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From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77-140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.

Catalytic membrane reactors have the potential to render the process of oxidative coupling of methane economically viable. Here, the results from operando XRD-CT studies of three different catalytic membrane reactors, employing BaCo0.4Fe0.4Zr0.2O3-δ (BCFZ) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite membranes with Mn-Na-W/SiO2 and La-promoted Mn-Na-W/SiO2 catalysts, are presented. It is shown that synchrotron X-ray tomographic diffraction imaging allows the extraction of spatially-resolved diffraction information from the interior of these working catalytic membrane reactors and makes it possible to capture the evolving solid-state chemistry of their components under various operating conditions (i.e. temperature and chemical environment).

The study of porous materials under high-pressure conditions is crucial for the understanding and development of structure–property relationships. Zeolites are a diverse class of microporous materials with an extensive range of properties and applications, which can be attributed to their unique pore systems, channel dimensions and mechanical stability. Due to their complex frameworks and compositions, determining the structures of zeolite materials at high pressures is particularly challenging. Here, we present our method which involves geometric modelling, implemented in GASP (Geometric Analysis of Structural Polyhedra) software, to obtain crystal structures at high-pressures when Rietveld refinements alone fall short. We show that GASP can be used to simulate theoretical compression mechanisms of zeolite frameworks at the atomic level, providing optimised structural models as reasonable starting points for full crystal structure analysis. Finally, we discuss the potential of geometric modelling to predict high-pressure behaviour of other known and hypothetical structures, which may find use in future applications. We anticipate that GASP software will be a powerful tool that can assist in determining the crystal structures of zeolite frameworks at high pressures.

The structural transformation of FePO4 from the trigonal berlinite phase to the orthorhombic CrVO4 phase has been studied using neutron diffraction at high pressure and high-temperature. The berlin...

A high-pressure neutron powder diffraction study was conducted on perdeuterated para-xylene (C8D10). para-Xylene crystallizes in the monoclinic crystal system (space group P21/n) at ambient temperature and ca. 0.1 GPa. The structure is consistent with the known low-temperature form. No further phase transitions were observed in the pressure range 0.11(1)-4.72(2) GPa. A complementary high-pressure single-crystal diffraction experiment was performed on hydrogenous para-xylene confirming the assigned space group P21/n. An isothermal equation of state was obtained [bulk modulus, B0 = 3.5(4) GPa] and structural changes of the material have been investigated as a function of pressure. This experimental study is supported by dispersion-corrected density functional theory calculations.

We revisit the pressure-induced order-disorder transition between phases II and IV in ammonium bromide

NaNiO

We present the in situ neutron diffraction data of a water and silica mixture at high pressure and temperature. We show initially the formation of ice VI at 1.5 GPa at 290 K in the presence of crys...

The compression behaviours of La2NiO4+δ and Pr2NiO4+δ have been studied up to a pressure of 2.8 and 2.2 GPa respectively. Using neutron diffraction, the mechanism of compression, and the behaviour of the NiO6 and La/PrO9 polyhedra in these layered perovskite materials have been determined. Their compression mechanisms have then been compared to related materials (La2-xPrxNiO4, Pr2-xNdxNiO4, La2-xSrxNiO4 and Pr2-xCaxNiO4) where the unit-cell volume has been reduced by controlling the composition (x), which acts as an 'effective chemical pressure'. Understanding the effects of both has implications for materials design; pressure can be used to finely tune a property, which theoretically may then be emulated using chemical doping.

We have performed high–pressure neutron diffraction studies on the layered oxide, Ca2Mn3O8. Studies up to approximately 6 GPa at temperatures of 120 and 290 K demonstrate that there are no structural phase transitions within this pressure range. Fits of the unit–cell volume to a Birch-Murngahan\ud equation of state gives values for the bulk modulus of 137(2) GPa and 130(2) GPa at temperatures of 290 K and 120 K respectively possibly suggesting that Ca2Mn3O8 is more compressible at\ud lower temperature. Furthermore, compression along the principal axes are anisotropic on the local\ud scale. Comparison of individual bond lengths and bond angle environments further demonstrate that\ud compression is complex and likely results in a shearing of the layers.

