High-pressure variable-temperature neutron powder diffraction was employed to investigate the synthesis and structural properties of arsenolite (As4O6), a cubic arsenic(III) oxide polymorph, and its inclusion compound with deuterium. The compound forms with the stoichiometry As4O6 & centerdot;2D(2), consistent with earlier findings. At 295 K and 1.96(2) GPa, D-2 molecules occupy the 16c site (0, 0, 0) and are aligned along the <111> direction. The diffraction data suggest that the D-2 molecules at this site become rotationally disordered upon decompression to 0.78(2) GPa at 120 K, while no occupancy is observed at the 8a position (1/8, 1/8, 1/8) under any of the studied conditions. The kinetic data of the high-pressure inclusion compound synthesis at 1.56(2) and 1.96(2) GPa were analyzed using the Avrami model. The Avrami exponent, 0.135(16), remains invariant with pressure, demonstrating that the reaction topochemistry is unaffected by compression. Similarly, the reaction rates, 0.29(3) and 0.23(3) min(-0.135) at 1.56(2) and 1.96(2) GPa, respectively, exhibit negligible pressure dependence and suggest a surprisingly low activation volume of 1.6 cm(3) mol(-1) for the reaction. These results provide detailed insight into the structural dynamics and kinetics of deuterium inclusion in arsenolite under high pressure.

Metal oxides containing cations with an s 2 electron lone pair possess favorable band gaps for photocatalysis. Understanding the stereochemical role of the lone pair is important for tuning their physical properties. Here, we present a detailed investigation of the pressure-dependent structure of the well-known photocatalyst BiVO4. We probe this behavior using a combination of neutron powder diffraction (NPD), neutron pair distribution function (NPDF), and density functional theory (DFT) calculations. Our analysis reveals that that the Bi3+ lone pair is suppressed in the high-pressure tetragonal phase, forming a symmetric electron distribution. This contrasts with the tetragonal phase at high temperature and ambient pressure, where localized lone pair distortions have been observed. Additional NPD measurements on LaNbO4, which lacks a cation lone pair, demonstrate the structural distortions expected in fergusonite structured metal oxides, while revealing new pressure-dependent behavior below 7.3 GPa that warrants further investigation.

The locations and occupancies of deuterium atoms in molybdenum deuteride were studied using time-of-flight neutron powder diffraction under pressures up to ∼6.2 GPa. We confirmed the P63/mmc space group and determined the overstoichiometric deuterium content to give a composition of MoD1.15, showing that our data are sensitive to deuterium positions and occupancies. In MoD1.15, the majority of the interstitial deuterium atoms occupy the octahedral sites, and the remainder occupy the tetrahedral sites and exhibit relatively short interatomic distances of 1.49 and 1.90 Å to molybdenum atoms.

We present a combination of neutron powder-diffraction measurements demonstrating negative linear compressibility and irregular thermal expansion in 7Li2CO3. This is shown to be due to an interplay between the tilting of the rigid carbonate group and the shear strain in the unit cell, which leads to a first-order transition from monoclinic to hexagonal symmetry. The phase evolution is shown to be highly sensitive to the level of hydrostaticity in the sample. Under hydrostatic conditions, the sample begins transformation at 8.5 GPa leading to a change from tetrahedral to octahedral Li coordination. Symmetry adapted basis mode analysis, combined with density functional theoretical (DFT) calculations and Raman spectroscopy, is used to show that this transition is reverse proper ferroelastic in nature.

