Structural phase transition and magnetic-field effect on the modulated structure in GdBaCo2O5+δ (δ < 0.5)

We investigated the crystal structures of an ordered perovskite-type cobaltate, GdBaCo2O(5+δ) (δ < 0.5), at elevated temperatures by transmission electron microscopy. Above the magnetic ordering temperature, we observed a first-order structural phase transition between the low-temperature tetragonal 3a(p) × 3a(p) and high-temperature orthorhombic 1a(p) × 2a(p) superstructure phases (where a(p) is the perovskite-unit cell). Upon the application of a magnetic field, an incommensurate phase emerges around the structural phase-transition temperature, which indicates a magnetic-field-induced structural phase transition via no magnetic ordering in the ordered perovskite-type cobaltate.

A synchrotron X-ray diffraction analysis of near-stoichiometric LiNbO3

Single-crystal diffraction data was collected at 120 and 294 K for an approximately spherical LiNbO3 crystal, 16 μm in diameter, using focused λ=0.7516 (1) Å wavelength synchrotron X-radiation and a fast avalanche photodiode (APD) detector. Extinction was significantly less than that recorded for previous single-crystal X-ray diffraction experiments. Anharmonic atomic displacements of Nb and Li were refined using a Gram-Charlier expansion to the fourth order. There was no significant change in the topography of the difference electron density (Δρ) map after anharmonic refinement. An accumulation of electrons present at the vacant site in the 294 K Δρ map was not evident in the 120 K Δρ map, indicating a migration of Li atoms to the vacant site as the temperature increased. The orientation of the electron density around Nb flipped by 90° going from 120 to 294 K.

Annular dynamical disorder of the rare earth ions in a La2Zr2O7 pyrochlore via single crystal synchrotron X-ray diffraction

The reciprocal lattices of a range of rare earth titanate and zirconate pyrochlores have recently been shown to exhibit an extremely characteristic diffuse intensity distribution identical in form to that characteristic of β-cristobalite implying the existence of β-cristobalite-like orientational disorder of essentially rigid, (O2)(R3+)4 tetrahedra within the pyrochlore structure type. A single crystal X-ray diffraction study of La2Zr2O7 has been carried out utilizing synchrotron radiation in order to investigate this proposed orientational disorder and to determine an unknown oxygen atom fractional co-ordinate. An annular distribution of residual electron density is found around the average La ion positions in {111} planes perpendicular to the local O2–La–O2 directions and interpreted as implying that the La ions never in fact occupy the average 16d positions but rather are dynamically disordered around an annulus perpendicular to that direction in an exactly analogous manner to that which occurs in β-cristobalite.

Fabrication and optical properties of Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) thin films on Si substrates using the PLD method

Epitaxial 0.67Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3)-0.33PbTiO(3) (PMN-PT) thin films with electro-optic effects were fabricated on (PMN-PT) thin films with electro-optic effects were fabricated on (La0(0.5)Sr0(0.5))CoO(3) (LSCO)/CeO(2)/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500 degrees C on the LSCO/CeO(2)/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600 degrees C, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMNPT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM) = 0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r(13) and r(33) were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r(13) = 17 pm/V at transverse electric field (TE) mode and r(33) = 55 pm/V at transverse magnetic field (TM) mode.

Synchrotron X-ray and ab initio studies of beta-Si3N4

Almost absorption- and extinction-free single-crystal synchrotron X-ray diffraction data were measured at 150, 200 and 295 K for beta-Si3N4, silicon nitride, at a wavelength of 0.7 A. The true symmetry of this material has been the subject of minor controversy for several decades. No compelling evidence favouring the low-symmetry P6(3) model was identified in this study.

A synchrotron X-ray study of triclinic LiCa2Nb3O10 with perovskite-type slabs

The triclinic superstructure of a small crystal of LiCa(2)Nb(3)O(10), lithium dicalcium triniobium decaoxide, has been investigated by synchrotron X-ray diffraction. The unit cell is an almost rectangular parallelepiped, although there is a 0.245 degrees offset from orthogonality for beta. The structure essentially belongs to a homologous series of Li[Na(n-3)Ca(2)Nb(n)O(3n+1)] with n = 3, where the moiety in square brackets has a perovskite-type slab structure. The superstructure has a doubled unit-cell volume with respect to the tetragonal aristotype. The NbO(6) octahedra are rotated about axes parallel to [110] by approximately 10 degrees. Adjacent slabs are connected by Li atoms and are geometrically related by 4(2) pseudosymmetry lying parallel to c. There are twice as many sites as Li atoms, providing a variation of population at these Li sites.