The short-range structure of 20BaO–80TeO2 glass was studied in situ by high pressure neutron diffraction and high pressure Raman spectroscopy. Neutron diffraction measurements were performed at the PEARL instrument of the ISIS spallation neutron source up to a maximum pressure of 9.0 ± 0.5 GPa. The diffraction data was analysed via reverse Monte Carlo simulations and the changes in the glass short-range structural properties, Ba–O, Te–O and O–O bond lengths and speciation were studied as a function of pressure. Te–O co-ordination increases from 3.51 ± 0.05 to 3.73 ± 0.05, Ba–O coordination from 6.24 ± 0.19 to 6.99 ± 0.34 and O–O coordination from 6.00 ± 0.05 to 6.69 ± 0.06 with an increase in pressure from ambient to 9.0 GPa. In situ high pressure Raman studies found that the ratio of intensities of the two bands at 668 cm−1 and 724 cm−1 increases from 0.99 to 1.18 on applying pressure up to 19.28 ± 0.01 GPa, and that these changes are due to the conversion of TeO3 into TeO4 structural units in the tellurite network. It is found that pressure causes densification of the tellurite network by the enhancement of co-ordination of cations, and an increase in distribution of Te–O and Ba–O bond lengths. The original glass structure is restored upon the release of pressure.

The crystal structure of the small organic molecule, alloxan, has been explored using high-pressure neutron diffraction; its already efficiently-packed structure provides a 'chemical head-start' on the pressure experiment. At the highest pressure measured, alloxan reaches a density of 2.36 g cm-3-unprecedented for a C, H(D), N, O-containing organic material of appreciable molecular weight. Its crystal structure is stable until ca. 6.5 GPa above which the sample starts to undergo amorphisation.

According to previous theoretical work, the binary oxide CuO can become a room-temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present Letter provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.

A method for subtracting the pairwise correlations of a pressure-transmitting medium from neutron pair distribution functions obtained under hydro­static compression is presented and applied to Ni, MgO and α-quartz., High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. High-pressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressure-transmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydro­static compression. The method applies a molecular-dynamics-informed empirical correction and a non-negative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected high-pressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of α-quartz under hydro­static pressure is presented, extracting compression behaviour of the real-space structure.

We have mapped the phase diagram of BaTiO3 more extensively than previous attempts using high-pressure neutron-powder diffraction. The mapping of the phase diagram has been performed using isothermal compression at fixed temperatures (175, 225, 290, 480 K) within each of the known crystallographic phases, up to ∼6 GPa using a large volume press. The crystallographic structure of each phase has been measured, and the determined absolute atomic displacements of all atoms within the cell have provided detailed information on the order of the phase transitions observed, and the behaviour of the ferroelectric dipole moment. Publisher PDF

Both the trigonal (Berlinite-type, phase-I), and orthorhombic (CrVO4-type, phase-II) forms of FePO4 have been studied at high-pressure using neutron powder diffraction. Phase-II was prepared by a high-pressure, high-temperature synthetic route, and recovered to ambient conditions. We report the first high-pressure structural study of this phase up to 8.4GPa at room temperature. It is shown that with increasing pressure, the FeO6 octahedra become more regular and decrease in volume, while the PO4 tetrahedra become less regulars and increase in volume. For phase-I, similar changes in volume are determined, though without changes in distortion. At 2GPa a signature of amorphisation has been observed for phase-I with the appearance of broad phase-II reflections. To support the results of the high-pressure study, Raman spectroscopic and SQUID magnetometry studies have been performed.

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a Pbcn-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials., Water’s phase diagram exhibits several hydrogen-disordered phases which become ordered upon cooling, but the behavior of ice VI is still debated. The authors, using high-pressure neutron diffraction, identify structural distortions that transform ice VI into ice XIX, here identified as a hydrogen disordered phase.

Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite La2NiMnO6 using neutron scattering over a temperature range of 4.2–300 K at ambient...

