High-temperature Neutron Powder Diffraction Research Articles

Neutron powder diffraction study at high temperature of the Ruddlesden–Popper phase Sr 3Fe 2O 6 + δ

The crystal and oxygen defect structure of the n = 2 Ruddlesden–Popper phase Sr 3Fe 2O 6 + δ have been studied by in situ high temperature neutron powder diffraction in the temperature range 20 ≤ T ≤ 900 °C in air. The analysis of the neutron diffraction data revealed the presence of structural oxygen vacancies on both the O(1) sites linking the octahedra along the c axis and the O(3) sites in the FeO 2 planes of the perovskite layers. The oxygen vacancies on the O(3) site increase with temperature up to ∼ 0.25 per formula unit at T = 900 °C. This result supports previously proposed oxygen ion diffusion mechanism in Sr 3Fe 2O 6 + δ that involves the migration of vacancies from an O(3) site to an adjacent O(1) site. The total linear expansion along the c axis α c = 17.7(5) · 10 − 6 K − 1 mainly affects the perovskite block while the width of the rock salt layers remains stable with temperature. The total volumetric expansion α V / 3 = 20(1) · 10 − 6 K − 1 is around the average of the TEC values (14.8–27.1 K − 1) reported for the perovskite system La 1 − x Sr x Co 1 − y Fe y O 3 − δ .

Phase Transitions of the Magnetoelectric A2NiMoO6 (A = Ba, Sr) and Ca2NiWO6 by Neutron Diffraction

The magnetic ferroelectric double perovskites A 2 NiMoO 6 (A = Ba, Sr), and Ca 2 NiWO 6 have been characterised using neutron powder diffraction (NPD) data. The Ba 2 NiMoO 6 compound has previously been reported in cubic or hexagonal symmetry which led to investigation of polymorphism. High temperature NPD data of the Sr 2 NiMoO 6 compound indicates a continuous structural phase transition from tetragonal to cubic symmetry at approximately 550 K. The reduction in symmetry is linked to spontaneous polarization allowing the structural perturbation to be described as an improper ferroelectric phase transition. The Ca 2 NiWO 6 compound shows similar behaviour as Sr 2 NiMoO 6 above 700 K, possibly P21/n → (I2/m) → 14/m → Fm-3m.

Structure phase transition in FeSr 2YCu 2O 6+δ

FeSr 2YCu 2O 6+ δ exhibits superconductivity around 50 K, when it is properly annealed in N 2 atmosphere and subsequently in O 2 atmosphere, although as-synthesized FeSr 2YCu 2O 6+ δ does not exhibit superconductivity. To understand the effect of annealing on superconductivity, we have performed in-situ high-temperature neutron powder diffraction measurements in N 2 atmosphere. We have found that the N 2 annealing at 650 °C promoted the atomic ordering of Cu and Fe and that the phase transition occurred from tetragonal structure to orthorhombic superstructure at 750 °C. The atomic ordering of Cu and Fe did not change at all during the following annealing process. Consequently, this compound exhibits superconductivity by supplying the carrier onto the CuO 2 sheet.

In situ high temperature neutron powder diffraction study of oxygen-rich La2NiO4+δin air: correlation with the electrical behaviour

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 700 °C on O2-treated (350 °C, 200 bar) La2NiO4+δ. A structural transition from the orthorhombic Fmmm to the tetragonal F4/mmm space group takes place at about 150 °C. The reversibility of this transition has been determined to be strongly dependent on the sample oxygen content. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O2-treated La2NiO4+δ exhibits a dirty-metal (high T)-to-semiconducting (low T) transition as a function of temperature, displaying a maximum value of 82 S cm−1 at around 400 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.19(1) A), revealing a major anisotropic component for the electronic transport. The interstitial oxygens occupy the 16j and 16e positions in the low and high temperature phases, respectively. The refined oxygen occupancy from NPD data is in quite good agreement with the thermogravimetric data. Good thermal stability of the oxygen content has been observed in the studied temperature range, as required for practical applications.

In situ Neutron Diffraction Study on Fast Oxide Ion Conductor LaGaO3‐Based Perovskite Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Space group and crystal structure of the Perovskite CaTiO 3 from 296 to 1720 K

The structural phase transitions of the calcium titanate perovskite CaTiO 3 were investigated by the Rietveld analysis of high-temperature neutron and X-ray powder diffraction data in the temperature range of 296–1720 K. The present work demonstrates that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/ mcm and of I4/ mcm-cubic Pm 3 ¯ m at 1498±25 K and 1634±13 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/ mcm structures. The lattice parameters discontinuously change at the Pbnm– I4/ mcm transition point, while a continuous change is observed for the I 4 / mcm – Pm 3 ¯ m transformation. These results indicate that the Pbnm– I4/ mcm transition is of first order and that the I 4 / mcm – Pm 3 ¯ m transformation is of either second or higher order.

