David B. Currie

Structure and oxygen stoichiometry in Sr3Fe2O7−y, 0 ≤ y ≤ 1.0

Abstract Powder neutron diffraction has been used to determine the structure of four compounds from the series Sr 3 Fe 2 O 7− y , y = 0, 0.25, 0.42, and 1.0. All these compounds crystallize with structures based on that of Sr 3 Ti 2 O 7 with oxygen systematically removed from a site linking the FeO 6 octahedra together. Complete removal of oxygen from this site produces an unusual, square-pyramidal coordination around each iron(III) ion and a stoichiometry of Sr 3 Fe 2 O 6 ; this compound is isostructural with La 2 SrCu 2 O 6 . A bond valence parameter, R 0 , for Fe 4+ has been derived as 1.788(2) A.

Structure and magnetic properties of Sr2FeO4 and Sr3Fe2O7 studied by powder neutron diffraction and Mössbauer spectroscopy

Sr2FeO4 and Sr3Fe2O7 have been studied using low-temperature powder neutron diffraction, magnetometry and 57Fe Mossbauer spectroscopy. Sr2FeO4 orders antiferromagnetically at 60 K whilst the Neel temperature of Sr3Fe2O7 is 110 K. At room temperature the Mossbauer spectrum of Sr3Fe2O7 is interpreted in terms of a disproportionation, Fe4+↔ Fe3++ Fe5+, though neutron diffraction measurements show a single, crystallographic iron site. Neutron diffraction data collected below the Neel temperatures from Sr2FeO4(4 K) and Sr3Fe2O7(6 and 90 K) only show broad weak magnetic reflections indicative of antiferromagnetic ordering in two dimensions. Mossbauer data collected below TN from Sr2FeO4 indicate the presence of at least four magnetically different Fe4+ sites.

The synthesis and structure of Sr2FeO4

Abstract Strontium ferrate(IV), Sr2FeO4, crystallizes with a tetragonal unit cell, I4 mmm , a = 3.8642(1), c = 12.3968(2) A, and V = 185.1 A3. The structure of this compound is of the K2NiF4 type as refined by Rietveld analysis of powder X-ray diffraction data. This material contains iron(IV) in a very slightly distorted octahedral environment (4 × 1.93) + (2 × 1.95).

Reinvestigation of the mixed-metal periodates M′MIO6(M′= alkali metal, M = Ge, Sn, Pb)

The periodate compounds M′MIO6(M′= K, Rb, NH4, Cs, sometimes Na; M = Ge, Sn, Pb) have been studied by vibrational spectroscopy, X-ray powder diffraction and EXAFS. The structures are based upon an h.c.p. array of oxide ions with one third of the octahedral holes in every second layer filled with M′, and in the other layers one third of the holes are filled with I and one third with M. The Ge compounds adopt the space group P312, but the tin and lead compounds belong to P6322, corresponding to a doubling of the axis perpendicular to the layers as a result of alternate stacking of IO6 and MO6 octahedra. The M–O, M′–O and I–O distances have been determined by a combination of X-ray powder diffraction and EXAFS and the results are discussed.

Synthesis of novel inorganic structures under unusual high pressure gas and hydrothermal conditions

The application of high pressures to synthesis in inorganic materials chemistry significantly widens the range and type of compound that can be prepared. Three different high pressure systems that allow, respectively, synthesis under flowing oxygen, nitrogen and hydrothermal conditions are described together with a new result obtained on each. Under hydrothermal conditions a framework material of the composition Ca 8 [AlGaSiO 6 ] 4 (OH) 8 has been prepared and structural analysis shows that this material contains uniquely linkages of the type Al–O–Ga. Under high pressure nitrogen, copper has been doped into the Li 3 N structure to levels higher than previously achieved and with high pressure oxygen a series of new phases of the type Ln 2 Ba 4 Cu 7–x Ni x O 14+δ (247) (Ln=lanthanide, yttrium) have been synthesised. The valuable role of high pressures in synthetic solid state chemistry is discussed.

