Local Structure and Dynamics in MPt(CN)6 Prussian Blue Analogues.

We use a combination of X-ray pair distribution function (PDF) measurements, lattice dynamical calculations, and ab initio density functional theory (DFT) calculations to study the local structure and dynamics in various MPt(CN)6 Prussian blue analogues. In order to link directly the local distortions captured by the PDF with the lattice dynamics of this family, we develop and apply a new "interaction-space" PDF refinement approach. This approach yields effective harmonic force constants, from which the (experiment-derived) low-energy phonon dispersion relations can be approximated. Calculation of the corresponding Grüneisen parameters allows us to identify the key modes responsible for negative thermal expansion (NTE) as arising from correlated tilts of coordination octahedra. We compare our results against the phonon dispersion relations determined using DFT calculations, which identify the same NTE mechanism.

Extensive Polymorphism in the Molecular Ferroelectric 18-Crown-6 Oxonium Tetrachloro-Gallium(III).

The materials property of ferroelectricity is intimately linked with symmetry-changing phase transitions. Characterizing such transitions is therefore essential for understanding molecular ferroelectrics. In this paper, we explore the temperature and thermal history dependence of polymorphic phase transitions in the multiaxial molecular ferroelectric 18-crown-6 oxonium tetrachloro-gallium(III). We have solved the structures of two previously suggested polymorphs (D and Y) ab initio from high-temperature powder diffraction data. We also report the structure of a new polymorph (X) using low-temperature powder diffraction data and identify a fifth (W) that can form on cooling. These polymorphs can be related using two distinct group-subgroup trees. Structure types A-C observed in this and related compounds can be derived from high-temperature polymorph D by group-subgroup relationships. The X and Y polymorphs can be described as child structures of a hypothetical polymorph Z using a molecular rotational distortion mode description. The ferroelectric properties of the various polymorphs can be rationalized based on our structural findings.

Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba3-x Sr x YGa2O7.5 0 ≤ x ≤ 3 Materials.

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba3-x Sr x YGa2O7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives.

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A3OhTd2O7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO2.5 compositions relative to that in the perovskite ABO3 or brownmillerite A2B2O5 structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O2- migration. The low- and high-temperature average and local structures of Ba3YGa2O7 (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and ß = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P1121/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O2- ions was identified. Ionic conductivities of Ba3YGa2O7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba3YGa2O7.5 retaining proton conductivity at high temperatures. Ba2.9La0.1YGa2O7.55 and Ba3YGa1.9Ti0.1O7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.

Amorphous Mixtures of Ice and C60 Fullerene.

Carbon and ice make up a substantial proportion of our universe. Recent space exploration has shown that these two chemical species often coexist such as on comets and asteroids and in the interstellar medium. Here, we prepare mixtures of C60 fullerene and H2O by vapor codeposition at 90 K with molar C60/H2O ratios ranging from 1:1254 to 1:5. The C60 percolation threshold is found between the 1:132 and 1:48 samples, corresponding to a transition from matrix-isolated C60 molecules to percolating C60 domains that confine H2O. Below this threshold, the crystallization and thermal desorption properties of H2O are not significantly affected by C60, whereas the crystallization temperature of H2O is shifted toward higher temperatures for the C60-rich samples. These C60-rich samples also display exotherms corresponding to the crystallization of C60 as the two components undergo phase separation. More than 60 vol % C60 is required to significantly affect the desorption properties of H2O. A thick blanket of C60 on top of pure amorphous ice is found to display large cracks due to water desorption. These findings may help us to understand the recently observed unusual surface features and the H2O weather cycle on the 67P/Churyumov-Gerasimenko comet.

Polymorph exploration of bismuth stannate using first-principles phonon mode mapping.

Accurately modelling polymorphism in crystalline solids remains a key challenge in computational chemistry. In this work, we apply a theoretically-rigorous phonon mode-mapping approach to understand the polymorphism in the ternary metal oxide Bi2Sn2O7. Starting from the high-temperature cubic pyrochlore aristotype, we systematically explore the structural potential-energy surface and recover the two known low-temperature phases alongside three new metastable phases, together with the transition pathways connecting them. This first-principles lattice-dynamics method is completely general and provides a practical means to identify and characterise the stable polymorphs and phase transitions in materials with complex crystal structures.

Average and Local Structure of Apatite-Type Germanates and Implications for Oxide Ion Conductivity.