We have measured the thermal behavior of four poly-aromatic hydrocarbons (PAHs) using high pressure gas flow DSC methods up to ∼7000 kPa. We have shown that there is a linear increase in melting te...

Pressure-induced polymerization (PIP) of aromatics is a novel method for constructing sp3 -carbon frameworks, and nanothreads with diamond-like structures were synthesized by compressing benzene and its derivatives. Here by compressing a benzene-hexafluorobenzene cocrystal (CHCF), H-F-substituted graphane with a layered structure in the PIP product was identified. Based on the crystal structure determined from the in situ neutron diffraction and the intermediate products identified by gas chromatography-mass spectrum, we found that at 20 GPa CHCF forms tilted columns with benzene and hexafluorobenzene stacked alternatively, and leads to a [4+2] polymer, which then transforms to short-range ordered H-F-substituted graphane. The reaction process involves [4+2] Diels-Alder, retro-Diels-Alder, and 1-1' coupling reactions, and the former is the key reaction in the PIP. These studies confirm the elemental reactions of PIP of CHCF for the first time, and provide insight into the PIP of aromatics.

We have determined the high pressure structure of perdeuterated ethanol by using neutron powder diffraction in the pressure region ∼2.5–4.0 GPa and compare it to that previously measured by single-crystal X-ray diffraction and the one suggested by ab initio structure calculations. In contrast to previous X-ray single-crystal studies, we see no evidence for a disordered proton around the hydroxyl group, but confirm the monoclinic symmetry in the measured pressure region. We also present the compression behaviour of the unit cell in this pressure range.

International audience; We report a quantitative comparison of measured intensities of neutron powder diffraction data collected in a single-crystal diamond anvil cell and in large-volume sintered diamond anvils. As expected from the difference in sample volumes, the latter provides 1–2 orders of magnitude higher intensities, depending on the anvil material. The remaining differences are due to effects of absorption and angular aperture.

We have determined the lattice parameters of perdeuterated melamine, C3N6D6, as a function of temperature (4.2–320 K) close to atmospheric pressure and as a function of pressure (0–5 GPa) a...

The effects of high pressure and low temperature have been explored on the ‘orange-plate’ (OP) form of the highly-polymorphic 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile molecular crystal structures. Neutron powder and single-crystal diffraction measured up to 9.3 GPa and 40 K, respectively, show that the crystal structure is robust, retaining P21/n symmetry over the pressure and temperature range studied, even though it is revealed to be quite soft, having a bulk modulus of 4.3(3) GPa. Intermolecular and lattice energies become progressively less favourable with pressure, relative to the thermodynamically-stable ‘yellow’ (Y, P21/n) form, showing that the kinetic barriers to transformation, stabilising the material, persist from ambient to extreme conditions.

We report a pressure-induced transition to a new crystalline phase of biurea (C2D6N4O2). Neutron-powder diffraction and Raman spectra, measured up to 3.89 GPa reveal this transition to be isosymmetric, where substantial intramolecular reorientation leads to an abrupt decrease in unit cell volume at ∼0.6 GPa and an increase in some intermolecular contact distances. DFT and vibrational energy calculations suggest that the ambient pressure phase is stabilised by zero point energy and entropy, until the pressure–volume–driven transition at 0.6 GPa.

