Variable Temperature Neutron Powder Diffraction Research Articles

Interstitial Oxide Ion Order and Conductivity in La1.64Ca0.36Ga3O7.32 Melilite

Solid oxide fuel cells (SOFCs) are a major candidate technology for clean energy conversion because of their high efficiency and fuel flexibility.1 The development of intermediate-temperature (500–750 °C) SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10−2 S cm−1 assuming an electrolyte thickness of 15 μm1). This conductivity, in turn, necessitates enhanced understanding of the mechanisms of oxide ion charge carrier creation and mobility at an atomic level. The charge carriers are most commonly oxygen vacancies in fluorites2, 3 and perovskites.3, 4 There are fewer examples of interstitial-oxygen-based conductors such as the apatites5, 6 and La2Mo2O9-based materials,7–9 so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.

Crystallographic and magnetic studies of mesoporous eskolaite, [formula omitted

Mesoporous eskolaite, Cr 2 O 3 prepared by using the evaporation method with mesoporous KIT-6 silica as a hard template has been analysed with SQUID magnetometry and variable temperature neutron powder diffraction (NPD). NPD analysis shows that the magnetic and crystallographic properties of the eskolaite structure within the walls of the mesoporous Cr 2 O 3 material are identical to that of “bulk” material, R 3 ¯ c . The refined magnetic moments on the Cr 3 + ions are lower than observed for bulk material due to the higher proportion of surface atoms, creating local disorder. This is corroborated by the magnetometry results, which suggest a small amount of ferromagnetism may be present at low temperatures.

Leakage and Proton Conductivity in the Predicted Ferroelectric CsBiNb2O7

The layered perovskite CsBiNb2O7 has been reinvestigated in the light of recent predictions of ferroelectric behavior. Variable temperature powder neutron diffraction experiments show that this material retains polar symmetry (space group P2(1)am) up to at least 900 degrees C, with only a slight decrease in net polarization up to this temperature, calculated using a simple ionic model. However, ac impedance measurements show only a modest dielectric constant (19-27) throughout the temperature range studied. Moreover, convincing evidence for ferroelectric behavior in polarization-electric field loops could not be obtained because of overriding electrical conductivity. Further measurements of electrical behavior, at different temperatures and under both wet and dry atmospheres, provide clear evidence that CsBiNb2O7 is hygroscopic, with water uptake resulting in significant protonic conductivity. Even in the dehydrated state the material is a semiconductor with a modest conductivity of 3.8 mu S cm(-1) at 250 degrees C. Although ferroelectric behavior is not observed in the samples studied here, we cannot rule out the possibility that ferroelectricity might be obtained in samples processed under appropriate conditions.

Ferroelectric-Paraelectric Transition inBiFeO3: Crystal Structure of the OrthorhombicβPhase

A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO3 undergoes a phase transition from the ferroelectric alpha phase (rhombohedral, R3c) to a paraelectric beta phase (orthorhombic, Pbnm) between 820 degrees C and 830 degrees C. Coexistence of both phases over a finite temperature interval, together with abrupt changes in key structural parameters, confirms that the transition is first order. The beta phase corresponds to a GdFeO3-type perovskite structure.

Structural changes in YBaCo4O7+δmonitored by variable-temperature neutron powder diffraction

Structural changes in YBaCo₄O_7+δmonitored by variable-temperature neutron powder diffraction

A powder neutron diffraction study of the metallic ferromagnet Co 3Sn 2S 2

Variable-temperature powder neutron diffraction data reveal that Co 3Sn 2S 2 crystallizes in the shandite structure (space group R 3 ¯ m , a = 5.36855(3) Å, c = 13.1903(1) Å at 300 K). The structural relationship between Co 3Sn 2S 2 and the intermetallic compound CoSn, both of which contain Kagomé nets of cobalt atoms, is discussed. Resistivity and Seebeck coefficient measurements for Co 3Sn 2S 2 are consistent with metallic behaviour. Magnetic susceptibility measurements indicate that Co 3Sn 2S 2 orders ferromagnetically at 180(10) K, with a saturation moment of 0.29 μ B per cobalt atom at 5 K. The onset of magnetic ordering is accompanied by marked anomalies in the electrical transport properties.

