The pressure response of Jahn-Teller-distorted Prussian blue analogues.

Jahn-Teller (JT) distorted CuII-containing compounds often display interesting structural and functional behaviour upon compression. We use high-pressure X-ray and neutron diffraction to investigate four JT-distorted Prussian blue analogues: Cu[Co(CN)6]0.67, CuPt(CN)6, and ACuCo(CN)6 (A = Rb, Cs), where the first two were studied in both their hydrated and dehydrated forms. All compounds are less compressible than the JT-inactive MnII-based counterparts, indicating a coupling between the electronic and mechanical properties. The effect is particularly strong for Cu[Co(CN)6]0.67, where the local JT distortions are uncorrelated (so-called orbital disorder). This sample amorphises at 0.5 GPa when dehydrated. CuPt(CN)6 behaves similarly to the MnII-analogues, with phase transitions at around 1 GPa and low sensitivity to water. For ACuCo(CN)6, the JT distortions reduce the propensity for phase transitions, although RbCuCo(CN)6 transitions to a new phase (P2/m) around 3 GPa. Our results have a bearing on both the topical Prussian blue analogues and the wider field of flexible frameworks.

Room-Temperature Type-II Multiferroic Phase Induced by Pressure in Cupric Oxide.

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.

High- versus Low-Spin Ni2+ in Elongated Octahedral Environments: Sr2NiO2Cu2Se2, Sr2NiO2Cu2S2, and Sr2NiO2Cu2(Se1-x S x )2.

Sr2NiO2Cu2Se2, comprising alternating [Sr2NiO2]2+ and [Cu2Se2]2- layers, is reported. Powder neutron diffraction shows that the Ni2+ ions, which are in a highly elongated NiO4Se2 environment with D4h symmetry, adopt a high-spin configuration and carry localized magnetic moments which order antiferromagnetically below ∼160 K in a √2a × âˆš2a × 2c expansion of the nuclear cell with an ordered moment of 1.31(2) µB per Ni2+ ion. The adoption of the high-spin configuration for this d 8 cation in a pseudo-square-planar ligand field is supported by consideration of the experimental bond lengths and the results of density functional theory (DFT) calculations. This is in contrast to the sulfide analogue Sr2NiO2Cu2S2, which, according to both experiment and DFT calculations, has a much more elongated ligand field, more consistent with the low-spin configuration commonly found for square-planar Ni2+, and accordingly, there is no evidence for magnetic moment on the Ni2+ ions. Examination of the solid solution Sr2NiO2Cu2(Se1-x S x )2 shows direct evidence from the evolution of the crystal structure and the magnetic ordering for the transition from high-spin selenide-rich compounds to low-spin sulfide-rich compounds as a function of composition. Compression of Sr2NiO2Cu2Se2 up to 7.2 GPa does not show any structural signature of a change in the spin state. Consideration of the experimental and computed Ni2+ coordination environments and their subtle changes as a function of temperature, in addition to transitions evident in the transport properties and magnetic susceptibilities in the end members, Sr2NiO2Cu2Se2 and Sr2NiO2Cu2S2, suggest that simple high-spin and low-spin models for Ni2+ may not be entirely appropriate and point to further complexities in these compounds.

Radiation effects, zero thermal expansion, and pressure-induced phase transition in CsMnCo(CN)6.

The Prussian blue analogue CsMnCo(CN)6 is studied using powder X-ray and neutron diffraction under variable temperature, pressure, and X-ray exposure. It retains cubic F4̄3m symmetry in the range 85-500 K with minimal thermal expansion, whereas a phase transition to P4̄n2 occurs at ∼2 GPa, driven by octahedral tilting. A small lattice contraction occurs upon increased X-ray dose. Comparisons with related systems indicate that the CsI ions decrease the thermal expansion and suppress the likelihood of phase transformations. The results improve the understanding of the stimuli-responsive behaviour of coordination polymers.

Phase behaviour of ammonium bromide-d4under high pressure and low temperature; an average and local structure study.