Superconductivity and crystal structure of Ln2Ba4Cu7O14+δ (Ln=Er, Y, Ho, Dy, Nd): A high-resolution powder neutron-diffraction study

Abstract The crystal structure of Ln 2 Ba 4 Cu 7 O 14+δ (Ln=Y, lanthanide) is derived from alternate blocks of the LnBa 2 Cu 3 O 7- x (123) and LnBa 2 Cu 4 O 8 (124) structures thus incorporating structural regions of oxygen non-stoichiometry and fixed stoichiometry, respectively. Samples containing a range of different lanthanide sizes and varying oxygen contents have been prepared by a high oxygen pressure method; seven have been examined using very high resolution powder neutron diffraction (PND); all have been examined using powder X-ray diffraction (XRD). AC susceptibility measurements indicate that removal of oxygen from the structure can result in either enhancement or reduction of T c , depending on the initial oxygen content of the as-prepared material. The variations in T c , changes in the detailed structure, oxygen content, and non-stoichiometry of the single chain copper site are discussed.

Structure of the mixed-valence lithium cuprate Li3Cu2O4 by powder neutron diffraction

The structure of Li3Cu2O4 has been refined from time-of-flight powder neutron diffraction data in order to define accurately the copper–oxygen coordination geometry and lithium-ion positions. The structure consists of slightly distorted, square-planar CuO4 units sharing opposite edges to form infinite chains running parallel to the b axis. Lithium ions were found to occupy two types of site in the structure: (1) a distorted tetrahedral site found previously, and (2) two disordered sites displaced from a mean octahedral position. The disorder in the lithium ions is similar to that observed in ferroelectric materials such as LiTaO3 below their Curie temperature. Bond lengths around copper support its non-integral valence, and bond-valence calculations are performed for all cation sites.

Synthesis, spectroscopic and structural studies of alkali metal–nickel periodates MNilO6(M = Na, K, Rb, Cs or NH4)

The dark red-purple nickel(IV) periodataes MNilO6(M = Na, K, Rb, Cs or NH4) have been prepared by peroxohexaoxodisulfate oxidation of mixtures of nickel(II) salts, MNO3 and periodate ions in hot aqueous solution. Powder X-ray diffraction studies showed that the structures adopted the space group P312, with hexagonal close-packed layers of oxide ions. One-third of the octahedral holes in every second layer contained M ions, and in the other layers one-third of the octahedral holes contained I and one-third Ni. The Ni–O,I–O, Ni ⋯ I and Ni ⋯ Ni distances have been determined by a combination of nickel and iodine K-edge extended X-ray absorption fine structure (EXAFS) data. IR, Raman and UV/VIS spectroscopic data for MNiIO6 were obtained. Iodine K-edge EXAFS data have also been obtained for the related periodates M′M″IO6(M″= K or Rb; M″= Ge, Sn, Pb or Mn).

Synthesis and properties of alkali-metal lithium periodatoferrate(III) and periodatocobaltate(III) complexes

The pale-yellow LiMFeIO 6 (M=K, Rb or Cs) have been prepared by reaction of MIO 4 , Li + and [Fe(H 2 O) 6 ] 3+ in hot aqueous solution at pH 9. Olive-green LiMCoIO 6 were prepared from MIO 4 , Li + and [Co(H 2 O) 6 ] 2+ in hot alkaline solution using ozonised oxygen as oxidant. These insoluble powders have been characterised by analysis, IR and Raman, UV–VIS and EPR spectroscopy and magnetic measurements. PXRD studies revealed that they are amorphous, but structural data have been obtained via Fe or Co and I K-edge EXAFS, and are consistent with a layered periodate structure type similar to that found previously in MM′IO 6 (M′=Mn, Ni, Ge, etc). 57 Fe Mossbauer data are reported for LiKFeIO 6 .

The crystal structures of Dy2Ba4Cu7O14+δ and Nd2Ba4Cu7O14+δ: A high resolution powder neutron diffraction study

Abstract The structures of Dy 2 Ba 4 Cu 7 O 14+δ and Nd 2 Ba 4 Cu 7 O 14+δ (247), which can be considered to be derived from alternate blocks of the LnBa 2 Cu 3 O 7 (123) and LnBa 2 Cu 4 O 8 (124) structures, have been refined using high resolution powder neutron diffraction data. Accurate structural parameters, determined by this oxygen-sensitive technique, are presented for these superconducting oxides. In both materials oxygen was found to be randomly distributed over the formally single chain and inter -chain sites of the type seen in 123 compounds; no crosslinking of the 124 double chains was observed. Notable differences in oxygen contents were found between the two samples; the neodymium sample exhibited a high average copper oxidation state as a result of considerable occupancy of both of the chain and inter -chain sites. The orthorhombic distortions of these materials are markedly reduced compared to those recorded for the corresponding 123 and 124 materials. A flattening of the copper-oxygen planes sandwiching the lanthanide site with increasing lanthanide size, as previously found in Ln123 and Ln124 compounds, is observed.