Structural studies of magnesium nitride fluorides by powder neutron diffraction

Samples of ternary nitride fluorides, Mg₃NF₃ and Mg₂NF have been prepared by solid state reaction of Mg₃N₂ and MgF₂ at 1323–1423 K and investigated by powder X-ray and powder neutron diffraction techniques. Mg₃NF₃ is cubic (space group: Pm3m) and has a structure related to rock-salt MgO, but with one cation site vacant. Mg₂NF is tetragonal (space group: I4₁/amd) and has an anti-LiFeO₂ related structure. Both compounds are essentially ionic and form structures in which nitride and fluoride anions are crystallographically ordered. The nitride fluorides show temperature independent paramagnetic behaviour between 5 and 300 K.     

In situ neutron powder diffraction studies of lithium-ion batteries

Neutron powder diffraction (NPD) offers many advantages in the analysis of battery materials. Understanding the relationship between the structural transformations of electrode materials and their electrochemical performance within lithium-ion batteries is crucial for further development of these technologies and is the overall goal of in situ NPD experiments. In this work, we present NPD data of electrode materials within batteries that are collected in situ during electrochemical cycling, including the commercially available materials LiCoO₂, LiMn₂O₄, LiFePO₄ and graphite and the YFe(CN)₆ and FeFe(CN)₆ materials that are not commercially available. Using these data, we illustrate the experimental approach and requirements for the collection of in situ NPD data of sufficient quality for detailed structural analyses of the electrode components of interest within batteries.     

Structural refinement of delithiated LiVO2 by neutron diffraction

Lithium has been extracted from the layered compound LiVO₂ by chemical oxidation with bromine. Previous X-ray data have shown that in Li_1−xVO₂ lithium extraction beyond x ≈ 0.33 is accompanied by migration of one-third of the vanadium ions into the lithium-deficient layer to stabilize the structure; little information about the location of the lithium ions could be gathered from this data. The neutron diffraction data presented in this paper show that at a composition Li_0.22VO₂, determined by atomic absorption spectroscopy, the residual lithium ions are distributed over the octahedral sites of the original lithium layer; the possibility that a small fraction of the lithium ions partially occupy the tetrahedral sites in this layer cannot be discounted. No significant occupation by lithium of the tetrahedral or octahedral vacancies in the vanadium-rich layer could be detected.     

Structural investigation of Ca2MnO4 by neutron powder diffraction and electron microscopy

The first member of the Ruddlesden-Popper family, Ca₂MnO₄, has been revisited. Coexistence of two structures has been shown from electron microscopy at room temperature and neutron diffraction data have evidenced two antiferromagnetic structures at low temperature. Two forms, with an orthorhombic Aba2 ( and c≈12 Å) and a tetragonal I4₁/cad ( and c≈24 Å) symmetries, were found to coexist coherently within the same matrix.     

Structural studies of water and other hydrogen—bonded liquids by neutron diffraction

The structural characteristics of hydrogen-bonded liquids may be studied by neutron diffraction. A brief review of the experimental techniques is given with particular emphasis on H/D isotope substitution and temperature difference techniques. These methods are illustrated with reference to recent work on water in its normal and supercooled liquid phases. The temperature variation is linked to the behaviour of the hydrogen bonds and the evolution towards the continuous random network of amorphous ice. A brief report is made for some other hydrogen-bonded liquids, contrasting the strong correlations in hydrogen fluoride which are not well understood with the study of methanol which gives good agreement with computer simulation results. Formic acid has also been investigated and novel techniques used in the analysis of the H/D datasets. The review ends with an overview of current issues and a consideration of future developments.     

Structure of oxygen clathrate hydrate by neutron powder diffraction

Oxygen hydrate was prepared by reacting oxygen with deuteriated ice at high pressure. Its structure was examined with powder neutron diffraction. It was found to crystallize in the cubic space group witha=17.070(1) Å. The formation of the type II structure rather than type I can be attributed to the larger Langmuir constant of sorption for the oxygen molecules in the 12-hedral cages. The encaged oxygen molecules are orientationally disordered. A comparison of the oxygen thermal parameters in both cages suggests the existence of local potential minima in the 16-hedral cages.Published as NRCC 25940     

Elastic properties of materials by pulsed neutron powder diffraction

We describe the use of pulsed neutron powder diffraction to measure the mean-square thermal displacements of atoms in polycrystalline materials at various temperatures. Data for aluminum, Pu_0.95Al_0.05, Ce_0.90Th_0.10, titanium, hafnium and CeRh₃B₂ are presented. A simple Debye model accounts for the observations, and a Debye temperature Θ_DW can be extracted from the data. Θ_DW is atom-specific and anisotropic, and is related to the elastic constants of the material. This technique provides a means of obtaining information on the elastic behavior of materials for which the traditional methods are not applicable.     

Structural stabilities and electrochemistry of Na2FeSiO4 polymorphs: first-principles calculations

Journal of Solid State Electrochemistry - Searching for new cathode materials is very important in the field of Na-ion batteries. Transition metal sodium orthosilicates have attracted much...     

Structural transformation and oxidation of Sr2MnO3.5+x determined by in-situ neutron powder diffraction

The oxidation of the n = 1 Ruddlesden-Popper phase, Sr₂MnO_3.5+x, where 0 ≤ x ≤ 0.5 has been investigated using a combination of in-situ diffraction techniques. In agreement with previous reports the room temperature structure of Sr₂MnO_3.5+x was determined to be monoclinic crystallising in space group P2₁/c. On heating in air the material undergoes rapid oxidation at a relatively modest temperature, ∼275 °C. The oxidation process is coincident with a significant change in the structure, with the material now adopting a tetragonal I4/mmm structure. In the oxygen deficient phase where x > 0 the Mn coordination is square pyramidal, with a sixth partially occupied oxygen position giving rise to octahedral coordination. Oxidation of Sr₂MnO_3.5+x results in the filling of the partially occupied O4 positions and a resulting increase in symmetry, with the Mn coordination now adopting solely a distorted octahedral environment.