Brett A. Hunter

Powder diffraction studies of synthetic calcium and lead apatites

The crystal structures of M10(PO4)6X2, where M = Ca or Pb and X = OH¯, F¯, Cl¯ or Br¯, have been determined by Rietveld refinement of powder synchrotron X-ray and neutron diffraction data. All the compounds are hexagonal with space group P 63/m. For the calcium compounds, the lattice parameters are a = 9.4302(5), 9.3475(3), 9.5902(6), 9.6482(6) and c = 6.8911(2), 6.8646(1), 6.7666(2), 6.7788(2) A, for X = OH¯, F¯, Cl¯, Br¯, respectively. For the lead compounds, the corresponding lattice parameters are a = 9.8612(4), 9.7547(5), 9.9767(4), 10.0618(3) and c = 7.4242(2), 7.2832(2), 7.3255(1), 7.3592(1) A. In these compounds there are two cation sites, a channel of M(I) atoms and a triangle of M(II) atoms. The anion interacts most strongly with the M(II) atoms with the interaction dictating the position of the anion relative to the M(II) triangle. In Ca10(PO4)6X2, the F¯ ion sits within the triangle planes, while the larger OH¯ and Cl¯ anions are disordered above and below the M(II) triangles. The even larger Br¯ is midway between two triangles at (0, 0, ). Despite the larger size of the isostructural lead compounds, no anions are found in the triangles. The F¯, Cl¯ and Br¯ ions are at (0, 0, ) and the OH¯ ion is disordered at (0, 0, z). This difference in behaviour is possibly related to the lead 6s electrons. In this paper, the experimental results are presented and possible reasons for the observed differences are discussed.

Structural phase transitions in the layered bismuth oxide BaBi4Ti4O15

Abstract The structure of the n =4 Aurivillius oxide BaBi 4 Ti 4 O 15 has been studied at room temperature using powder neutron diffraction, and from 300 to 1000 K using synchrotron X-ray diffraction methods. The structure is orthorhombic (space group A 2 1 am ) at 300 K and transforms to a tetragonal ( I 4/ mmm ) structure near 700 K.

Dependence of superconducting transition temperature on doping and structural distortion of the CuO2 planes in La2−xMxCuO4 (M=Nd, Ca, Sr)

We have separated the structural and doping effects on the superconducting transition temperature in La2−xMxCuO4 (M=Nd, Ca, Sr). At a fixed doping level, the highest Tc is found for flat and square CuO2 planes in the tetragonal structure. Tc is reduced proportionally to the structural distortions of the CuO2 planes in the orthorhombic structure. The local minimum of Tc vs doping is observed for both orthorhombic Ca- and Sr-substituted materials around x∼0.12. This minimum may be related to the intrinsic dependence of the electronic density of states at EF due to a weakly split singularity of DOS.

Cation disorder in the ferroelectric Aurivillius phase PbBi2Nb2O9 : an anamolous dispersion X-ray diffraction study

Abstract The effect of high temperature annealing on cation disorder in PbBi 2 Nb 2 O 9 has been studied using a combination of powder neutron and anomalous dispersion X-ray diffraction methods. High resolution diffraction data shows that the orthorhombic cell volume is greater in rapidly cooled samples compared to slowly cooled samples. Rietveld analysis of the diffraction data gave the following cell parameters, space group A 21 am, a =5.4909(1) and 5.4879(1), b =5.4998(1) and 5.4989(1), c =25.5313(2) and 25.5390(2) A for quenched and slowly cooled samples respectively. Anomalous dispersion X-ray diffraction methods have shown this is a result of disorder of the Pb 2+ and Bi 3+ cations. In the rapidly cooled sample a near statistical distribution of Pb 2+ and Bi 3+ in the 4 a perovskite type sites was observed, the occupancies being 33 and 67% of Pb 2+ and Bi 3+ respectively. This occurs since both Pb 2+ and Bi 3+ have lone pair electrons and at high temperatures these cations have a similar preference for the 4 a and 8 b (Bi 2 O 2 ) type sites. In the slowly cooled sample the 4 a sites are occupied by 53 Pb 2+ and 47% Bi 3+ suggesting Bi has a preference for the 8 b sites.

Observations on pyrochlore oxide structures

Abstract The structures of three defect pyrochlore oxides were refined using powder neutron or synchrotron X-ray diffraction data: Bi 1.89 GaSbO 6.84 ( Fd 3 m ; a = 10.3807(4) A); Pb 2 (Ga 0.5 Sb 1.5 )O 6.5 and Pb 2 (Ni 0.333 Sb 1.667 )O 6.5 ( F 43 m ; a = 10.4433(2) and 10.5052(2) A, respectively). Based on these and previously reported structures, some generalizations on the structure of defect pyrochlores are made. It was found that the contraction of the lattice in metallic Bi 2 B 2 O 7−y -type oxides, relative to that of the isostructural insulating oxides, is not a result of B–B bonding. Rather, there is a contraction of the BiO 8 polyhedron in the metallic oxides, due to the decreased stereochemical activity of Bi 6s electrons, as a consequence of their mixing with B d orbitals. In A 2 B 2 O 6.5 compounds, where A is an easily polarizable cation, oxygen-vacancy ordering accompanied by A-cation displacement can occur. The extent of the displacement of the A-type cation is shown to be strongly dependent on the cell parameter.

