Edward Prince

Superconducting Tl2.0Ba2.0CuO6+δ: A high resolution neutron powder and single crystal x-ray diffraction investigation

Abstract The structure of the orthorhombic form of Tl2Ba2CuO6+δ(Tc=90 K), synthesized in sealed gold tubes using BaO2, has been refined using high resolution neutron powder diffraction data collected at 4 K. No significant deficiency in metal stoichiometry could be inferred from the crystal structure analysis. The space group (Fmmm) used in this study gives essentially the same results as the alternative (Abma, Amaa) proposed in earlier work. The large amount of disorder that exists in the Tl2O2-slab is mirrored in the large thermal parameters for this structural unit compared to those for the CuO2 sheet. A site at ( built1 4 built1 4 built1 4 ) is occupied by an amount of oxygen to give overall stoichiometry Tl2Ba2CuO6.10, suggesting that oxygen non-stoichiometry plays an important role in producing high Tc superconductivity in this system. Analysis of single crystal X-ray diffraction data of tetragonal Tl2Ba2CuO6+ δ is totally consistent with the neutron powder diffraction results.

Neutron powder diffraction study of solid solution Li1+xTi2−xInxP3O12: I. 0.0 ≤ x ≤ 0.4

Abstract Two polycrystalline samples of Li 1+ x Ti 2− x In x P 3 O 12 prepared with compositions x = 0.25 and x = 0.40 have been analyzed by powder neutron diffraction. The x values found in the refinement were systematically lower, 0.12 and 0.15, respectively, than those required by chemical analysis. The discrepancy in the x values is explained by formation of a parasitic phase coexisting with the main phase. The structure framework of the analyzed samples (actually x = 0.12 and 0.15) is of the NASICON type, with Ti and In atoms randomly distributed among octahedral sites. Lithium atoms are distributed over octahedral and eight-coordinated sites. The octahedral Li(1) site is almost filled, with a relatively large Li(1)O distance, 2.29 A, leaving Li(1) ions enough room to jump into Li(2) sites. The latter sites are only partially occupied by Li(2) ions. A large temperature factor associated with a split position inside the eight-coordinated cavity suggests that the Li(2) ions are in a highly disordered configuration.

Neutron diffraction determination of full structures of anhydrous LiX and LiY zeolites

Abstract Virtually monoionic LiX and LiY zeolites have been prepared by LiOH titration of parent NH 4 zeolites. Structural studies have been performed at room temperature on the anhydrous zeolites, Li 80.7 H 4.9 Na 0.4 Al 86 Si 106 O 384 and Li 46.0 H 5.8 Na 5.1 K 0.1 Al 57 Si 135 O 384 , by powder neutron diffraction profile refinement in order to locate Li + cations. The cell parameters are 24.6716(10) and 24.4498(12) A for LiX and LiY, respectively. Three positions have been found for Li + , sites I′ and II in the six-ring windows of the sodalite unit and site III′ in the supercage for the additional Li + of LiX.

Structure of the lithium insertion compound Li2Fe2(MoO4)3 from neutron powder diffraction data

The structure of Li2Fe2(MoO4)3 has been determined by a profile refinement of neutron powder diffraction data. Li2Fe2(MoO4)3 was prepared at ambient temperature by the reaction of solid ferric molybdate, Fe2(MoO4)3, with a LiI/acetonitrile solution. The title compound is orthorhombic with a = 12.8947(5), b = 9.4957(4), and c = 9.3477(3) A, in the space group Pbcn. Its structure is related to that of garnet and langbeinite with corner-sharing Fe2+O6 octahedra and Mo6+O4 tetrahedra. The lithium ions reside in general positions and these sites are ordered and fully occupied. Lithium is tetrahedrally coordinated to oxygen by bridging the edges of two FeO6 octahedra. Bond valence calculations are entirely consistent with the structure.

The structure of cesium-exchanged zeolite-RhO at 293K and 493K determined from high resolution neutron powder data

Abstract The crystal structure of Cs-exchanged zeolite Rho (143m) has been solved using neutron powder data collected at 293K and 493K. The model differs from that proposed by Robson, Shoemaker, Ogilvie, and Manor (1973) for the hydrogen form. Structure modeling studies (DLS) suggested a starting point for the refinement. Cs at ( 1 2 , O, O) is in the elliptically distorted double 8-ring blocking absorption. An increase in temperature decreases the elliptical distortion of the double 8-ring which is restored at lower temperature.

