Locating hydrogen positions in the autunite mineral metatorbernite [Cu(UO2)2(PO4)2·8H2O]: a combined approach using neutron powder diffraction and computational modelling

Metatorbernite [Cu(UO2)2(PO4)2·8H2O] is a promising remediation material for environmental uranium contamination. Previous X-ray diffraction studies have been unable to definitively locate hydrogen positions within metatorbernite, which are key to determining the hydrogen-bond network that helps to stabilize the structure. Here, hydrogen positions have been determined using a combination of neutron powder diffraction and the computational modelling technique ab initio random structure searching (AIRSS). Atomic coordinates determined through Rietveld analysis of neutron powder diffraction data are in excellent agreement with the minimum energy configuration predicted by AIRSS; thus, simulations confirm that our proposed model likely represents the global minimum configuration. Two groups of water molecules exist within the metatorbernite structure: free water and copper-coordinating water. Free water molecules are held within the structure by hydrogen bonding only, whilst the coordinating water molecules bond to copper in the equatorial positions to produce a 4 + 2 Jahn–Teller octahedra. The successful agreement between neutron powder diffraction data and AIRSS suggests that this combined approach has excellent potential for the study of other (trans)uranium materials in which hydrogen bonding plays a key role in phase stability.

Synthesis and Structure of Oxygen Deficient Lead-Technetium Pyrochlore, the First Example of a Valence V Technetium Oxide

The structure of lead-technetium pyrochlore has been refined in space group Fd3¯m with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O7-d. Displacive disorder of the Pb cations is evident from the refinements, as has been observed in Bi2Tc2O7-d. X-ray absorption spectroscopic measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the presence of disorder leading us to conclude that this pyrochlore is the first example of a valence V technetium oxide.

Unveiling the crystal structure of ‘antimonic acid’: short- and long-range perspectives

The elusive crystal structure of the socalled “antimonic acid” has been investigated by means of robust and state-of-the-art techniques. The synergic results of solidstate magicangle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochloretype structure of stoichiometric formula (H3O)1.20(7)H0.77(9)Sb2O6. Some protons belong to heavily delocalized H3O+ subunits, while some H+ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O − H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bondvalence energy landscape analysis. Xray absorption spectroscopy results corroborate the structural data around Sb5+ ions at shortrange order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

The crystal structure of ZrCr2D≈4 at 50 K ≤ T ≤ 200 K

Many Laves phases take up considerable amounts of hydrogen to form metallic Laves phase hydrides. They frequently undergo phase transitions driven by ordering phenomena for the hydrogen atom distribution. The cubic Laves phase ZrCr2 takes up hydrogen to form a hydride with almost four hydrogen atoms per formula unit, which undergoes a phase transition to a monoclinic modification at a critical temperature Tc = 250.2 K. Its crystal structure was refined based on neutron powder diffraction data on the deuteride (ZrCr2D3.8 type [T = 1.6 K, C2/c]) at four temperatures in the range 50 K ≤ T ≤ 200 K. The monoclinic low-temperature modification features a strongly distorted square anti-prism ZrD8 and three CrD4 polyhedra with almost fully occupied deuterium sites in saddle-like, distorted tetrahedral and planar configurations. Zr–D distances are in the range 201.4(7) pm ≤ d(Zr–D) ≤ 208.5(8) pm and Cr–D distances in the range 172.9(7) pm ≤ d(Cr–D) ≤ 182.4(8) pm.

Unravelling the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ mixed ionic electronic conductors

Mixed ionic electronic conducting ceramics Nd6−y WO12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or H2S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6−y WO12−δ, the ionic/electronic transport mechanism in Nd6−y WO12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12−δ and molybdenum-substituted Nd5.7W0.75Mo0.25O12−δ prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm 3 m and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6−y WO12−δ. X-ray absorption spectroscopy as a function of partial pressure pO2 in the near-edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd6−y WO12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.

