Ian E. Grey

The crystal structure of β-U4O9−y

The structure of the ..beta.. form of U/sub 4/O/sub 9-y/, an anion-excess fluorite-related phase stable above 80/sup 0/C, has been determined from the analysis of single-crystal neutron diffraction data collected at 230 and 500/sup 0/C. The excess anions are accommodated in 13-member clusters which are centered on the 12-fold positions of the I 43d space group. The clusters are formed by the corner-sharing of octahedral groupings of UO/sub 8/ square antiprisms, which enclose a cuboctahedron of anions with an additional oxygen at the center. The unit cell composition is U/sub 256/ O/sub 572/, i.e., U/sub 4/O/sub 9-y/ with y = 0.062. 18 references, 3 figures, 3 tables.The structure of the β form of U4O9−y, an anion-excess fluorite-related phase stable above 80°C, has been determined from the analysis of single-crystal neutron diffraction data collected at 230 and 500°C. The excess anions are accommodated in 13-member clusters which are centered on the 12-fold positions of the I43d space group. The clusters are formed by the corner-sharing of octahedral groupings of UO8 square antiprisms, which enclose a cuboctahedron of anions with an additional oxygen at the center. The unit cell composition is U256O572, i.e., U4O9−y with y = 0.062.

The stability and structure of Csx[Ti2−x4□x4]O4, 0.61 < x < 0.65

The stability of the phase Csx[Ti2−x4□x4]O4, □ = vacancy, in the Cs2OTiO2 system has been studied in the temperature range 800−1120°C. The phase has a narrow range of homogeneity with 0.61 < x < 0.65. Structure refinements were carried out for X-ray intensity data from two crystals obtained from melts with TiO2:CsO0.5 molar ratios of 2.0 and 3.2. The crystals have orthorhombic symmetry, Immm, a0 = 3.83, b0 = 17.00, c0 = 2.96 A. The refinements are consistent with the structural model of Hervieu and Raveau (Rev. Chim. Miner. 18, 642 (1981)) in which (010) corrugated layers of titanium-oxygen octahedra, containing disordered titanium vacancies, are separated by planes of cesium ions. Long-exposure precession photographs showed diffuse superlattice diffraction effects in the form of continuous streaks parallel to b∗0. These have been analyzed in terms of models for two-dimensional ordering of the interlayer cesium ions. Annealing of melted samples below 700°C causes a transformation of the I-centered structure to a C-centered structure of the Kx(MyTi2−y)O4 type in which 3-D ordering of cesium occurs.The stability of the phase Cs/sub x/(Ti/sub 2-x/4/square/sub x/4/)O/sub 4/, square = vacancy, in the Cs/sub 2/O-TiO/sub 2/ system has been studied in the temperature range 800-1120/sup 0/C. The phase has a narrow range of homogeneity with 0.61 < x less than or equal to 0.65. Structure refinements were carried out for X-ray intensity data from two crystals obtained from melts with TiO/sub 2/: CsO/sub 0.5/ molar ratios of 2.0 and 3.2. The crystals have orthorhombic symmetry, Immm, a/sub 0/ = 3.83, b/sub 0/ = 17.00, c/sub 0/ = 2.96 A. The refinements are consistent with the structural model of Hervieu and Raveau in which (010) corrugated layers of titanium-oxygen octahedra, containing disordered titanium vacancies, are separated by planes of cesium ions. Long-exposure precession photographs showed diffuse superlattice diffraction effects in the form of continuous streaks parallel to b/sub 0/*. These have been analyzed in terms of models for two-dimensional ordering of the interlayer cesium ions. Annealing of melted samples below 700/sup 0/C causes a transformation of the I-centered structure to a C-centered structure of the K/sub x/(M/sub y/Ti/sub 2-y/)O/sub 4/ type in which 3-D ordering of cesium occurs.

TiO2-II. Ambient pressure preparation and structure refinement

Abstract The ∝-PbO2 form of TiO2 forms as a finely crystalline intermediate phase when Ti3O5 is dissolved in sulphuric acid at elevated temperatures. Its structure has been refined using the Rietveld method applied to X-ray powder data. A structural model is proposed for the oxidation and solid-state transformation of Ti3O5 to TiO2-II, controlled by the close match between (110)Ti3O5 and (101)TiO2 atomic layers.

New cesium titanate layer structures

A phase study of the Cs2OTiO2 system in the composition range 75–100 mole% TiO2 and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs2Ti5O11 and Cs2Ti6O13. The former compound undergoes a reversible hydration reaction below 200°C to form Cs2Ti5O11 · (1 + x)H2O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H2O) packing between the layers. In Cs2Ti6O13, the layers are continuous in two dimensions, whereas in Cs2Ti5O11 and Cs2Ti5O11 · (1 + x)H2O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.