The widespread use of biodiesel as a renewable fuel offers many potential advantages, but at the same time presents challenges for modern internal combustion engines, particularly for those that involve high-pressure injection of fuel into the combustion chamber. At the typical elevated pressures used in such engines, biodiesel can crystallise and block fuel filters and injection nozzles, thereby causing engine failure. In this study, optical studies and Raman spectroscopy of a typical biodiesel sample contained in a diamond-anvil cell show that biodiesel initially crystallises at ca. 0.2 GPa and then undergoes a series of structural changes on further increase of pressure. On account of the complex composition of biodiesel, this study focused on one of its main components – methyl stearate. Using a combination of Raman spectroscopy, X-ray powder diffraction and neutron powder diffraction, it was shown that methyl stearate exhibits rich polymorphic behaviour when subjected to elevated pressures up to 6.3 GPa. Under non-hydrostatic conditions, pressures as low as 0.1 GPa converted Form V to crystallites of Form III that typically adopt plate-like morphologies. This observation has implications for the pressure-induced crystallisation of biodiesel containing high proportions of methyl stearate because of the potentially serious consequences for blocking of injection nozzles in engines. Four phase transitions over the pressure range of 0.1 GPa to 6.3 GPa were also observed. Form III was recovered on decompression to ambient pressure. High-pressure neutron powder diffraction studies of a perdeuterated sample showed that Form V persisted up to 3.11 GPa. This contrast in behaviour between the X-ray and neutron studies may be a consequence of deuteration, or of compression under non-hydrostatic versus hydrostatic conditions.

We use in situ high-pressure neutron powder diffraction to study elastic properties of Fe3O4 magnetite nanoparticles of different sizes. It is found that nanoparticles are elastically softer than the bulk. Apart from the smallest nanoparticle of diameter 8 nm, the atomic and magnetic structures do not change significantly with nanoparticle size or pressure. The 8 nm sample appears to take a disordered spinel structure instead of the inverse spinel structure of the bulk and larger nanoparticles, as seen in bond lengths and magnetic structures. Synchrotron x-ray total scattering was used to support this interpretation. Furthermore, this study suggests that the influence of magnetic disorder at the nanoparticle surface is significant for the size of 8 nm.

Ice III is a hydrogen-disordered phase of ice that is stable between about 0.2 and 0.35 GPa. Upon cooling, it transforms to its hydrogen-ordered counterpart ice IX within the stability region of ice II. Because of this metastability, detailed studies of the ice III to ice IX phase transition have so far not been carried out. Using ammonium fluoride doping to prevent the formation of ice II, we now present a detailed study on this phase transition using in-situ powder neutron diffraction. The a and c lattice constants are found to expand and contract, respectively, upon hydrogen ordering yielding an overall negative volume change. Interestingly, the anisotropy in the lattice constants persists when ice IX is fully formed and negative thermal expansion is observed. Analogous to the isostructural keatite and b-spodumenes, the negative thermal expansion can be explained through the build-up of torsional strain within in the a-b plane as the helical ‘springs’ within the structure expand upon heating. The reversibility of the phase transition was demonstrated for the first time upon heating. The ammonium fluoride doping induces additional residual hydrogen disorder in ice IX and is suggested to be a chemical way for ‘excitation’ of the ice-rules configurational manifold. Compared to ices II and VIII, the induced hydrogen disorder in ice IX is smaller which suggests a higher density of configurational states close to the ground state. This study highlights the importance of dopants for exploring water’s phase diagram and underpins the highly complex solid-state chemistry of ice.

Author(s): Wu, S; Yin, J; Smart, T; Acharya, A; Bull, CL; Funnell, NP; Forrest, TR; Simutis, G; Khasanov, R; Lewin, SK; Wang, M; Frandsen, BA; Jeanloz, R; Birgeneau, RJ | Abstract: The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, BaFe2X3(X=S, Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. Here we report x-ray, neutron diffraction, and muon spin relaxation experiments on BaFe2Se3 under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was found at a pressure of 3.7(3) GPa. Neutron diffraction measurements at 6.8(3) GPa and 120 K show that the block magnetism persists even at these high pressures. A steady increase and then fast drop of the magnetic transition temperature TN and greatly reduced moment above the pressure Ps indicate potentially rich and competing phases close to the superconducting phase in this ladder system.