Ice XIX: The second hydrogen-ordered polymorph related to ice VI

We here report ex situ calorimetry and high-resolution neutron powder diffraction data of a novel ice polymorph produced at 1.8 GPa and recovered to ambient pressure at 80 K. Ice XIX, previously called ice β-XV by us, is shown to be partially hydrogen-ordered and crystallising in a √2 x √2 × 1 supercell with respect to the parent ice VI phase. Our powder data match two nearly degenerate structural models in space-groups \(P\stackrel{-}{4}\) and Pcc2, in which the water molecules are partially antiferroelectrically ordered. Key to the synthesis of deuterated ice XIX is the use of DCl as dopant and the use of a D2O/H2O mixture, where the small H2O fraction nucleates ice XIX. This provides the basis to study the first order-order transition known in ice physics, from ice XIX to its sibling ice XV at ambient pressure. It proceeds via a transition state, ice VI‡, which contains a disordered H-sublattice, whereas both ice XIX and ice XV are fully crystalline.

Revisiting Goodenough-Kanamori rules in a new series of double perovskites LaSr1−xCaxNiReO6

The magnetic ground states in highly ordered double perovskites LaSr1−xCaxNiReO6 (x = 0.0, 0.5, 1.0) are studied in view of the Goodenough-Kanamori rules of superexchange interactions in this paper. In LaSrNiReO6, Ni and Re sublattices are found to exhibit curious magnetic states separately, but no long range magnetic ordering is achieved. The magnetic transition at ~255 K is identified with the independent Re sublattice magnetic ordering. Interestingly, the sublattice interactions are tuned by modifying the Ni-O-Re bond angles through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction data reveals long range ferrimagnetic ordering of the Ni and Re magnetic sublattices along the crystallographic b-axis. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration.

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

The crystal structure of cubic C-type samarium sesquioxide, Sm2O3

Cubic C-type samarium sesquioxide, Sm2O3, was prepared by thermal decomposition of samarium carbonate. Its crystal structure was refined based on neutron powder diffraction data [space group Ia3̅, a = 1092.77(6) pm] in order to accurately determine the oxygen atom positions. Isotopically enriched 154Sm2O3 was used for the neutron diffraction work because of the enormous absorption cross section of the natural isotopic mixtures for thermal neutrons. Crystal structure refinements by the Rietveld technique confirm the cubic bixbyite type and yield structure data of high accuracy. Interatomic distances between Sm1 (Wyckoff site 8a) and oxygen are 6 × 235.0(2) pm, and between Sm2 (Wyckoff site 24d) and oxygen 2 × 231.6(2) pm, 2 × 233.7(2) pm, 2 × 240.2(2) pm with a weighted average of d̅(Sm–O) = 235.1(2) pm.

Structure and magnetic properties of W-type hexaferrites

W-type hexaferrites (WHFs) (SrMe 2Fe16O27, Me = Mg, Co, Ni and Zn) are hard magnetic materials with high potential for permanent magnet applications owing to their large crystalline anisotropy and high cation tunability. However, little is known with regards to their complex structural and magnetic characteristics. Here, the substitution of metals (Me = Mg, Co, Ni and Zn) in WHFs is described and their crystal and magnetic structures investigated. From joined refinements of X-ray and neutron powder diffraction data, the atomic positions of the Me atoms were extracted along with the magnetic dipolar moment of the individual sites. The four types of WHFs exhibit ferrimagnetic ordering. For Mg, Ni and Zn the magnetic moments are found to be ordered colinearly and with the magnetic easy axis along the crystallographic c axis. In SrCo2Fe16O27, however, the spontaneous magnetization changes from uniaxial to planar, with the moments aligning in the crystallographic ab plane. Macromagnetic properties were measured using a vibration sample magnetometer. The measured saturation magnetization (M s) between the different samples follows the same trend as the calculated M s extracted from the refined magnetic moments of the neutron powder diffraction data. Given the correlation between the calculated M s and the refined substitution degree of the different Me in specific crystallographic sites, the agreement between the measured and calculated M s values consolidates the robustness of the structural and magnetic Rietveld model.