Structural Phase Transitions in the Ferroelectric Oxide SrBi 2 Ta 2 O 9

The structure of SrBi 2 Ta 2 O 9 between room temperature and 1100°C has been studied using high resolution powder neutron diffraction methods. At room temperature the structure is orthorhombic (space group A 2 1 am ) and this undergoes an apparently continuous transition to a paraelectric orthorhombic phase in space group Amam near 300°C in addition to a transition above 500°C to a tetragonal structure in space group I 4/ mmm . This sequence of transitions is consistent with a group theoretical analysis. The low temperature ferroelectric phase is characterised by both rotations of the TaO 6 and displacement of the Ta atoms from the center of these octahedra, the paralectric phase contains a single tilt of the TaO 6 octahedra while there are no rotations of the TaO 6 octahedra in the high temperature tetragonal structure.

Dependence of superconducting transition temperature on doping and structural distortion of the CuO{sub 2} planes in La{sub 2{minus}{ital x}}M{sub {ital x}}CuO{sub 4} (M=Nd, Ca, Sr)

By comparing structural and superconducting properties of La{sub 2{minus}{ital x}{minus}{ital y}}Ca{sub {ital x}}Nd{sub {ital y}}CuO{sub 4} to La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4} we have separated the effects of structure and doping on the superconducting {ital T}{sub {ital c}}. At a fixed doping level, the highest {ital T}{sub {ital c}} is found for flat and square CuO{sub 2} planes in the tetragonal structure. {ital T}{sub {ital c}} is reduced by the structural distortions of the CuO{sub 2} planes in the orthorhombic structure. The local minimum of {ital T}{sub {ital c}} vs doping observed around {ital x}{approximately}0.12 indicates the presence of a weakly split singularity of the electronic density of states for the orthorhombic material. {copyright} {ital 1996 The American Physical Society.}

Neutron diffraction study of the LaNi5–D system during activation

Abstract The microstructural changes occurring during the initial absorption of deuterium by virgin LaNi 5 at 40°C have been investigated using in-situ neutron powder diffraction. Rietveld profile refinement was used to determine the α and β phase proportions, lattice parameters and microstrains. In absorption, we found that in the two-phase region (i) the lattice parameters of the α and β phases were (within resolution) independent of the phase proportions; (ii) the α-phase diffraction peaks remained essentially unbroadened relative to the virgin metal; (iii) the β-phase peaks were relatively broad with the usual anisotropy of breadth. These findings imply that, as nuclei of β phase form for the first time in a particle that is wholly α phase, the lattice expansion causes pure β crystallites containing a very high density of lattice defects to fracture off the particle, i.e., decrepitate. Hence the nanoscale mixing and strong mechanical interaction between the α and β phases noted in multiply cycled material are not observed during the initial absorption of D atoms, because the lattice parameter misfit cannot be accommodated. In desorption, and subsequently, there is sufficient accommodation of the lattice parameter mismatch between the α and β phases for them to coexist in the same powder particle.

Polymorphism and Phase Transitions in Bis(glycinato)copper(II). A Powder Diffraction Study

Powder diffraction methods have been used to establish the structures of four known polymorphs of bis(glycinato)copper(II). The two cis-modifications are orthorhombic, and the two trans-complexes are monoclinic. Refinements of the structures, including the location of the hydrogen/deuterium atoms have been achieved utilizing Rietveld analysis of powder data. The cis-monohydrate complex crystallizes in the space group P212121 with lattice parameters a 10.8053(3), b 5.2101(1), and c 13.4983(4) A. Upon dehydration, contraction along the a- and c-axes and elongation of the b-axis is observed. The anhydrous cis-complex also crystallizes in the space group P212121 with lattice parameters a 10.0673(7), b 5.3152(4), and c 13.212(1) A. The trans-hydrated complex crystallizes in the space group I2/a with lattice parameters a 14.8218(3), b 5.2321(1), and c 9.6408(2) A, and β 87.243(1)°. Dehydration of the complex affords the anhydrous modification which crystallizes in the space group P21/c with lattice parameters a 7.0831(6), b 5.1459(4), and c 9.4431(9) A, and β 107.506(4)°. In all four modifications a network of hydrogen bonds stabilizes the complex. This study illustrates the current capabilities of powder diffraction techniques.

Phase transformation in CuRh2O4 : A powder neutron diffraction study

The structure of the spinel-type oxide CuRh{sub 2}O{sub 4} has been determined between 25 and 650 C using powder neutron diffraction methods. A first-order phase transition from a tetragonal to a cubic structure occurred at ca. 575 C. The high-temperature cubic structure is a normal spinel (Fd{bar 3}m, a = 8.5487(2) {angstrom}, x = 0.2601(4)), whereas at room temperature the structure can be described as a distorted spinel (I{bar 4}{sub 1}/amd, = 6.1707(4) {angstrom}, c = 7.9183(7) {angstrom}). In both forms, Cu and Rh exclusively occupy the tetrahedral and octahedral sites, respectively. The room temperature tetragonal distortion is ascribed to the strong preference of Cu{sup 2+} for a square planar geometry. From valence bond sum analysis, it was concluded that the transition is not electronic in origin.