We employ a combined computational and experimental approach to systematically assess the hydrostatic properties of methanol-ethanol (MeOH-EtOH) mixtures of varying compositions, with the aim of evaluating their suitability as pressure-transmitting mediums (PTMs). PTMs are essential for enabling the characterization of materials properties at high pressure, perhaps most prominently in the context of diffraction measurements, to provide uniform compression and avoid strain on the sample. Molecular dynamics (MD) simulations indicate that the hydrostatic limit and several structural and dynamic properties of the widely used 4:1 MeOH-EtOH volume ratio do not exhibit any significant deviations from the monotonic trends observed as a function of MeOH content within the mixture. These findings are in agreement with X-ray pair distribution function measurements, which show no peculiar structural behaviour for the 4:1 composition. Experimental measurements of the hydrostatic limit confirm this result and demonstrate that, as previously reported, the role of ethanol is primarily to delay MeOH crystallization. However, we find that the same role can be fulfilled by other small molecules, such as propan-2-ol. Additional simulations of several MeOH-X binary mixtures suggest that these results might hold for a variety of similar mixtures. Thus, our findings indicate that the 4:1 ratio is neither peculiar nor optimal in terms of PTM performance; instead, its popularity seems to be mostly due to the influence of previous literature. Indeed, we find that the 9:1 MeOH-EtOH mixture is characterized by a hydrostatic limit which is superior (by nearly 1 GPa) to that observed for the 4:1 ratio. These findings offer a promising alternative PTM composition which is readily available, and pave the way towards future work aimed at the rational design of novel PTMs.

Rutile-structured materials can exhibit negative linear compressibility (NLC) following ferroelastic phase transitions, expanding in one direction under uniform compression. We investigate this phenomenon in structural analogues-transition metal dicyanamides (dca) and tricyanomethanides (tcm) with single and double rutile-like structures, respectively. The pressure-induced structural behaviour of Cu(tcm)2 and Cu(dca)2 are studied using high-pressure diffraction. Both systems undergo anisotropic deformation upon compression, with Cu(dca)2 exhibiting NLC of -6.5(10) TPa-1 along the c-axis, while Cu(tcm)2 shows zero linear compressibility (ZLC) along the a-axis. This difference is attributed to the single rutile-like network with flexible dca- linkers in Cu(dca)2, in contrast to the more constrained doubly interpenetrating structure of Cu(tcm)2 with rigid tcm- linkers. We also study the interplay between structural features and electronic effects arising from the Jahn-Teller distortion in both materials, in controlling their compression behaviour.

The high-pressure behaviour of Na_2CuF_4 is explored by powder neutron diffraction and density functional theory (DFT) calculations. A first-order phase transition is observed to take place between 2.4 - 2.9 GPa, involving a reorientation of the Jahn-Teller (JT) long axes of the (CuF6) octahedra (and therefore the d_z^2 Cu orbitals), in agreement with our DFT calculations which suggest a transition at 2.8 GPa. The transition can be described as being between a state of ferro-orbital order and one of A-type antiferro-orbital order, reflecting a shift in the associated electronic instability from being in the zone-center to zone boundary of the first Brillouin zone of the parent structure, with pressure. This change results in a decoupling of magnitude of the associated Jahn-Teller distortion of the Cu-F bond lengths from the lattice strain. This scenario is supported by our observations that the compressibility of the pre-transition phase is highly anisotropic, whilst in the post-transition phase it becomes almost isotropic, and that we observed no further decrease of the magnitude the JT distortion up to 5 GPa, or melting of the OO in our DFT calculations up to at least 5 GPa.

The effect of pressure and temperature on the unit-cell volume of NaF has been measured by X-ray powder diffraction at ambient pressure between 12 and 300 K and neutron powder diffraction up to 5 GPa between 140 and 350 K. These data have been combined with high-pressure volume data at 300 and 950 K to 25 GPa and adiabatic bulk modulus data to 650 K to define an equation of state for NaF relating molar volume to both temperature and pressure. The model combines a fourth-order Birch-Murnaghan equation of state at 295 K with a Mie-Grüneisen-Debye model for thermal pressure. The parameters of the model set at 295 K and ambient pressure are as follows: reference unit-cell volume V 0 = 14.9724 (5) cm3 mol-1, isothermal bulk modulus K 0T  = 46.79 (14) GPa, first derivative of the bulk modulus K'0T  = 5.72 (12), second derivative of the bulk modulus K''0T  = -0.43 (4) GPa-1, Debye temperature T MGD = 459 (3) K, and Anderson Grüneisen parameters γ0 = 1.547 (11) and q = 0.94 (18).