The oxidation state of iron in some BaFeS phases: A Mössbauer and electrical resistivity investigation of Ba2FeS3, Ba7Fe6S14, Ba6Fe8S15, BaFe2S3, and Ba9Fe16S32

Abstract Mossbauer spectroscopy, electrical resistivity, and magnetic susceptibility results are used in conjunction with crystal structure information to characterize the oxidation state of iron in five phases formed in the BaFeS system. The compounds have as a common feature FeS4 tetrahedra which articulate by edge and corner sharing into infinite chains or columns. In Ba2FeS3 and Ba7Fe6S14 iron is divalent in the first compound and in the latter the ratio of Fe (II) Fe (III) is 2:1 as expected by stoichiometry. The electrons are localized and Fe(II) and Fe(III) are in definite locations in the trinuclear [ Fe 3 S 6 S 2 2 ] unit. Delocalization of electrons occurs in Ba6Fe8S15, BaFe2S3, and Ba9Fe16S32 and these compounds have low electrical resistivities and display only one quadrupole doublet in the room temperature Mossbauer spectrum. The isomer shift values of 0.2 mm/sec and 0.6 mm/sec are diagnostic of high spin Fe(III) and Fe(II), respectively, when they are in tetrahedral coordination with sulfur; intermediate values are found when electron delocalization occurs.

An X-ray and Mössbauer study of the FeTi2O5Ti3O5 system

Abstract Mossbauer spectra and X-ray diffraction data have been recorded for compounds in the solid solution series FexTi3−xO5, 0 ⩽ x ⩽ 1. Compounds with x > 0.35 have the orthorhombic pseudobrookite structure, and those with x

Infinitely adaptive structures, Ba1+xFe2S4, in the BaFeS system

Abstract X-ray and electron diffraction results are reported for compounds with compositions given by Ba 1+ x Fe 2 S 4 , 0.062 ⩽ x ⩽ 0.143. The composition range is spanned by an infinite number of ordered phases, Ba p Fe 2 q S 4 q , p and q integers, with superstructures based on the NH 4 CuMoS 4 structure type. The one-dimensional ordering of barium atoms in these structures is a function of temperature of preparation, and the system is thus a suitable model for the testing of current theories relating high temperature structures to periodic atomic thermal vibrations.

Shear structure compounds (Cr,Fe)2Tin−2O2n−1 derived from the α-PbO2 structural type

Abstract Chromium iron titanates, with general formula (Cr,Fe) 2 Ti n −2 O 2 n −1 , n = 3, 4, and 5 have been prepared by reacting the component oxides in air at 700–1650°C. The compounds have been characterized by X-ray crystallographic techniques and, where necessary, lattice parameters have been confirmed by selected area electron diffraction. The structures of the compounds are closely related to that of α-PbO 2 and may be derived from it by crystallographic shear parallel to (110) α-PbO 2 . Preliminary results are reported for two series of ordered intergrowth phases, (M 3 O 5 ) n (M 4 O 7 ) m and (M 4 O 7 ) n (M 5 O 9 ) m .

Stability relations in the pseudobrookite solid solution FeyTi3−yO5

Abstract The stability relations in the pseudobrookite solid solution Fe y Ti 3− y O 5 have been studied in the temperature range 1350–1623°K, using the quench method and controlled gaseous buffers. The temperature/oxygen-fugacity relationships for three univariant assemblages containing the pseudobrookite solid solution have been established, and the composition/temperature curve delineating the pseudobrookite solid solution and its decomposition products has been defined. This curve exhibits two maxima at temperatures of 1413 and 1619°K and corresponding compositions of FeTi 2 O 5 and Fe 0.30 Ti 2.70 O 5 . The intervening minimum occurs at a temperature of 1339°K and a composition of Fe 0.9 Ti 2.1 O 5 . A qualitative explanation for the shape of the stability curve is given in terms of the relative contributions to the free energy from configurational entropy and enthalpy (internal strain).

Formation and characterization of new magnesium aluminum hydroxycarbonates

Abstract The removal of aluminum from strong caustic soda solutions (simulated paper pulp liquors) was studied using the addition of calcined magnesite to precipitate magnesium aluminum hydroxycarbonate phases. For aluminum concentrations higher than about 150 ppm the precipitated phase was hydrotalcite, whereas for lower aluminum concentrations, new hydroxycarbonate phases formed. For solution temperatures above 70°C the phase which formed was characterized as a 1:1 unit cell intergrowth of brucite and hydrotalcite. The new phase is isostructural with coalingite, having a rhombohedral unit cell, R 3 m, a = 3.108(4) A, c = 38.3(1) A, and a composition of [Mg 0.88 Al 0.12 (OH) 2 ] 2 · [(CO 3 ) 0.12 (H 2 O) 0.64 ]. With variation of solution temperature and concentration, other phases responsible for aluminum removal were formed and identified, including higher order brucite-hydrotalcite intergrowths and aluminum-containing brucite.