Monoclinic superstructure of SrMgF4 with perovskite-type slabs

Crystals of Ce-doped SrMgF4, strontium magnesium tetrafluoride, have been found to have a monoclinic P2(1) structure with doubled a and tripled c cell lengths compared with the orthorhombic Cmcm structure previously reported in the literature. The perovskite-type slabs, composed of corner-sharing MgF6 octahedra and Sr atoms, are stacked along the b axis. The six crystallographically independent MgF6 octahedra are rotated so as to provide long periodicities along a and c. The coordination numbers and bond distances around the six crystallographically independent Sr atoms are slightly different in each case. In the superstructure, the Sr atoms lie on local mirror planes which are thought to originate at the high-temperature phase transition.

Synchrotron X-ray study of Er3Al5O12 and Yb3Al5O12 garnets

Structure factors for Er(3)Al(5)O(12) and Yb(3)Al(5)O(12) garnets were measured using focused synchrotron X-radiation, with lambda = 0.7500 (2) and 0.7000 (2) A, respectively. The difference electron density maps for Er(3)Al(5)O(12) and Yb(3)Al(5)O(12) were similar, as expected. This was attributed to the 4f electrons being shielded, which reduces their effectiveness in chemical bonding and the relative position of the rare-earth atoms in the periodic table. The symmetry of the difference electron density around the rare-earth atoms was found to reflect that of the cation geometry, emphasizing the importance of second nearest-neighbor interactions. This is consistent with the view that oxide-type structures may be regarded as a packed array of cations with anions in the interstices.

Synchrotron X-ray imaging of the electron density in RFeO3 (R = Y, Ho) using an APD detector

Structure factors for small hydrothermally grown yttrium and holmium orthoferrites, RFeO(3) (R = Y, Ho), were measured with focused synchrotron radiation at wavelengths of 0.70 and 0.84 A using both scintillation and high-speed avalanche photodiode (APD) detectors. Resulting APD Deltarho images showed striking correlations between aspherical electron densities around Fe and rare-earth metals. Approximate high symmetry in the Deltarho images indicates that cations deform the electron density far more strongly than the O atoms. The Ho-Fe magnetic interactions appear to affect the electron density distribution of the Fe atoms and the magnetic phase transitions. Space group Pnma, orthorhombic, YFeO3 (APD): Mr = 192.76, a = 5.5931 (3), b = 7.6102 (4), c = 5.2806 (3) A, V = 224.77 (2) A(3), Z = 4, Dx = 5.695 Mg m(-3), mu(0.84) = 15.56 mm(-1), F(000) = 356, T = 293 K, R = 0.045, wR = 0.073, S = 4.83 (9) for 1282 unique reflections; HoFeO3 (APD): Mr = 268.78, a = 5.5922 (3), b = 7.6157 (5), c = 5.2798 (3) A, V = 224.86 (2) A(3), Z = 4, D(x) = 7.939 Mg m(-3), mu(0.84) = 61.98 mm(-1), F(000) = 356, T = 293 K, R = 0.036, wR = 0.037, S = 3.07 (6) for 1284 unique reflections.

Re-Refinement of Composite Modulated Nb2Zr x−2O2x+1 (x = 8) Using Synchrotron Radiation Data

The crystal structure of Nb2Zr_{x-2}O_{2x + 1}, x = 8, has been re-refined as a composite modulated structure using synchrotron radiation data measured at energies below the Zr K absorption edge. The structure comprises one-atom thick layers of two substructures stacked alternately along the b direction. The two component substructures are, in general, mutually incommensurable along their common a-axis directions and are referred to as the metal substructure (M), space group Amma, aM = 5.1348 (2), bM = 4.9590 (2), cM = 5.2908 (3) Å, and the oxygen substructure (O), space group Imam, aO = 2.4164, bO = bM = b, cO = cM = c, respectively. The four basis vectors used to index reciprocal space, based on the more strongly scattering metal substructure, are a^{\ast}_{M}, b*, c* and q M . The primary modulation wavevector q M = ^{\ast}_{M} − 2a^{\ast}_{M} + b* - 1/8 a^{\ast}_{M} + b* for x = 8. The overall superspace-group symmetry is Amma(\alpha10)0s0. Two sets of intensity data recorded at room temperature, 8 eV (\lambda_n = 0.6892 Å) and 1450 eV (\lambda_f = 0.7492 Å) below the Zr K absorption edge, were refined to an overall R(\lambda_n) = 0.0377 and R(\lambda_f) = 0.0439. The displacive modulation wave amplitudes obtained from these refinements agree well with those previously obtained for an x = 12 member. Difference Fourier maps show that metal-atom ordering is virtually negligible.