A linked structural and electronic transition in Na0.63CoO2

A structural transition at 158 °C in Na0.63CoO2 is associated with a resistivity anomaly. Raman spectroscopy suggests a change in Na ordering at the transition and, for the first time, we have identified the nature of this rearrangement using variable temperature neutron powder diffraction. On heating, some Na ions migrate to the thermodynamically less stable (Na1) site from Na2, consistent with loss of an incommensurate superstructure. It is suggested that the change in Na order is linked to a change in the electronic band structure, which is reflected in the resistivity anomaly.

Oxygen Vacancy Ordering Phenomena in the Mixed‐Conducting Hexagonal Perovskite Ba7Y2Mn3Ti2O20.

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Order–disorder transition in the complex lithium spinel Li 2CoTi 3O 8

Li 2CoTi 3O 8 has an ordered Li 2BB′ 3O 8 spinel structure, space group P4 332, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order–disorder phase transition in Li 2CoTi 3O 8 which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯ m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li, B cations.

Oxygen Vacancy Ordering Phenomena in the Mixed-Conducting Hexagonal Perovskite Ba7Y2Mn3Ti2O20

A 21R-type mixed conducting oxygen-deficient hexagonal perovskite Ba7Y2Mn3Ti2O20 was synthesized and its structure characterized by variable temperature neutron powder diffraction. The structure is characterized by the presence of both BaO2 and BaO3 layers. In the oxygen-stoichiometric material, the vacancies are ordered in c‘-BaO2 layers. Extra oxygen is accommodated by intergrowth of the vacancy-ordered c‘-BaO2 layers with cubic c-BaO3-x layers (x ≅ 1), which have disordered 2/3 oxygen occupancy. This material displays mixed electronic and oxide ionic conduction at high temperature, and the role of the two distinct and partially occupied anion sites within the BaO2 layers in permitting the oxygen mobility is discussed.

Anion Composition Control and Magnetic Short- and Long-Range Order in Transition Metal Oxide Hydrides

Lanthanide transition metal oxide hydride phases, LnSrCoO3+αHβ (Ln = Pr, Nd), with variable H and O content, have been prepared by topotactic reduction of the parent n = 1 Ruddlesden−Popper phase, LnSrCoO4, with CaH2 via a LnSrCoO3.5 (Co2+) intermediate. The hydride anions occupy the site bridging the cations along the shorter in-plane direction in the CoOH plane. Substitution of oxide for hydride anions is incomplete leading to the compositions NdSrCoO3.084(4)H0.796(3) (A), NdSrCoO3.210(3)H0.580(6) (B), and PrSrCoO3.160(4)H0.680(7) (E) as determined by Rietveld refinement using powder neutron diffraction data. Diffraction, thermal gravimetry, and X-ray absorption spectroscopy show that the Co oxidation state remains close to +2 despite variation in hydride content. Variable temperature neutron powder diffraction studies show that the lanthanide size and H content strongly influence the Neel temperature (410 K (A), 375 K (B), and 445 K (E)). All the materials are antiferromagnets. Magnetic short-range ord...

Synthesis, Structure, and Magnetic Properties of Sr2NiOsO6 and Ca2NiOsO6: Two New Osmium-Containing Double Perovskites

Two new double perovskite oxides, Ca(2)NiOsO(6) and Sr(2)NiOsO(6), have been prepared as polycrystalline powders by solid state synthesis. The two oxides were structurally characterized by variable-temperature powder neutron diffraction. Ca(2)NiOsO(6) was found to adopt a monoclinic structure (P2(1)/n), while Sr(2)NiOsO(6) was found to be tetragonal (I4/m). Magnetic susceptibility measurements indicate that Ca(2)NiOsO(6) orders in a canted antiferromagnetic state at about 175 K while Sr(2)NiOsO(6) orders antiferromagnetically at about 50 K.

