second order Birch-Murnaghan. This work demonstrates the importance of understanding structural changes under pressure for zero thermal expansion materials, as changes in pressure impart a more significant impact on the crystallographic volume compared to temperature.

Three high-pressure phases of trimethylacetonitrile, a compound of potential interest in the context of Titan's atmospheric chemistry, have been investigated using single-crystal X-ray diffraction, periodic density functional theory, and CrystalExplorer intermolecular energy calculations. A disordered tetragonal P4/nmm phase is formed between 0.07 and 0.29 GPa (denoted hp-I). Compression to 0.43 GPa forms an ordered orthorhombic Pnma phase (hp-II), which transforms to a monoclinic P2(1)/m phase (hp-III) at 1.52 GPa. The hp-III phase persists to at least 3.34 GPa. Phase transitions are driven by densification of the crystal and facilitated by rearrangement of the supramolecular hydrogen-bonding network, with 180 degrees reorientation of half the molecules. Compression of each phase is associated with slight shortening of the intermolecular hydrogen bonds, with gradual destabilization of the cohesive energy to 3.34 GPa.

The growing pressure on school curricula has meant crystals and the science of crystallography have been cut from or made optional for many educational programs. This omission is a serious disservice to the history and understanding of modern sciences, given that crystallography underpins many of the greatest advancements in science over the past century, is a critical component of many modern research papers and patents, and has 29 Nobel Prizes awarded in the field. This contribution describes a simple activity to target classroom and public engagement with crystallography, using marshmallows or equivalent sweets/candy to represent atoms and cocktail sticks to represent bonds, together with examples of how crystals are studied and how they are useful. Though it has a simple basis, this activity can be extended in numerous ways to reflect the aims of the demonstrator, and a few of these are described.

The thermal transformation of the tetragonal-zircon (tz-) to tetragonal-scheelite (ts-)BiVO4 was studied by in situ synchrotron X-ray diffraction, thermogravimetric analysis, and Fourier-transformed infrared spectroscopy. Upon heating, the tetragonal zircon polymorph of BiVO4 (tz-BiVO4) transitioned to the ts-polymorph between 693-773 K. Above 773 K, single phase ts-BiVO4 was observed before transitioning to the monoclinic fergusonite (mf-) polymorph upon cooling. An anomaly in thermal expansion was observed between 400-500 K, associated with the loss of intercalated H2O/NH4 + from the coprecipitation procedure. Heating tz-BiVO4 resulted in contraction of the V-O bond distance and VO4 polyhedra volume, ascribed to rotation of the tetrahedra groups. Attempts to study this by neutron diffraction failed due to the large incoherent scatter from the hydrogenous species. Efforts to remove these species while maintaining the tz-BiVO4 structure were unsuccessful, suggesting they play a role in stabilizing the tz-polymorph. The local structure of both mf-BiVO4 and tz-BiVO4 were investigated by X-ray pair distribution function analysis, revealing local distortions.

Neutron powder diffraction data has been used to quantify the monoclinic (space group I2/a) to tetragonal (I4(1)/a) phase transition that occurs at 775 degrees C in HoNbO4 and 1300 degrees C in HoTaO4. In both cases, deviation from second-order behavior is evident. The LnTaO(4) (Ln = Tb-Er) family of oxides has the potential to adopt one of monoclinic, I2/a or P2/c, structures depending on the synthesis conditions. The monoclinic P2/c polymorph of HoTaO4 undergoes an irreversible first-order phase transition to the high-temperature I4(1)/a scheelite-type structure upon heating, with the monoclinic I2/a phase recovered upon cooling. This is the first direct evidence of this irreversible phase transition and implies a maximum heating temperature to synthesize the P2/c phase for potential ionic conductivity applications. Heating a green powder mixture of Ho2O3 + Ta2O5 revealed a complex series of phase transformations, including the observation of a weberite-type Ho3TaO7 intermediate between 1200 and 1390 degrees C that was not observed upon cooling. Coupled with electrochemical impedance spectroscopy measurements, this diffraction data provides a structural model that explains the higher mobility of charge carriers in LnTaO(4) materials that can be used to identify dopants and improve their ionic conductivity and applicability. Undoped HoNbO4 and HoTaO4 are poor conductors, and the activation energy of tetragonal HoNbO4 is greater than that of the monoclinic polymorphs. Oxygen ion and proton conductivities of the undoped structures occur via interstitial oxygen sites (similar to 10(-6) S cm(-1) at 800 degrees C), providing a potential avenue to improve their application in practical devices such as solid oxide fuel cells.