We revisit the pressure-induced order-disorder transition between phases II and IV in ammonium bromide-d4using neutron diffraction measurements to characterise both the average and local structures. We identify a very sluggish transition that does not proceed to full conversion and local structure correlations indicate a slight preference for ammonium cation ordering along ⟨110⟩ crystallographic directions, as pressure is increased. Simultaneous cooling below ambient temperature appears to facilitate the pressure-induced transition. Variable-temperature, ambient-pressure measurements across the IV → III → II transitions show slower conversion than previously observed, and that phase III exhibits metastability above ambient temperature.

Pressure Tuning the Jahn-Teller Transition Temperature in NaNiO2.

NaNiO2 is a layered material consisting of alternating layers of NaO6 and Jahn-Teller-active NiO6 edge-sharing octahedra. At ambient pressure, it undergoes a broad phase transition from a monoclinic to rhombohedral structure between 465 and 495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 and 490 K. The 290 and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixed-phase throughout. The unit-cell volume was fitted to a second-order Birch-Murnaghan equation of state, where B = 119.6(5) GPa at 290 K. We observe at 490 K that the fraction of the Jahn-Teller-distorted phase increases with pressure, from 67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in conjunction with neutron diffraction measurements at 490 K on removing pressure from 5.46(9) to 0.342(13) GPa, we show that the Jahn-Teller transition temperature increases with pressure. Our results are used to present a structural pressure-temperature phase diagram for NaNiO2. To the best of our knowledge, this is the first diffraction study of the effect of pressure on the Jahn-Teller transition temperature in materials with edge-sharing Jahn-Teller-distorted octahedra and the first variable-pressure study focusing on the Jahn-Teller distortion in a nickelate.

Recovering local structure information from high-pressure total scattering experiments.

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 hydro-static 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 hydro-static pressure is presented, extracting compression behaviour of the real-space structure.

Probing the Influence of Defects, Hydration, and Composition on Prussian Blue Analogues with Pressure.

The vast compositional space of Prussian blue analogues (PBAs), formula AxM[M'(CN)6]y·nH2O, allows for a diverse range of functionality. Yet, the interplay between composition and physical properties-e.g., flexibility and propensity for phase transitions-is still largely unknown, despite its fundamental and industrial relevance. Here we use variable-pressure X-ray and neutron diffraction to explore how key structural features, i.e., defects, hydration, and composition, influence the compressibility and phase behavior of PBAs. Defects enhance the flexibility, manifesting as a remarkably low bulk modulus (B0 ≈ 6 GPa) for defective PBAs. Interstitial water increases B0 and enables a pressure-induced phase transition in defective systems. Conversely, hydration does not alter the compressibility of stoichiometric MnPt(CN)6, but changes the high-pressure phase transitions, suggesting an interplay between low-energy distortions. AMnCo(CN)6 (AI = Rb, Cs) transition from F4̅3m to P4̅n2 upon compression due to octahedral tilting, and the critical pressure can be tuned by the A-site cation. At 1 GPa, the symmetry of Rb0.87Mn[Co(CN)6]0.91 is further lowered to the polar space group Pn by an improper ferroelectric mechanism. These fundamental insights aim to facilitate the rational design of PBAs for applications within a wide range of fields.

A simple system for neutron diffraction at 4 K and elevated pressures.

We describe a unique cryogen-free closed-cycle refrigerator system using a beryllium-copper VX1 variant of the Paris-Edinburgh press, which enables approximately 3 GPa to be generated on a sample volume of 66 mm3, over the temperature range of 4 K-300 K. The main advantage of this system is its versatility; it has been designed to be fully compatible with the PEARL neutron powder-diffraction instrument at the ISIS facility, but is also compatible with several other instruments at the facility with minor modifications. We provide a full description of the system, along with representative data collected on PEARL from MnF2 at 13 K and 2.4 GPa.

Using applied pressure to guide materials design: a neutron diffraction study of La2NiO4+δ and Pr2NiO4+δ.

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.

Alloxan under pressure-squeezing an extremely dense molecular crystal structure.

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.

High-Pressure Study of the Elpasolite Perovskite La2NiMnO6.