Materials with the apatite structure have a range of important applications in which their function is influenced by details of their local structure. Here, we describe an average and local structural study to probe the origins of high-temperature oxide ion mobility in La10(GeO4)6O3 and La8Bi2(GeO4)6O3 oxygen-excess materials, using the low-conductivity interstitial oxide-free La8Sr2(GeO4)6O2 as a benchmark. For La10 and La8Bi2, we locate the interstitial oxygen, Oint, responsible for conductivity by Rietveld refinement and relate the P63/m to P1̅ phase transitions on cooling to oxygen ordering. Local structural studies using neutron total scattering reveal that well-ordered GeO5 square pyramidal groups form in the structure at low temperature, but that Oint becomes significantly more disordered in the high-conductivity, high-temperature structures, with a transition to more trigonal-bipyramid-like average geometry. We relate the higher conductivity of Bi materials to the presence of several Oint sites of similar energy in the structure, which correlates with its less-distorted low-temperature average structure.

Shape-persistent porous organic cage supported palladium nanoparticles as heterogeneous catalytic materials.

Porous Organic Cages (POCs) are an emerging class of self-assembling, porous materials with novel properties. They offer a key advantage over other porous materials in permitting facile solution processing and re-assembly. The combination of POCs with metal nanoparticles (NPs) unlocks applications in the area of catalysis. In this context, POCs can function as both the template of ultra-small NPs and a porous, but reprocessable, heterogeneous catalyst support. Here, we demonstrate the synthesis of ultra-small Pd NPs with an imine linked POC known as 'CC3', and show that hydrogen gas can be used to form metallic NPs at ∼200 °C without the reduction of the organic cage (and the accompanying, unwanted loss of crystallinity). The resulting materials are characterized using a range of techniques (including powder diffraction, scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy) and shown to be recrystallizable following dissolution in organic solvent. Their catalytic efficacy is demonstrated using the widely studied carbon monoxide oxidation reaction. This demonstration paves the way for using ultra-small NPs synthesized with POCs as solution-processable, self-assembling porous catalytic materials.

Overcoming chemical equilibrium limitations using a thermodynamically reversible chemical reactor.

All real processes, be they chemical, mechanical or electrical, are thermodynamically irreversible and therefore suffer from thermodynamic losses. Here, we report the design and operation of a chemical reactor capable of approaching thermodynamically reversible operation. The reactor was employed for hydrogen production via the water-gas shift reaction, an important route to 'green' hydrogen. The reactor avoids mixing reactant gases by transferring oxygen from the (oxidizing) water stream to the (reducing) carbon monoxide stream via a solid-state oxygen reservoir consisting of a perovskite phase (La0.6Sr0.4FeO3-δ). This reservoir is able to remain close to equilibrium with the reacting gas streams because of its variable degree of non-stoichiometry and thus develops a 'chemical memory' that we employ to approach reversibility. We demonstrate this memory using operando, spatially resolved, real-time, high-resolution X-ray powder diffraction on a working reactor. The design leads to a reactor unconstrained by overall chemical equilibrium limitations, which can produce essentially pure hydrogen and carbon dioxide as separate product streams.

Brownmillerite-Type Sr2ScGaO5 Oxide Ion Conductor: Local Structure, Phase Transition, and Dynamics.

Brownmillerite-type Sr2ScGaO5 has been investigated by a range of experimental X-ray and neutron scattering techniques (diffraction, total scattering, and spectroscopy) and density functional theory calculations in order to characterize its structure and dynamics. The material undergoes a second-order phase transition on heating during which a rearrangement of the (GaO4/2)∞ tetrahedral chains occurs, such that they change from being essentially fully ordered in a polar structure at room temperature to being orientationally disordered above 400 °C. Pair distribution function analysis carried out using neutron total scattering data suggests that GaO4 tetrahedra remain as fairly rigid units above and below this transition, whereas coordination polyhedra in the (ScO6/2)∞ layers distort more. Inelastic neutron scattering and phonon calculations reveal the particular modes that are associated with this structural change, which may assist ionic conductivity in the material at higher temperatures. On the basis of the correlations between these findings and the measured conductivity, we have synthesized a derivative compound with increased conductivity and suggest a possible conduction mechanism in these brownmillerite-type solid electrolytes.

Understanding the Behavior of the Above-Room-Temperature Molecular Ferroelectric 5,6-Dichloro-2-methylbenzimidazole Using Symmetry Adapted Distortion Mode Analysis.