The structures and high-temperature phase transition of CaUNb2O8 were studied in situ using synchrotron X-ray and neutron powder diffraction. Rietveld refinements provided an accurate description of the crystal structures of both the monoclinic fergusonite-type I2/b structure observed at room temperature and the tetragonal scheelite-type I41/a structure found at high temperatures. Bond valence sum analysis showed Nb5+ to be octahedrally coordinated in the monoclinic fergusonite-type structure, akin to other ANbO4 materials. Rietveld analysis of the variable temperature data allowed for the determination of accurate unit cell parameters and atomic coordinates, as well as revealing a reversible phase transition around ∼750 °C. The Nb-O bond distances display anomalous behavior, with a discontinuity in the longer Nb-O(1') distance coinciding with the phase transition suggestive of a reconstructive phase transition. Mode analysis identified the Γ2+ mode as the primary mode that drives the phase transition; this is linearly coupled to the induced spontaneous strain within the monoclinic fergusonite-type structure. Analysis of the temperature dependence of the Nb(z) positional parameter, as well as of the ϵ1-ϵ2 and ϵ6 strain parameters, showed that the phase transition is not strictly second order, with the critical exponent β ≠ 1/2. This study demonstrates the complex structural features of mixed cation metal oxides at elevated temperatures.

Reflections and projections are regular tools of any crystallographer's trade.However, they are also essential tools when thinking about the 75th anniversary of the IUCr and in exploring how welcoming and inclusive our union is.When beginning a crystallographic career, like any field, it can be incredibly influential to see those you identify with, both historic and current, actively succeeding.Crystallography has a reputation for being more inclusive than other science areas, but the reality is and the data show that we still have a long way to go.So rather than question the problem, we must move on and seek solutions, ask ourselves 'What concrete actions should the IUCr take to ensure our union truly reflects our world?We can all reflect on how to make crystallography a science that everyone feels they can participate in, and use this as a projection for the future.

Tuning the coefficient of thermal expansion (CTE) of functional materials is paramount for their practical implementation. The multicomponent nature of metal-organic frameworks (MOFs) offers an opportunity to finely adjust negative thermal expansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust the NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentane-1,3-dicarboxylate to form the MOFs CUB-5 and 3DL-MOF-1, respectively, where each linker has low torsional energy barriers. The core of these nonconjugated linkers is decoupled from the carboxylate functionalities, which frees the relative movement of these components. This results in enhanced NTE compared to the analogous, conjugated system; VT-PXRD results were used to calculate the CTE for 3DL-MOF-1 (alpha(L) = -13.9(2) x 10(-6) K-1), and CUB-5 (alpha(L) = -14.7(3) x 10(-6) K-1), which is greater than the NTE of MOF-5 (alpha(L) = -13.1(1) x 10(-6) K-1). These results identify a new route to enhanced NTE behaviors in IRMOF materials influenced by low energy molecular torsion of the linker.

Synchrotron X-ray and neutron powder diffraction methods have been used to obtain accurate long-range average structures of some double scheelite compounds of the type NaA(BO4)2 (A = La, Pr, Nd, Sm, Lu, and Bi; B = Mo, W) at room temperature. Phase pure samples were synthesized using standard solid-state methods. Rietveld re-finements using combined synchrotron X-ray diffraction (SXRD) and neutron diffraction (NPD) revealed a random distribution of the Na and A-type cations regardless of the presence of 6s2 lone pairs (such as Bi3+) and the difference in oxidation states and ionic radii between the cations. The NaA(BO4)2 (A = La, Pr, Nd, Sm, Lu, and Bi) series displayed linear trends in lattice parameters and AO8 polyhedra volume with the ionic radius of the A-type cation for the lanthanoids, but a deviation from the trend was observed for A = Bi3+. The NaBi(BO4)2 structure has a smaller than expected unit cell volume than based on extrapolation from the corresponding NaLn(BO4)2 series, possibly due to short-range ordering of the 6s2 lone pair electrons.