Temperature Powder Neutron Diffraction Studies Research Articles

Evolution of crystal structure of PbMoO4 between 5 and 300 K: A low temperature powder neutron diffraction study

Evolution of crystal structure of PbMoO4 (Wulfenite, tetragonal, I41/a) from 5 to 300 K by in situ low temperature powder neutron diffraction studies is reported. The study revealed no structural transition in the complete range of temperature. The unit cell parameters show anisotropic expansion with about 2.2 times higher expansion along c-axis compared to that along a-axis. From the refined structural parameters, it is emphasized that the expansion of Pb–O bonds in PbO8 (bisdisphenoid) units and their orientations along a- and c-axes are the controlling factors. The MoO4 tetrahedra behaves like a rigid unit in the temperature range of this study. The expansion of unit cell volume is fully arising from the expansion of PbO8 bisdisphenoids. The analyses of anisotropic displacement parameters also show almost smooth variation with temperature down to 5 K. The evolutions of structural parameters of PbMoO4 with temperature suggest it for a promising material for cryogenic applications.

Observation of a new cryogenic temperature dielectric relaxation in multiferroic Bi7Fe3Ti3O21

In this communication, we report details of structural, magnetic, and dielectric properties of multiferroic Bi7Fe3Ti3O21 at cryogenic temperatures. The low temperature impedance spectroscopic studies showed a relaxor-like dielectric anomaly which follows Vogel-Fulcher relation with freezing temperature (TVF) of 33 K. Temperature and field dependent magnetization indicate the onset of a possible antiferromagnetic ordering below 150 K. Variable temperature powder neutron diffraction studies indicate no structural change down to 22 K. The appearance of the low temperature dielectric anomaly along with evidence of magnetic ordering at low temperatures suggests the presence of magnetic domains of mesoscopic scale within the bulk matrix.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2(x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28−xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD2O and pO2 at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10−6 K−1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

Variable Temperature Powder Neutron Diffraction Study of SmNiO3 through Its M−I Transition Using a Combination of Samarium and Nickel Isotopic Substitution

Neutron powder diffraction studies of 154Sm58NiO3, 154Sm60NiO3, and 154Sm62NiO3, at a range of temperatures through the M−I transition at approximately 128 °C, have been performed on the new general materials diffractometer (GEM) at ISIS, RAL. With combined data-set Rietveld analysis, using samples containing different nickel isotopes with contrasting scattering lengths, it has been found that extremely high quality structural parameters can be determined, even though total data collection times are more than an order of magnitude shorter than those previously used for this system. Rietveld analysis shows that the evolution of the structural parameters over the temperature range are smooth and that no symmetry change or abrupt structural transition occurs at the M−I transition. This is consistent with evolution of the high-temperature metallic material within the low-temperature insulating phase over the temperature range 108−128 °C. The key effects of thermal motion on the M−I transition have been extrac...

Variable temperature powder neutron diffraction study of the Verwey transition in magnetite Fe3O4

Abstract Neutron powder diffraction data have been collected from a sample of polycrystalline Fe 3 O 4 . High resolution data collected at 60 K are fitted using a simple rhombohedral distortion of the cubic unit cell. This model accounts for all of the peak splittings that occur upon cooling through the Verwey transition temperature ( T V ). Lower resolution data collected between 2 and 280 K are fitted using the same model, which gives T V =110±5 K. These data show a change in the thermal expansion due to the softening of phonons above T V and a change in crystallite extinction arising from twinning at the transition.

Neutron diffraction study of superconducting RESr 2Cu 3 − xM xO z (M  Re, W)

Superconductivity has recently been reported in RESr 2Cu 2.85Re 0.15O z (RE = Y, Tb, Dy, Ho, Er and Tm) and RESr 2Cu 2.80W 0.20O z (RE = Y, Dy, Ho and Er) with T c ranging from 22 to 41 K. Room temperature powder neutron diffraction studies have been carried out for obtaining structural parameters and their relation with superconductivity in these materials. Reitveld refinements were carried out adopting the space groups P4/mmm (tetragonal). Occupancy of the rare earths is less than 1 indicating vacancy at the rare earth sites. Cu(2)-O distance in RESr 2Cu 3 − x M x O z (M = Re and W) is smaller than that of REBa 2Cu 3O 7 − δ which is consistent with the smaller ionic radius of Sr 2+. Our studies show that the M-cations (Re or W) occupy the Cu(1) chain site.

High temperature powder neutron diffraction studies of structural transformations in transition metal hemicarbides M 2C 1− x: II. Compounds of the VIth group (Mo 2C 1− x and W 2C 1− x

This second part is devoted to the hemicarbides of the VIth transition metal group (Mo 2C and W 2C); a new structural transformations ζ → (ϵ-type) is identified in Mo 2C 1− x compounds, while this transition is confirmed in W 2C.