Copper(I) tricyanomethanide, Cu(tcm), is a flexible framework material that exhibits the strongest negative area compressibility (NAC) effect ever observed─a remarkable property with potential applications in pressure sensors, artificial muscles, and shock-absorbing devices. Under increasing pressure, Cu(tcm) undergoes two sequential phase transitions (tetragonal → orthorhombic → monoclinic): It has an initial tetragonal structure (I41md) at ambient conditions, but this structure only persists within a narrow pressure range; at 0.12(3) GPa, a pressure-induced ferroelastic phase transition occurs, transforming Cu(tcm) into a low-symmetry orthorhombic structure (Fdd2). The orthorhombic phase has a NAC of -108(14) TPa-1 in the b-c plane between 0.12(3) and 0.93(8) GPa. The NAC behavior is associated with framework hinge motion in a flexible framework with "wine-rack" topology. At 0.93(8) GPa, Cu(tcm) undergoes a second phase transition and transforms into a layered monoclinic structure (Cc) with topologically interpenetrating honeycomb networks. The monoclinic phase of Cu(tcm) exhibits a slight negative linear compressibility (NLC) of -1.1(1) TPa-1 along the a axis and a zero area compressibility of Kac = Ka + Kc = 0.0(4) TPa-1 in the a-c plane over the pressure range of 0.93-2.63 GPa. In contrast to the orthorhombic phase, its mechanism is understood as the pressure-driven dampening of layer "rippling," which acts to increase the cross-sectional area of the layer at higher hydrostatic pressures. These findings have implications for understanding the underlying mechanism of NAC phenomenon in framework materials.

The structural behaviour of CsCaCl3 has been investigated using neutron diffraction as a function of pressure and temperature.

A highly piezochromic co-crystal of the energetic molecule NTO with bipyridine. Colour change attributed to direct compresion of the crystal lattice. No phase transition observed between 0–8 GPa. Impact sensitivity reduced compared to NTO.

The response of the trimethylammonium–iodinechloride and diiodide (TMA–ICl/I2) crystal structures have been examined under high pressure using neutron powder diffraction.

Jahn–Teller (JT) distorted Cu II -containing compounds often display interesting structural and functional behaviour upon compression.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A remarkable degree of polymorphism is exhibited in the high-pressure and low-temperature crystal structures of 2,2,2-trifluoroethanol, with the observation of four ordered phases (forms 1–4) and a cubic rotor phase (form 5).

Ammonium perchlorate (AP) is an energetic material that functions as an oxidiser in solid rocket propellant.The performance of energetic materials are often affected by crystal structure which in turn is influenced by temperature/pressure.We present a neutron diffraction study of ammonium perchlorate as a function of pressure and temperature.Upon compression at 290 K under hydrostatic conditions we observe a first-order phase transition from Phase I (ambient phase, space group: Pnma) to Phase II (high-pressure phase, space group: P212121) at ~4 GPa as previously reported.[2]We present both equation of states and structural behaviour for both phases up to a pressure of ~11.5 GPa.Previous high-pressure Raman experiments suggested the existence of a third phase (Phase III) at ~10 GPa.[3]However, our large volume neutron diffraction experiments on perdeuterated AP show no evidence for such a phase transition to even up to 11.5 GPa.Previous variable-temperature studies of AP has revealed a high-temperature polymorph of AP (Phase V, space group: Fm3 ̅ m) at 511 K but no low-temperature forms have been identified down to 10 K. [4,5] Contrary to this, upon cooling at ambient pressure, we observe a second order phase transition at ~130 K which has only previously been suggested by Raman spectroscopy.[5]Upon further cooling, we observe a previously unreported phase transition at ~29 K.The structural behaviour for Phase I was followed down to a temperature of 31 K.