M. Grazia Francesconi

An investigation of the high temperature reaction between the apatite oxide ion conductor La9.33Si6O26 and NH3

There is growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs). However, the possible reaction between the electrolyte and ammonia, and its potential effect on performance, has received little attention. In this paper, we report an investigation of the high temperature (950 °C) reaction of the apatite-type oxide ion conductor, La9.33Si6O26, and ammonia. The results show that such treatment leads to nitridation of the sample, with evidence for Si loss leading to an increased La:Si ratio in the final product. From neutron diffraction studies, the composition of the final product was determined to be La9.7(1)Si6O22.6(2)N2.7(2), with structural and 29Si NMR data suggesting the presence of N both within the apatite anion channels, and bonded to Si. An interesting feature of the structural studies are the relatively low atomic displacement parameters compared to the comparable apatite oxide systems, La9.33 + xSi6O26 + 3x/2, which can be related to the lack of interstitial anions in the oxynitride. Further studies on samples heated in ammonia at lower temperatures (600, 800 °C) suggest lower N incorporation, particularly for the 600 °C treatment. Considering the correlation of ionic conductivity, and interstitial oxide ion content in apatite systems, the data suggests the potential use of apatite-type electrolytes in SOFCs utilising NH3 as the fuel should be limited to temperatures <800 °C.

Fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y (Ln=La, Nd; A=Ca, Sr)

The low-temperature (200–350 °C) fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y(Ln=La, Nd; A=Ca, Sr) using F2 gas, CuF2 , and NH4F is reported. The incorporation of large levels of fluoride ions is observed for each of these fluorinating agents. The general characteristics of each method are discussed, and it is shown that for this system, fluorination mainly occurs by insertion of fluorine for reaction with F2 , substitution of fluorine for oxygen for NH4F, and a mixture of the two processes for CuF2 . For A=Sr, it is assumed that fluorine inserts mainly between the two CuO2 layers, since large expansions of the unit cell along the c direction are observed. No evidence for bulk superconductivity has so far been observed after fluorination.

Synthesis and characterisation of the quaternary nitride-fluoride Ce2MnN3F2−δ

Ce(2)MnN(3)F(2-delta) has been synthesised via low-temperature fluorination of the ternary nitride Ce(2)MnN(3). To the best of our knowledge, Ce(2)MnN(3)F(2-delta) is the first quaternary nitride-fluoride to be reported. The reaction of Ce(2)MnN(3) with 10% F(2)/N(2) at 95-115 degrees C yields the fluorinated phase Ce(2)MnN(3)F(2-delta), with tetragonal symmetry (P4/nmm a = 3.8554(4) A and c = 13.088(4) A based on neutron powder diffraction), accompanied by starting material. Rietveld refinement supports a staged fluorine insertion reaction (into alternate rocksalt layers) to give a product with stoichiometry Ce(2)MnN(3)F(2-delta). A comparison with the formation of Sr(2)TiO(3)F(2) and the isostructural LaSrMnO(4)F indicates that two F(-) anions are inserted but no F(-)/N(3-) substitution takes place. Magnetic susceptibility measurements show a transition from Pauli paramagnetic behaviour, in Ce(2)MnN(3), to paramagnetic behaviour upon fluorination. The effective magnetic moment in Ce(2)MnN(3)F(2-delta), mu(eff) = 5.38 mu(B), is consistent with an intermediate value between that of Mn(3+) (4.9 mu(B)) and Mn(2+) (5.9 mu(B)) supporting the proposed stoichiometry, Ce(2)MnN(3)F(2-delta).

Magnetic coupling and long-range order in the spin–chain sulfide Ba2CoS3

In this paper, we report on the magnetic properties of Ba2CoS3, a spin–chain compound recently found to be the first Co2+-containing one-dimensional sulfide to show metallic-like conductivity and negative magnetoresistance. We carried out an in-depth experimental investigation of the local structure of the cobalt atoms, and ab initio calculations of the resulting electronic configuration of Co2+. From theoretical considerations, the intra-chain coupling was predicted to be antiferromagnetic. Experimentally, several estimates of this magnetic coupling were derived by analysing the temperature dependence of the magnetic susceptibility. Magnetic and heat capacity measurements also provided evidence of a three-dimensional antiferromagnetic ordering, a feature indicative of a noticeable inter-chain coupling in this quasi-1D system.