The exploitable properties of many functional materials are intimately linked with symmetry-changing phase transitions. These include properties such as ferroelectricity, second harmonic generation, conductivity, magnetism and many others. We describe a new symmetry-inspired method for systematic and exhaustive evaluation of the symmetry changes possible in molecular systems using molecular distortion modes, and how different models can be automatically tested against diffraction data. The method produces a quantitative structural landscape from which the most appropriate structural description of a child phase can be chosen. It can be applied to any molecular or molecular-fragment containing material where a (semi) rigid molecule description is appropriate. We exemplify the method on 5,6-dichloro-2-methylbenzimidazole (DC-MBI), an important molecular ferroelectric. We show that DC-MBI undergoes an unusual symmetry-lowering transition on warming from orthorhombic Pca21 ( T ≲ 400 K) to monoclinic Pc. Contrary to expectations, the high temperature phase of DC-MBI remains polar.

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7.

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to ß and α structures on cooling. The results include the first reliable structural model for ß-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, ß = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O' anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations.

Infinitely Adaptive Transition-Metal Ordering in Ln2O2MSe2-Type Oxychalcogenides.

A number of Ln2O2MSe2 (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln2O2](2+) sheets and antifluorite-like [MSe2](2-) sheets, have recently been reported in the literatures. The available MSe4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe2](2-) layers contain MSe4/2 tetrahedra that are exclusively edge-sharing (stripe-like), exclusively corner-sharing (checkerboard-like), or mixtures of both. This paper reports 60 new compositions in this family. We reveal that the transition-metal arrangement can be systematically controlled by either Ln or M doping, leading to an "infinitely adaptive" structural family. We show how this is achieved in La2O2Fe1-xZnxSe2, La2O2Zn1-xMnxSe2, La2O2Mn1-xCdxSe2, Ce2O2Fe1-xZnxSe2, Ce2O2Zn1-xMnxSe2, Ce2O2Mn1-xCdxSe2, La2-yCeyO2FeSe2, La2-yCeyO2ZnSe2, La2-yCeyO2MnSe2, and La2-yCeyO2CdSe2 solid solutions.

Synthesis, characterisation and properties of rare earth oxyselenides A4O4Se3 (A = Eu, Gd, Tb, Dy, Ho, Er, Yb and Y).

Rare earth oxyselenides A4O4Se3 (A = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y) were synthesised using solid state reactions and three new structure types (ß, γ, and δ) were observed. A4O4Se3 materials adopt either the α (A = Nd, Sm), ß (A = Eu), γ (A = Gd, Tb) or δ (A = Dy, Ho, Er, Yb, Y) structure depending on the rare earth radius. Each structure type contains alternating [A2O2](2+) and Se(2-)/Se2(2-) layers. Different ordered and disordered arrangements of Se(2-) and [Se-Se](2-) give the Se layer flexibility and lead to the four different structure types observed. The volume coefficients of expansion for A4O4Se3 ranged from +1.746(9) × 10(-5) to +2.237(3) × 10(-5) K(-1) from 12 to 300 K; no structural phase transitions were observed in this temperature range. Diffuse reflection spectra show A4O4Se3 are semiconductors with band gap Eg 1.02-1.46 eV. Gd4O4Se3, Dy4O4Se3, and Tb4O4Se3 samples show antiferromagnetic ordering with Néel temperature, TN, of 7-9 K. DFT calculations confirm the two different valence states of Se(2-) and Se2(2-) in Eu4O4Se3.

Synthesis, structural characterization, and physical properties of the new transition metal oxyselenide Ce2O2ZnSe2.

The quaternary transition metal oxyselenide Ce2O2ZnSe2 has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within [ZnSe2](2-) layers. The color of the compound changes as a function of cell volume, which can vary by ∼0.4% under different synthetic conditions. At the highest, intermediate, and lowest cell volumes, the color is yellow-ochre, brown, and black, respectively. The decreased volume is attributed to oxidation of Ce from 3+ to 4+, the extent of which can be controlled by synthetic conditions. Ce2O2ZnSe2 is a semiconductor at all cell volumes with experimental optical band gaps of 2.2, 1.4, and 1.3 eV for high, intermediate, and low cell volume samples, respectively. SQUID measurements show Ce2O2ZnSe2 to be paramagnetic from 2 to 300 K with a negative Weiss temperature of θ = -10 K, suggesting weak antiferromagnetic interactions.