Variable temperature neutron powder diffraction study to determine the magnetic interactions inSr2LnRuO6 (Ln = Ho and Tb)

Neutron powder diffraction has been used to study the two title compounds,Sr2HoRuO6 and Sr2TbRuO6, in order to determine the crystal and magnetic structures of the materials. Both materialshave a distorted double perovskite structure and at temperatures below 30–40 K long-rangemagnetic order is observed. The magnetic structure for each compound consists of twointerpenetrating Type I arrangements, one for the ruthenium sublattice and one for therare earth sublattice, which are antiferromagnetically coupled to each other and order atthe same temperature. From variable temperature measurements the combination ofmagnetic interactions and their relative strengths was deduced. The antiferromagneticRu–O–O–Ru interaction is approximately constant in the compounds, but it is a factor of2.5 and 0.7 times stronger than the antiferromagnetic Ru–O–Ho and Ru–O–Tb interactionsrespectively.

Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO 3–CaZrO 3 system

Phase equilibria in the (1− x)BaZrO 3– xCaZrO 3 system were analyzed using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. The proposed phase diagram features two extended two-phase fields containing mixtures of a Ba-rich cubic phase and a tetragonal, or orthorhombic Ca-rich phase, all having perovskite-related structures. The symmetry differences in the Ca-rich phases are caused by different tilting patterns of the [ZrO 6] octahedra. In specimens quenched from 1650°C, CaZrO 3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO 3 is approximately 30 at% . The BaZrO 3–CaZrO 3 system features at least two tilting phase transitions, Pm3 m→ I4/ mcm and I4/ mcm→ Pbnm. Rietveld refinements of the Ba 0.8Ca 0.2ZrO 3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3 m→ I4/ mcm transition corresponds to a rotation of octahedra about one of the cubic axes; successive octahedra along this axis rotate in opposite directions. In situ variable-temperature electron diffraction studies indicated that the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately −120°C at 5 at% Ca to 225°C at 20 at% Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO 3 to 53 for Ba 0.9Ca 0.1ZrO 3, and then decreases to 50 for Ba 0.8Ca 0.2ZrO 3. This later specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment. Strongly non-monotonic behavior was also observed for the temperature coefficient of resonant frequency; however, in this case, the maximum occurred at a lower Ca concentration, 0.05⩽ x⩽0.1. The non-linear behavior of the dielectric properties was attributed to two competing structural effects: a positive effect associated with substitution of relatively small Ca cations on the A-sites, resulting in stretched Ca–O bonds, and a negative effect, related to the distortion of the A-site environment (bond strain relaxation) upon octahedral tilting.

Metal-Insulator Transition Induced by Short Range Magnetic Ordering in Mono-layered Manganite

The structural, magnetic, and transport properties of a mono-layered manganite La 0.7Sr1.3MnO4+δ were investigated using variable temperature neutron powder diffraction as well as magnetization and transport measurements. The compound adopts the tetragonal I4/mmm symmetry and exhibits no magnetic reflection in the temperature region of 10 K ≤ T ≤ 300 K. A weak ferromagnetic (FM) transition occurs about 130 K, which almost coincides with the onset of a metal-insulator (M-I) transition. Extra oxygen that occupies the interstitial site between the [(La,Sr)O] layers makes the spacing between the [MnO 2] layers shorten, which enhances the inter-layer coupling and eventually leads to the M-I transition. We also found negative magneto resistance (MR) below the M-I transition temperature, which can be understood on the basis of the percolative transport via FM metallic domains in the antiferromagnetic (AFM) insulating matrix.