Water and the many phases of ice display a plethora of complex physical properties and phase relationships1–4 that are of paramount importance in a range of settings including processes in Earth’s hydrosphere, the geology of icy moons, industry and even the evolution of life. Well-known examples include the unusual behaviour of supercooled water 2 , the emergent ferroelectric ordering in ice films 4 and the fact that the ‘ordinary’ ice Ih floats on water. We report the intriguing observation that ice II, one of the high-pressure phases of ice, disappears in a selective fashion from water’s phase diagram following the addition of small amounts of ammonium fluoride. This finding exposes the strict topologically constrained nature of the ice II hydrogen-bond network, which is not found for the competing phases. In analogy to the behaviour of frustrated magnets 5 , the presence of the exceptional ice II is argued to have a wider impact on water’s phase diagram, potentially explaining its general tendency to display anomalous behaviour. Furthermore, the impurity-induced disappearance of ice II raises the prospect that specific dopants may not only be able to suppress certain phases but also induce the formation of new phases of ice in future studies.

This study details the structural characterization of glycolide-h4 as a function of pressure to 6 GPa using neutron powder diffraction on the PEARL instrument at ISIS Neutron and Muon source. Glycolide-h4, rather than its deuterated isotopologue, was used in this study due to the difficulty of deuteration. The low background afforded by zirconia-toughened alumina anvils nevertheless enabled the collection of data suitable for structural analysis to be obtained to a pressure of 5 GPa. Glycolide-h4 undergoes a reconstructive phase transition at 0.15 GPa to a previously identified form (II), which is stable to 6 GPa.

We describe the capability to measure high-resolution neutron powder diffraction data to a pressure of at least 8 kbar. We have used the HRPD instrument at the ISIS neutron source and a piston-cyli...

The formation of the spin-ice pyrochlore Ho2Ge2O7 by two different high temperature, high pressure routes has been explored using in situ neutron diffraction. The first route involves the solid-state reaction of Ho2O3 and GeO2, and formation of the pyrochlore phase is observed at 994(27) °C and 3.81(2) GPa, which are significantly milder conditions than those previously reported. The second route involves the hydrothermal synthesis of the tetragonal Ho2Ge2O7 pyrogermanate from Ho(NO3)3·5H2O and GeO2 and its subsequent transformation to the pyrochlore phase, which is observed at 683(23) °C and 3.89(3) GPa. The lowering of the formation temperature of high pressure phases by employment of a precursor of appropriate stoichiometry may also have applications in the wider field of solid-state chemistry.

We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C0 at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move. It has a network that is unrelated to any experimentally known ice, silica, or zeolite network but is instead related to two Zintl compounds. Both hydrates are found to be stable in electronic structure calculations, with hydration ratios in very good agreement with experiment. peerReviewed

The thermoelastic and structural properties of SrZrO 3 perovskite in the Pnma ( Pbnm ) phase have been studied using neutron powder diffraction at 82 temperatures between 11 K and 406 K at ambient pressure, and at sixteen pressures between 0.07 and 6.7 GPa at ambient temperature. The bulk modulus, derived by fitting the equation of state to a second order Birch-Murnaghan equation-of-state, 157(5) GPa, is in excellent agreement with that deduced in a recent resonant ultrasound investigation. Experimental axial compressional moduli are in agreement with those calculated from the elastic stiffness coefficients derived by ab-initio calculation, although the experimental bulk modulus is significantly softer than that calculated. Following low temperature saturation for temperatures less than 40 K, the unit cell monotonically increases with a predicted high temperature limit in the volume expansivity of ∼2.65 × 10 −5 K −1 . Axial linear thermal expansion coefficients are found to be in the order α b c a for all temperatures greater than 20 K with the b axis indicating a weak, low temperature negative expansion coefficient at low temperatures. The thermoelastic properties of SrZrO 3 can be approximated by a two-term Debye model for the phonon density of states with Debye temperatures of 238(4) K and 713(6) K derived in a self-consistent manner by simultaneously fitting the isochoric heat capacity and the unit cell volume. Atomic displacement parameters have been fitted to a modified Debye model in which the zero-point term is an additional refinable variable and shows the cations and anions have well separated Debye temperatures, mirroring the need for two Debye-like distributions in the vibrational density of states. The temperature dependence of the crystal structure is presented in terms of the amplitudes of the seven symmetry-adapted basis vectors of the aristotype phase that are consistent with space group Pbnm , thus permitting a direct measure of the order parameter evolution in SrZrO 3 . The temperature variation of the in-phase tilt, which is lost at the phase transition at 973 K, is consistent with tricritical behaviour, in agreement with published results based on high temperature crystallographic data.