Synchrotron X-ray Studies of LiNbO3 and LiTaO3

The structural geometry, vibrations and deformation density Δρ for lithium niobate, LiNbO3, and lithium tantalate, LiTaO3, are derived from synchrotron radiation diffraction measurements. Electron density is transferred from the Nb (Ta) atom towards the large O3 triangle near the Li atom. Spontaneous polarizations of 0.43 (LiNbO3) and 0.33 cm−2 (LiTaO3), calculated with Hirshfeld charges, agree qualitatively with experimental values of 0.71 and 0.50 −2, respectively. Both strong Li...Nb (Ta) interactions in the structure are markedly shorter than all Li...Li, Nb...Nb or Ta...Ta vectors, as expected if the Li and Nb (Ta) cations carry charges of opposite sign. The Li atom's negative charge plays a pivotal role in the polarization responsible for ferroelectricity. LiNbO3, space group R3c, rhombohedral, M r = 147.8, a = 5.493 (2) Å, α = 55.89 (3)°, V = 106.02 (4) Å3, Z = 2, D x = 4.629 Mg m−3, F(000) = 136, μ(0.7 Å) = 5.01 mm−l, y min = 0.27, T = 293 K, R = 0.015, wR = 0.015, S = 3.276 (84) for 773 unique reflections. LiTaO3, R3c, rhombohedral, M r = 235.9, a = 5.471 (2) Å, α = 56.16 (3)°, V = 105.51 (4) Å3, Z = 2, D x = 7.412 Mg m−3, F(000) = 200, μ(0.7 Å) = 50.03 mm−1, y min = 0.32, T = 293 K, R = 0.016, wR = 0.022, S = 1.372(35) for 773 unique reflections.

Synchrotron X-ray electron density in the layered LaOCl structure

The deformation density Δρ in lanthanum oxychloride, LaOCl, has been determined for a small, naturally faced single-crystal using 0.7 Å synchrotron X-radiation. Accumulation of electron density in the interlayer region stabilizes the structure that contains layers of positively charged Cl atoms. Space group P4/nmm, tetragonal, Mr = 190.36, a = 4.1198 (7), c = 6.883 (2) Å, V = 116.8 (5) Å3, Z = 2, Dx = 5.411 Mg m−3, μ 0.7 = 17.28 mm−1, F(000) = 164, T = 293 K, R = 0.011, wR = 0.011, S = 1.98 (7) for 416 unique reflections.

A synchrotron X-ray study of the electron density in Y2BaCuO5

The deformation density (Δρ) for Y2BaCuO5, barium diyttrium cuprate, determined by single-crystal X-ray diffraction with synchrotron radiation, is affected to only a limited degree by the bonding interactions involving the O anions. Electron density is strongly depleted along the cation–cation contacts within the mirror plane in the structure and is transferred to regions between mirror planes that do not lie along short cation–cation vectors. The structural geometry for the CuO5 moiety, with the Cu atom in the +2 state, closely resembles that of the Cu2O5 group in YBa2Cu3O7−δ , for which the +3 state involvement for Cu has been suggested. Space group Pnma, orthorhombic, Mr = 458.68, a = 12.1793 (7), b = 5.6591 (5), c = 7.1323 (4) Å, V = 491.6 Å3, Z = 4, Dx = 6.197 Mg m−3, λ = 0.9 Å, μ 0.9 = 28.893 mm−1, F(000) = 812, T = 293 K, R = 0.020, wR = 0.020, S = 3.09 (5) for 2225 unique reflections.