The Preparation of Magnetic Iron Oxide Nanoparticles in Microreactors

The preparation of magnetic iron oxide nanoparticles within microreactors is reported. The proportion of γ-Fe2O3 and Fe3O4 in the sample was determined, an important parameter for reproducibility in applications.

Stereostructural behaviour of N-N atropisomers: Two conglomerate crystallisations and a crystallisation-induced deracemisation

The solid state behaviour of a number of compounds which show hindered rotation around an N–N bond, in some cases leading to axial chirality is described. A diacyl hydrazine, bisanthranoly hydrazine, 1 crystallises in the chiral space group P212121, presenting an example of conglomerate crystallisation. A tetra-acyl hydrazine derived from lactic acid, 2, shows kinetic resolution by crystallisation, as of the two isomers observed in the solution NMR, only one crystallises, again in the space group P212121. Two cyclic acyl hydrazines in the form of biquinazolinones are studied: 2,2′diphenyl-3,3-biquinazolinone, 3 crystallises in the achiral space group Pbca, while 3,3′-dimethyl-2,2′-biquinazoline-4′-thio-4-one, 4 crystallises in the chiral space group P21 giving another example of a conglomerate crystallisation. The single crystal structures of each of the species have been compared to powder XRD data to confirm that the single crystal structures are representative of the bulk material.

Defects in the new oxide-fluoride Ba2PdO2F2: the search for fluoride needles in an oxide haystack

The defect chemistry and fluorine insertion properties of the new Ba2−xSrxPdO2F2 system have been investigated using atomistic simulation techniques. The interatomic potential model produces good agreement between simulated and observed structures for the “parent” oxides Ba2PdO3 and Sr2PdO3, and the oxide-fluorides Ba2PdO2F2 and BaSrPdO2F2. The perfect lattice simulations confirm the most favourable structure type is the T′ (Nd2CuO4) structure for the oxide-fluoride phases comprised of square planar Pd, which accords with recent EXAFS studies. The fluorination reaction in the precursor oxide Ba2PdO3, involving substitution of two fluorine ions for one oxygen ion, is highly favourable. The formation of fluorine interstitials and holes in Ba2PdO2F2 is an unfavourable process in accord with the observed resistance to excess fluorine, and in contrast to the related cuprate superconductor Sr2CuO2F2+δ.

Using High Pressure to Prepare Polymorphs of the Ba2Co1–xZnxS3 (0 ≤ x ≤ 1.0) Compounds

In this work, high pressure was used as a tool to induce structural transition and prepare metastable polymorphs of ternary sulfides. Structural transformations under high pressure of compounds belonging to the Ba(2)Co(1-x)Zn(x)S(3) (0 ≤ x ≤ 1.0) series were studied using X-ray diffraction and electron microscopy. All members of the Ba(2)Co(1-x)Zn(x)S(3) series show the Ba(2)CoS(3)-type one-dimensional structure, but, after heating under pressure, the Ba(2)CoS(3) compound (x = 0) separates into BaS and the two-dimensional BaCoS(2-δ) (δ ≈ 0), while Ba(2)Co(1-x)Zn(x)S(3) compounds with x ≥ 0.25 maintain their one-dimensional features but rearrange into polymorphs showing the Ba(2)MnS(3)-type structure. All structural transformations can be linked to shortening in interchain metal-metal distances caused by the high pressure, and the role of the zinc in preventing loss of one-dimensionality is discussed.

Zeolite-like nitride–chlorides with a predicted topology

We describe the synthesis and structures of the first tantalum-containing nitride-chlorides, Ba(3)Ta(3)N(6)Cl and Ba(15)Ta(15)N(33)Cl(4), and the structurally related Ba(3)Si(3)N(5)OCl, and their relationship with a theoretical silaceous framework.

Negative magnetoresistance in Ba2CoS3

A small negative magnetoresistance and metallic-like behaviour has been detected for the first time in a one-dimensional sulfide containing Co2+.