Systematic and controllable negative, zero, and positive thermal expansion in cubic Zr(1-x)Sn(x)Mo2O8.

We describe the synthesis and characterization of a family of materials, Zr1-xSnxMo2O8 (0 < x < 1), whose isotropic thermal expansion coefficient can be systematically varied from negative to zero to positive values. These materials allow tunable expansion in a single phase as opposed to using a composite system. Linear thermal expansion coefficients, αl, ranging from -7.9(2) × 10(-6) to +5.9(2) × 10(-6) K(-1) (12-500 K) can be achieved across the series; contraction and expansion limits are of the same order of magnitude as the expansion of typical ceramics. We also report the various structures and thermal expansion of "cubic" SnMo2O8, and we use time- and temperature-dependent diffraction studies to describe a series of phase transitions between different ordered and disordered states of this material.

Structural characterization and physical properties of the new transition metal oxyselenide La2O2ZnSe2.

The quaternary transition metal oxyselenide La(2)O(2)ZnSe(2) has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within the [ZnSe(2)](2-) layers. This cation-ordered structure can be derived and described using the symmetry-adapted distortion mode approach. La(2)O(2)ZnSe(2) is an direct gap semiconductor with an experimental optical band gap of 3.4(2) eV, consistent with electronic structure calculations.

The superstructure determination of displacive distortions via symmetry-mode analysis.

For any crystal structure that can be viewed as a low-symmetry distortion of some higher-symmetry parent structure, one can represent the details of the distorted structure in terms of symmetry-adapted distortion modes of the parent structure rather than the traditional list of atomic xyz coordinates. Because most symmetry modes tend to be inactive, and only a relatively small number of mode amplitudes are dominant in producing the observed distortion, symmetry-mode analysis can greatly simplify the determination of a displacively distorted structure from powder diffraction data. This is an important capability when peak splittings are small, superlattice intensities are weak or systematic absences fail to distinguish between candidate symmetries. Here, the symmetry-mode basis is treated as a binary (on/off) parameter set that spans the space of all possible P1 symmetry distortions within the experimentally determined supercell. Using the average R(wp) over repeated local minimizations from random starting points as a cost function for a given mode set, global search strategies are employed to identify the active modes of the distortion. This procedure automatically yields the amplitudes of the active modes and the associated atomic coordinates. The active modes are then used to detect the space-group symmetry of the distorted phase (i.e. the type and location of each of the parent symmetry elements that remain within the distorted supercell). Once a handful of active modes are identified, traditional refinement methods readily yield their amplitudes and the resulting atomic coordinates. A final symmetry-mode refinement is then performed in the correct space-group symmetry to improve the sensitivity to any secondary modes present.

Giant deuteron migration during the isosymmetric phase transition in deuterated 3,5-pyridinedicarboxylic acid.

Deuterated 3,5-pyridinedicarboxylic acid exhibits reversible temperature-induced deuteron migration of a magnitude unprecedented in this class of compounds. We used a combination of variable-temperature powder and single-crystal neutron diffraction and density functional theory (DFT)-based computational methods to elucidate the origin of this remarkable behaviour. Single-crystal neutron diffraction shows that between 15 and 300 K, the deuteron moves by 0.32(1) Å and the structure changes from a low-temperature N-D···O form to a high-temperature N···D-O form. Variable-temperature powder neutron-diffraction data, which was fitted by using parametric Rietveld refinement, show that this deuteron migration is due to an isosymmetric, first-order phase transition that occurs by growth of the daughter phase in the parent-phase matrix. Similar phase transitions are observed in two selectively deuterated forms of the material. DFT calculations demonstrate the role of phonons and show that vibrational free-energy stabilisation, which plays a key role in the observed structural phase transitions, is more pronounced in the fully deuterated material and proportional to the mass of the molecule, that is, the level of deuteration. This is consistent with our experimental work, for which distinct crystallographic phase transitions were clearly observed for the three deuterated systems, but not for the fully protonated material.

A new iron oxyselenide Ce2O2FeSe2: synthesis and characterisation.

In this communication we report the synthesis, structural and preliminary physical characterisation of a new layered oxyselenide Ce(2)O(2)FeSe(2). This material, containing a 1D portion of the structure of the layered FeSe-related superconductors, is a semiconductor with a band gap of around 0.64 eV and orders antiferromagnetically at low temperatures.