A synchrotron X-ray study of the electron density in C-type rare earth oxides

Structure factors for small synthetic crystals of the C-type rare earth (RE) sesquioxides Y2O3, Dy2O3 and Ho2O3 were measured with focused λ = 0.7000 (2) Å, synchrotron X-radiation, and for Ho2O3 were re-measured with an MoKα (λ = 0.71073 Å) source. Approximate symmetry in the deformation electron density (Δρ) around a RE atom with pseudo-octahedral O coordination matches the cation geometry. Interactions between heavy metal atoms have a pronounced effect on the Δρ map. The electron-density symmetry around a second RE atom is also perturbed significantly by cation–anion interactions. The compounds magnetic properties reflect this complexity. Space group Ia{\bar 3}, cubic, Z = 16, T = 293 K: Y2O3, Mr = 225.82, a = 10.5981 (7) Å, V = 1190.4 (2) Å3, Dx = 5.040 Mg m−3, μ 0.7 = 37.01 mm−1, F(000) = 1632, R = 0.067, wR = 0.067, S = 9.0 (2) for 1098 unique reflections; Dy2O3, Mr = 373.00, a = 10.6706 (7) Å, V = 1215.0 (2) Å3, Dx = 8.156 Mg m−3, μ 0.7 = 44.84 mm−1, F(000) = 2496, R = 0.056, wR = 0.051, S = 7.5 (2) for 1113 unique reflections; Ho2O3, Mr = 377.86, a = 10.606 (2) Å, V = 1193.0 (7) Å3, Dx = 8.415 Mg m−3, μ 0.7 = 48.51 mm−1 F(000) = 2528, R = 0.072, wR = 0.045, S = 9.2 (2) for 1098 unique reflections of the synchrotron data set.

A synchrotron X-ray study of the electron density in SmFeO3

Structure factors for small, hydrothermally grown samarium orthoferrite, SmFeO3, were measured with focused λ = 0.7 Å synchrotron radiation. Approximate high symmetry in the Δρ images indicates that cations deform the electron density far more strongly than the O atoms. The most obvious effect is on distribution of the Fe atoms. The influence of the nearest low-symmetry (Cs ) O coordination on the electron density of the Sm cation is weak by comparison with that of the Sm–Fe interactions, as is illustrated by the high symmetry of the Δρ map near the Sm atom. The Sm–Fe interactions appear to affect the magnetic ordering and spin configuration of the Fe atoms. Space group Pnma, orthorhombic, Mr = 254.20, a = 5.6001 (3), b = 7.7060 (7), c = 5.3995 (6) Å, V = 233.01 (4) Å3, Z = 4, Dx = 7.246 Mg m−3, μ 0.7 = 28.5 mm−1, F(000) = 448, T = 293 K, R = 0.017, wR = 0.021, S = 3.83 (8) for 1329 unique reflections.

Structure of Mg6SO2(OH)14 determined by micro single-crystal X-ray diffraction

The structure of hexamagnesium sulfonyl tetradecahydroxide, Mg6SO2(OH)14, has been determined with an extremely small single-crystal (ca 0.5 × 100 × 2.5 μm) using synchrotron radiation [1.00 (1) Å]. The crystal is orthorhombic with space group Ccmm (No. 63), a = 15.895 (1), b = 3.105 (1), c = 13.367 (1) Å, V = 659.64 (3) Å3 and Z = 2. The final R and wR values are 0.073 and 0.083 for 105 crystallographically independent reflections. The structure consists of MO6 octahedra and SO4 tetrahedra. The MgO6 octahedra share their edges to form zigzag sheets parallel to (100). The SO4 tetrahedron shares two of the O atoms with the MgO6 octahedra at the turning of the sheet and is statistically located with a probability of 0.5. Fine streaks observed perpendicular to the b* axis on diffraction photographs indicate that the SO4 tetrahedra are distributed at random along the a and c axes, but alternately along the b axis. All the H atoms are considered to exist in the form of OH groups.

Selective en bloc resection of osteoid osteoma of the superior articular process of the sacral spine. A case report

Selective en bloc resection of an osteoid osteoma from the superior articular process of the sacral spine was performed, sparing the facet joint. Computed tomography was very useful, both in making a diagnosis and in facilitating precise planning of the operative procedure. The paraspinal approach readily facilitated complete excision of the tumoral nidus with minimal surgical dissection.