Magnetic ordering in the rutile molecular magnetsMII[N(CN)2]2(M=Ni,Co, Fe, Mn,Ni0.5Co0.5,andNi0.5Fe0.5)

Rietveld refinement of powder neutron diffraction data, combined with group theory considerations, is used to determine the magnetic structures of the binary metal dicyanamide, ${M}^{\mathrm{II}}[{\mathrm{N}(\mathrm{CN})}_{2}{]}_{2}$ where $M=\mathrm{Ni},$ Co, Fe, Mn, ${\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5},$ and ${\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{0.5}.$ Compounds with $M=\mathrm{Mn}$ or Fe show a canted antiferromagnetic arrangement of spin oriented in the ab crystallographic plane, with antiparallel components of the two sublattices along the a axis and parallel along the b axis. Symmetry considerations forbid an additional moment, whether compensated or not, to be present along the c axis. The compounds with fewer unpaired electrons (Co and Ni) are ferromagnets, with all moments oriented along the c axis. The mixed composition of ${\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5}$ displays the same collinear ferromagnetic structure as its parent compounds. However, the composition with $M={\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{0.5},$ whose parent compounds show different magnetic behavior, does not exhibit long-range magnetic ordering down to 1.7 K. Magnetostriction was observed for the ferromagnets for which we investigated the variable temperature powder neutron diffraction. The cobalt-rich compounds show more pronounced effects, consistent with their increasing magnetocrystalline anisotropy.

Crystal and magnetic structures of A2YRu1−xCuxO6with A = Sr, Ba and x = 0.05 to 0.15

A series of mixed ruthenium–copper oxides of general formulae A2YRu1−xCuxO6 with A = Sr, Ba and x = 0.05 to 0.15 were prepared by high temperature synthesis. Electrical and magnetic measurements confirmed the samples were superconducting. Variable temperature neutron powder diffraction studies were undertaken to determine the crystal and magnetic structures of the materials in the temperature range 2–100 K. Both series are double perovskites with the Sr compounds adopting a distorted structure with space group P21/n and the Ba materials in the cubic space group Fm3m. The neutron diffraction patterns clearly show the development of extra peaks at temperatures ∼30–40 K indicative of long-range magnetic order. Refinements of the data indicate that the Ru5+ ions adopt a Type I magnetic structure for both series with a magnetic moment at 2 K of ∼2.4(1) μB in the ab plane. The magnetic moment is found to increase slightly with increased copper doping in the Sr series, whereas the Ba series has a higher magnetic ordering temperature due to higher symmetry of the crystal structure.

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...

Structure and magnetism in BaLaMnO4 +/– δ (δ = 0.00, 0.10) and BaxSr1 – xLaMnO4. Disappearance of magnetic order for x > 0.30.

Polycrystalline samples of BaLaMnO4, oxygen deficient and oxygen excess BaLaMnO4 ± δ and the solid solution BaxSr1 − xLaMnO4 have been prepared and investigated. Magnetic susceptibility, powder X-ray diffraction and variable temperature powder neutron diffraction data are presented for these compounds. BaLaMnO4 and BaLaMnO3.89 crystallize in space group I4/mmm with unit cell constants a = 3.90252(6) A, c = 13.2660(3) A and a = 3.9089(1) A, c = 13.3243(5) A. The unit cell constants for the tetragonal BaLaMnO4.1 are a = 3.9314(2) A, c = 13.051(1) A. For the δ = 0.00 compound a small ferromagnetic response appears in the susceptibility below 300 K and the Curie–Weiss θ = +5 K but no evidence for long range magnetic order of any type is seen above 6 K in neutron diffraction data. The ferromagnetic response is supressed in the reduced form, BaLaMnO3.89, but is enhanced in the oxidized form, BaLaMnO4.1, with the ferromagnetic onset moving to 350 K. The solid solution BaxSr1 − xLaMnO4 (0.00 ≤ x ≤ 0.34) also crystallizes in space group I4/mmm. We find antiferromagnetic long range ordering for SrLaMnO4 (x = 0.0), ordering vector k = [1/2 1/2 0], with TN = 128 K and a Mn3+ moment of 3.3 μB, contrary to a previous report. The Neel temperatures and ordered moment magnitudes decrease as the Ba concentration is increased from x = 0.00 to x = 0.29. For x > 0.29 all signs of antiferromagnetic long range order vanish. An explanation for the destruction of three-dimensional (3D) magnetic long range ordering is proposed based on the distance dependence of the 2D in-plane correlation length, the interplanar exchange coupling and competing interactions.

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.