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 pressure and over a temperature range of 120-1177 K up to a maximum pressure of 6.6 GPa. X-ray diffraction was also used up to a maximum pressure of 64 GPa, over a temperature range of 300-720 K. The sample was found to exist in a mixed rhombohedral/monoclinic symmetry at ambient conditions, the balance of which was found to be strongly temperature- and pressure-dependent. Alternating current magnetometry and X-ray absorption near-edge structure measurements were made at ambient pressure to characterize the sample, suggesting that the transition-metal sites exist in a mixed Ni3+/Mn3+ and Ni2+/Mn4+ state at ambient temperature and pressure. Analysis of the magnetic properties of the sample shows that the Curie temperature can be enhanced by ∼12 K with 2 GPa applied pressure, but it is highly stable at pressures beyond this. We report a pressure-volume-temperature equation of state for this material over this combined temperature and pressure range, with an ambient temperature bulk modulus of ∼179(8) GPa. The previously reported transition from monoclinic to rhombohedral symmetry upon heating to 700 K is seen to be encouraged with applied pressure, transforming fully by ∼1.5 GPa. Raman spectroscopy data were collected up to ∼8 GPa and show no clear changes or discontinuities over the reported phase transition to rhombohedral symmetry or any indication of further changes over the range considered. The ambient-pressure Grüneisen parameter γth was determined to be γth = 2.6 with a Debye temperature of 677 K. The individual modal parameters γj at ambient temperature were also determined from the high-pressure Raman data.

Structure and physical properties of SeCo1-x Mn x O3.

We describe the high-pressure (4 GPa) high-temperature (∼1100 K) synthesis of the solid solution series SeCo1-x Mn x O3 (0 < x < 1) using H2SeO4 and transition metal oxide mixtures sealed in Pt capsules. Neutron powder diffraction has been performed to determine progression of the structure across the solution. All samples crystallise with orthorhombic Pnma symmetry, and octahedral tilting is determined to increase with Mn content. SQUID magnetometry measurements were performed, and reveal that the Néel temperature shifts only by approximately 1 K over the series.

In situ neutron diffraction study of the formation of Ho2Ge2O7 pyrochlore at high temperature and pressure.

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.

A novel compact three-dimensional laser-sintered collimator for neutron scattering.

Improvements in the available flux at neutron sources are making it increasingly feasible to obtain refineable neutron diffraction data from samples smaller than 1 mm(3). The signal is typically too weak to introduce any further sample environment in the 30-50 mm diameter surrounding the sample (such as the walls of a pressure cell) due to the high ratio of background to sample signal, such that even longer count times fail to reveal reflections from the sample. Many neutron instruments incorporate collimators to reduce parasitic scattering from the instrument and from any surrounding material and larger pieces of sample environment, such as cryostats. However, conventional collimation is limited in the volume it can focus on due to difficulties in producing tightly spaced neutron-absorbing foils close to the sample and in integrating this into neutron instruments. Here we present the design of a novel compact 3D rapid-prototyped (or "printed") collimator which reduces these limitations and is shown to improve the ratio of signal to background, opening up the feasibility of using additional sample environment for neutron diffraction from small sample volumes. The compactness and ease of customisation of the design allows this concept to be integrated with existing sample environment and with designs that can be tailored to individual detector geometries without the need to alter the setup of the instrument. Results from online testing of a prototype collimator are presented. The proof of concept shows that there are many additional collimator designs which may be manufactured relatively inexpensively, with a broad range of customisation, and geometries otherwise impossible to manufacture by conventional techniques.

High-pressure cell for neutron diffraction with in situ pressure control at cryogenic temperatures.

Pressure generation at cryogenic temperatures presents a problem for a wide array of experimental techniques, particularly neutron studies due to the volume of sample required. We present a novel, compact pressure cell with a large sample volume in which load is generated by a bellow. Using a supply of helium gas up to a pressure of 350 bar, a load of up to 78 kN is generated with leak-free operation. In addition, special fiber ports added to the cryogenic center stick allow for in situ pressure determination using the ruby pressure standard. Mechanical stability was assessed using finite element analysis and the dimensions of the cell have been optimized for use with standard cryogenic equipment. Load testing and on-line experiments using NaCl and BiNiO3 have been done at the WISH instrument of the ISIS pulsed neutron source to verify performance.