We describe the high-pressure (4 GPa) high-temperature (∼1100 K) synthesis of the solid solution series SeCo

Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite La

The type II clathrate hydrate (CH) THF·17 H

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, their phase transformations under pressure are more complex and less well understood. We have studied the structures of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Under pressure, these materials transform to a rhombohedral phase isostructural to cadmium guanidinium formate. This transformation accommodates the reduced cell volume while preserving the perovskite topology of the framework. Using density-functional theory calculations, we show that this behaviour is a consequence of the hydrogen-bonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression within their plane. Our results demonstrate more generally that identifying suitable host–guest hydrogen-bonding geometries may provide a route to engineering hybrid perovskite phases with desirable crystal structures. This article is part of the theme issue ‘Mineralomimesis: natural and synthetic frameworks in science and technology’.

The removal of residual hydrogen disorder from various phases of ice with acid or base dopants at low temperatures has been a focus of intense research for many decades. As an antipode to these efforts, we now show using neutron diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice VIII with ~31% residual hydrogen disorder illustrating the long-range hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by ~20 K with respect to the hydrogen-ordering temperature of pure ice VII after which the hydrogen-ordering took place slowly over a ~60 K temperature window. These findings demonstrate that ND4F-doping slows down the hydrogen-ordering kinetics quite substantially. The partial hydrogen order of the doped sample is consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue that local ferroelectric domains must exist between ionic point defects of opposite charge. In addition to the long-range effect of NH4F-doping on hydrogen-ordered water structures, the design principle of using topological charges should be applicable to a wide range of other 'ice-rule' systems including spin ices and related polar materials., 23 pages, 4 figures, 2 tables

We explore the polymorphism of p-aminobenzoic acid (pABA) under high-pressure conditions. We have been able to isolate a new high-pressure form (δ-pABA) at pressures exceeding 0.3 GPa from three different pressure-transmitting media, water, water : ethanol and pure ethanol. We explore the compression behaviour of α-pABA in each of these media using neutron powder diffraction and observe that the transition is kinetically hindered using the aqueous ethanol and ethanol solutions compared with the pure aqueous medium. δ-pABA is sufficiently stable to be recovered to ambient pressure to enable its characterisation via X-ray powder diffraction and differential scanning calorimetry. At ambient pressure we have observed that δ-pABA converts into α-pABA on heating beyond 70 °C.

Hyperhoneycomb iridate $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is a three-dimensional analog of two-dimensional honeycomb iridates, such as $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ shows a noncollinear spiral ordering of spin-orbital-entangled ${J}_{\mathrm{eff}}=1/2$ moments at low temperatures below 38 K, which is known to be suppressed under a pressure of $\ensuremath{\sim}2$ GPa. In addition, a structural transition is observed at ${P}_{\mathrm{S}}\ensuremath{\sim}4$ GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ above ${P}_{\mathrm{S}}$ was refined, which indicates the formation of ${\mathrm{Ir}}_{2}$ dimers on the zigzag chains, with an Ir-Ir distance of $\ensuremath{\sim}2.66\phantom{\rule{0.16em}{0ex}}\AA{}$, even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular-orbital state comprising the two local ${d}_{zx}$ orbitals in the Ir-${\mathrm{O}}_{2}$-Ir bond plane, which conflicts with the equal superposition of ${d}_{xy}, {d}_{yz}$, and ${d}_{zx}$ orbitals in the ${J}_{\mathrm{eff}}=1/2$ wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering measurements and the electronic structure calculations are fully consistent with the collapse of the ${J}_{\mathrm{eff}}=1/2$ state. The competition between the spin-orbital-entangled ${J}_{\mathrm{eff}}=1/2$ state and molecular-orbital formation is most likely universal in honeycomb-based Kitaev materials.

Pressure-induced polymerization (PIP) of aromatics is a novel method for constructing sp