Fast ion transport in the NASICON analog Na3Sc2(PO4)3: Structure and conductivity☆

The room temperature modification of stoichiometric NASICON(Sc) is monoclinic Cc. At 64°C there is a first order transition to a normal-conducting rhombohedral form R3c. Na(1) sites are fully occupied whereas Na(2) sites are partially occupied. At 167°C there is a transition to asuperionic phase, but the structure remains rhombohedral R3c. Vacancies are now shared equally by Na(1) and Na(2) sites. Fast Na-ion motion in stoichiometric Na3Sc2(PO4)3 arises from vacancy motion in a “dogleg” path between Na(1) and Na(2) sites.

Structure and proton conduction in pyrochlore-type antimonic acid: a neutron diffraction study

Abstract The structure of pyrochlore-type antimonic acid, HSbO 3 · 0.5H 2 O, has been investigated using powder diffraction of neutrons. The structure (space group Fd3m ) contains an [Sb 2 O 6 2− ] framework, with interstitial oxygens present as a mixture of H 2 O and H 3 O + . Three H-atom sites have been identified, (1) close to the [Sb 2 O 6 2− ] framework in hydroxyl groups (SbOH), (2) H in water molecules H-bonded to framework hydroxyls, and (3) H in H 3 O + ions H-bonded to O-atoms of the intercavity windows. An intraframework equilibrium of the type Sb  OH + H 2 O ⇋ Sb  O − + H 3 O + provides the mechanism for intracavity chemical exchange and proton conduction.

The structure of V9Mo6O40 determined by powder neutron diffraction

Abstract The crystal structure of V 9 Mo 6 O 40 has been determined by Rietveld profile analysis of neutron powder diffraction data. The structure is a monoclinically distorted variant of Nb 3 O 7 F (space group C 2) consisting of ReO 3 -type slabs three octahedra thick connected by edge sharing of component octahedra. The octahedra are considerably distorted due to off-center displacement of the metal atoms. Metal-oxygen bond lengths conform with those of similar compounds.

Neutron Rietveld analysis of structural changes in NASICON solid solutions Na1+xZr2SixP3−xO12 at elevated temperatures: x=1.6 and 2.0 at 320°C

Abstract Neutron Rietveld analyses of the structures of NASICON solid solutions as a function of composition have been extended to 320°C for the high-conductivity compositions x=1.6 and 2.0. The transformation from the room temperature monoclinic C2/c structure to the hexagonal R3c high temperature phase involves small atomic displacements, ranging from 0.385A for Na(2) down to shifts of only a few hundredths of an Angstrom for several framework ions. The Na(1) interstice remains fully occupied to the temperature presently examined. No evidence for partial occupancy of the Zr octahedron is found, a non-stoichiometry which is possible but not obligatory for NASICON. The distortions of the framework are largest at x=2.0 as at room temperature. The radius of the windows between Na sites at 320°C remain largest at the composition with x=2.0 for both a Na(1)-Na(2) jump and a Na(2)-Na(2) jump. The radii are significantly larger than the maximum value available among the three symmetry-independent paths in the room-temperature monoclinic structures for both types of diffusion paths.

Neutron powder diffraction study and physical characterization of zeolite D-ZK—5 deep-bed calcined at 500°C and 650°C

Abstract Zeolite ZK—5 samples, deep-bed calcined at 500°C and at 650°C, respectively, were studied by neutron powder diffraction, i.r. and n.m.r. In addition their methanol sorption was measured. The chemical composition of the starting material was (NH4)xCs4.5K0.003Al22.5Si73.5 o192. After calcination at 500°C and deuteration the composition was D8.5Cs4.5Al13Si83O192 (DZK500); 75% of the pore space was accessible to methanol. Sample DZK500 crystallizes in space group lm 3m , a = 18.633 A . The sample calcined at 650°C and subsequently deuterated (DZK650) had the composition D3.5Cs4.5Al8Si88O192, and only 44% of its pore space was accessible to methanol. Sample DZK650 crystallizes in space group lm 3m , a = 18.440 A . l.r. and n.m.r. studies show that 9.5–20.2 Al/u.c. in DZK500 and 14.5–19.2 Al/u.c. in DZK650 are removed from the framework. The neutron diffraction study shows Cs atoms located in eight rings and shows a peak in the difference Fourier map at 0, ∼0.45, and ∼0.07 in the γ cage which has been attributed to non-framework Al O atoms. The discrepancy between the number of Al O in the γ cage and that found by Al and Si n.m.r. is attributed to the presence of non-framework Al O in other sites or randomly distributed condensed AlOOH clusters.