High temperature powder neutron diffraction studies of structural transformations in transition metal hemicarbides M 2C 1− x: I. Compounds of the Vth group (V 2C, Nb 2C, Ta 2C)

The crystal chemistry of the metallic hemicarbides V 2C, Nb 2C, Ta 2C and Mo 2C, W 2C has been re-examined by in situ neutron diffraction up to 2200°C. The results concerning ordered modifications and structural transformations are reported in 2 parts, devoted to the compounds of the Vth and VIth groups respectively.

Low Temperature Powder Neutron Diffraction Studies of CsFeS2

Polarized neutron powder diffraction and high resolution powder neutron diffraction measurements with the unpolarized beam were performed in an attempt to determine the low temperature magnetic and crystal structure of CsFeS 2 . The results of the measurements reveal that the magnetic and the crystal structure of the low temperature phase as well as the transition behavior depend strongly on the state of the internal stress of the sample. Using a powder sample of the stress free as-grown fine crystallites, we determine the low temperature crystal structure as a triclinic one with lattice parameters at T =40 K as a =7.08 A, b =12.05 A, c =5.47 A, α=96.6°, β=100.9°, γ=88.5°. The results also suggest that the observed transition may be viewed as that of the Spin-Peierls singlet to the Neel state, which accompanies the simultaneous structural transformation.

The structure of a cubic hydrogen rhenium bronze, H 1.36ReO 3

The structure of D 1.36ReO 3 has been determined by a room temperature powder neutron diffraction study. The unit cell is body centered cubic (space group Im3) and contains eight formula weights ( a = 7.497 ± 0.001Å). A least squares method based on peak intensities was used to refine the structure. The rhenium atoms lie on the special perovskite sites and are surrounded by distorted octahedra of oxygen atoms which are displaced towards the vacant perovskite A sites. The deuterium atoms are statistically distributed over 72 possible sites in the unit cell, as OD bonds.

The solid solutions Na 3(1−x)Al x▭ 2xPO 4: room temperature powder neutron diffraction studies of the two compositions x=0.025 and x=0.2

The system Na 3(1−x)Al x▭ 2xPO 4, where ▭ represents a cation vacancy, has been examined under ambient conditions at the two compositions x=0.025 and x=0.2 by powder neutron diffraction. The presence of Al 3+ ions stabilises the high-temperature face-centred cubic phase of sodium orthophosphate, II-Na 3PO 4, with a 0=7.424(2) A ̊ (x=0.025) and a 0=7.421(2) A ̊ (x=0.2) at 25°C. At the lower dopant concentration, the structure corresponds closely to that of II-Na 3PO 4. The PO 3- 4 groups are orientationally disordered about the origin with sodium ions occupying the 1 4 1 4 1 4 and 1 2 1 2 1 2 sites in space group Fm3m. For x=0.2, vacancies are found solely at the 1 2 1 2 1 2 position. This is consistent with the interpretation of these phases in terms of a cation-excess anti-fluorite structure. The influence of the structural evolution on the sodium-ion conductivity is discussed and a basis for the formation of low melting-point glasses for x > 0.5 is proposed.

A neutron diffraction study of disorder in SrCl 2 at high temperature

High temperature powder neutron diffraction study of SrCl 2 is reported. The intensity of some reflections decreases drastically in the region 900–1100K. The neutron data have been interpreted by assuming that the chlorine ions are displaced slightly towards the empty octahedral sites.

Neutron diffraction evidence for anion sublattice disordering in alkali earth fluorides at high temperatures

High temperature powder neutron diffraction studies of calcium, strontium and barium fluorides indicate that there are large thermal motions associated with the anions. This suggests that the anion sublattice in these compounds with the fluoride structure may disorder at temperatures below the melting points. In S:F 2 this disordering appears at about 1370 K and for BaF 2 at about 1300 K. This is in good agreement with transitions of 1440 K and 1240 K, predicted from thermodynamic data.

The crystal structure of cubic hydrogen tungsten bronze

The structure of D 0.53WO 3 has been determined by a room temperature powder neutron diffraction study. The unit cell is body centered cubic ( Im3) and contains 8 formula weights ( a = 7.562 ± 0.002 Å). The average cell content was refined by a least squares method based on peak intensities. The tungsten atoms lie on the special sites of the perovskite structure and are surrounded by nearly regular octahedrons of oxygen atoms with the latter displaced from the normal perovskite positions along 〈110〉. The deuterium atoms constitute statistically distributed -OD bonds which are directed towards the nearest oxygen atom of a neighboring octahedron. The analogous hydrogen compound is best described as a nonstoichiometric oxide hydroxide WO 3− x (OH) x based on a distorted ReO 3 structure. The crystal structure is closely related to that of In(OH) 3 and Sc(OH) 3.