In Situ Neutron Diffraction Study on Fast Oxide Ion Conductor LaGaO3-Based Perovskite Compounds

The crystal structures of LaGaO3, LaGa0.9Mg0.1O2.95, and La0.9Sr0.1Ga0.9Mg0.1O2.9 have been investigated by high-temperature neutron powder diffraction in order to clarify a mechanism for high oxide ion conduction properties in LaGaO3-based compounds. Parent phase LaGaO3 crystallized in the orthorhombic (Pbnm) up to 150 °C and the rhombohedral (R3c) at between 160 and 800 °C. LaGa0.9Mg0.1O2.95 crystallized in the orthorhombic (Ibmm) up to 340 °C and the rhombohedral (R3c) at between 350 and 800 °C. On the other hand, La0.9Sr0.1Ga0.9Mg0.1O2.9 crystallized in the monoclinic (I2/a) up to 400 °C and the rhombohedral (R3c) at between 410 and 800 °C. The correlations between the crystal structures and conduction properties of LaGaO3-based materials were discussed on the basis of the analytical results.

Order-disorder octahedral tilting transitions inSrSnO3perovskite

High-temperature powder neutron diffraction has been used to characterize two structural phase transitions in the perovskite ${\mathrm{SrSnO}}_{3}$. A continuous orthorhombic Pnma to Imma symmetry transition occurs at 636 \ifmmode^\circ\else\textdegree\fi{}C, and a first-order transition to a tetragonal $I4∕\mathit{mcm}$ phase occurs at 800 \ifmmode^\circ\else\textdegree\fi{}C. Such octahedral tilting transitions are very common in perovskite oxides and are generally assumed to be displacive. However, a displacive description of the transitions in ${\mathrm{SrSnO}}_{3}$ gives physically unrealistic results, and a considerable dynamic order-disorder component is found. A more realistic order-disorder modeling shows that the population difference between majority and minority tilt states is an appropriate order parameter, and the onset of tilting fluctuations $\ensuremath{\sim}300\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ below the Pnma to Imma transition leads to an unusual divergence of the lattice parameters on warming towards the transition.

Structural Behavior of Oxygen Permeable SrFe0.2Co0.8Ox Ceramic Membranes with and Without pO2 Gradients

The oxygen partial pressure (pO2)‐dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe0.2Co0.8Ox with cubic perovskite structure have been investigated by high‐temperature neutron powder diffraction: one in “static” mode and one in simulated‐operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable pO2 levels. Rietveld analysis on data collected for the membrane without pO2 gradients showed that the perovskite is stable in pO2 down to ∼10−12 atm, and at ∼10−14 atm it starts to decompose into a three‐phase mixture containing layered intergrowth Ruddlesden–Popper phases Srn+1(Fe,Co)nOx with n=2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a pO2 gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a pO2 profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900°C were estimated to be 0.4–0.9 sccm/cm2 for the ∼1 mm thick membrane containing perovskite, depending upon pO2 gradient.

Phase equilibria in the Bi 2TeO 5Bi 2SeO 5 system and a high temperature neutron powder diffraction study of Bi 2SeO 5

Phase equilibria in the Bi 2TeO 5Bi 2SeO 5 system were studied by X-ray, DTA and second harmonic generation (SHG). The samples were synthesized by solid state reactions of the Bi, Te and Se oxides. The phase diagram is interpreted as a quasibinary peritectic one with wide ranges of solid solutions on the basis of both compounds. The SHG study showed Bi 2SeO 5 to undergo a phase transition at about 250 °C. Neutron diffraction (25–650 °C) showed no major changes in the structure of Bi 2SeO 5 at high temperatures. However, the analysis of the oxygen atom thermal factors and site occupancies suggested that the mechanism of the phase transformation is an order–disorder transition involving reorientation of the SeO 3 group.

Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalides—II: ordered phases within the (Ag X) x–( MX) 1− x and (Cu X) x–( MX) 1− x ( M=K, Rb and Cs; X=Cl, Br and I) systems

The crystal structures of 23 ternary phases present in the systems ( AX) x –( MX) 1− x ( A=Ag, Cu; M=K, Rb and Cs; X=Cl, Br and I) have been determined and/or refined using X-ray and neutron diffraction studies of powder samples. A total of 11 x=0.333 phases of stoichiometry M 2 AX 3 are found (Rb 2AgCl 3, Cs 2AgCl 3, Rb 2AgBr 3, Cs 2AgBr 3, K 2AgI 3, Rb 2AgI 3, Cs 2AgI 3, K 2CuCl 3, K 2CuBr 3, Rb 2CuBr 3 and Rb 2CuCl 3) which crystallize in one of two closely related crystal structures with space group Pnma. The x=0.4 composition is characterized by three compounds Cs 3Cu 2 X 5 ( X=Cl, Br, I) which all adopt space group Pnma, whilst the x=0.5 case comprises the two compounds CsAgCl 2 and CsAgBr 2, which possess space group Cmcm. The latter undergo phase transitions at 408(5) and 413(8) K, respectively, to higher symmetry structures in space group P4/ nmm. Five x=0.667 compounds of stoichiometry MCu 2 X 3 have been identified (CsCu 2Cl 3, CsCu 2Br 3, CsCu 2I 3, RbCu 2Br 3, and RbCu 2I 3). Together with CsAg 2I 3, these form a family of compounds, which crystallize in one of two closely related structures (space group Pbnm or Cmcm). Impedance spectroscopy measurements indicate that all the above-mentioned compounds have relatively low values of ionic conductivity ( σ<10 −4 Ω −1 cm −1 at room temperature). Contrary to previous reports, no evidence of high conductivity phases of composition CsCu 9Br 10 and CsCu 9I 10 was obtained during high-temperature powder neutron diffraction studies. Some general remarks concerning the structural properties of these systems are given, together with a discussion of the influence of the M + cations on the ionic conductivity. Finally, the structural information for all the ambient temperature phases has been used to derive improved bond valence parameters for Ag– X and Cu– X bonds.

Orbital order–disorder transition in La 1− xBa xMnO 3 in the low-doping region

We have investigated the Jahn–Teller transition accompanied by orbital order–disorder transition in La 1− x Ba x MnO 3 by high temperature powder neutron diffraction in the low-doping region (0≤ x≤0.1). The Jahn–Teller transition temperature which is about T JT ≈750 K for the stoichiometric LaMnO 3 decreases drastically on doping with Ba. We have shown that the previously identified metrically cubic orthorhombic O-phase for x=0 is metrically orthorhombic and the orthorhombicity increases with doping. The Jahn–Teller transition is accompanied only by the partial reduction of the distortion of the MnO 6 octahedra. From the distortion of the MnO 6 octahedra we have determined the orbital mixing coefficients. Negative thermal expansion has been observed in stoichiometric LaMnO 3 in a temperature range of about 75 K just below T JT ≈750 K. The negative thermal expansion has also been observed in La 1− x Ba x MnO 3 below T JT but the effect is reduced with doping.

Neutron diffraction study of KNO 3 at elevated temperatures

High-temperature powder neutron diffraction measurements of KNO 3 have shown co-existence of the α- and β-phases in the temperature range 112–132°C and presence of diffuse scattering in the β-phase.

Volume collapse inLaMnO3at the Jahn-Teller transition temperature

We have studied the volume collapse of ${\mathrm{LaMnO}}_{3}$ at the Jahn-Teller (JT) transition temperature ${T}_{\mathrm{JT}}=750\mathrm{K}$ which has recently been found in high-temperature powder x-ray and neutron diffraction experiments. We construct a model Hamiltonian involving the pseudospin of ${\mathrm{Mn}}^{3+}$ ${e}_{g}$ states, the staggered JT distortion and the volume strain coordinate. We show that the anharmonic coupling between these primary and secondary order parameters leads to the first-order JT phase transition associated with a comparatively large reduction of the unit cell volume of $\ensuremath{\Delta}V/V\ensuremath{\simeq}{10}^{\ensuremath{-}2}.$ We explain the temperature dependence of JT distortions and volume strain and discuss the volume change as function of the anharmonic coupling constant. A continuous change to a second-order transition as function of model parameters is obtained. This behavior is also observed under Ba doping.

Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Characterization of La2NiO4+δ Using in situ High Temperature Neutron Powder Diffraction.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Partial melting and HIP processing of Bi(2223): bulk and tapes

A detailed investigation of the equilibrium between the Bi,Pb(2223) phase and the melt was carried out by in-situ high temperature neutron powder diffraction both on sintered bulk samples and Ag-clad tapes. Stability, decomposition and re-formation of Bi,Pb(2223), as well as the evolution of secondary phases, were studied and the effect of the oxygen partial pressure was investigated. Bi,Pb(2223) melts incongruently to (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/, (Sr,Ca)/sub 2/CuO/sub 3/, and a Bi,Pb-rich liquid, and no precipitation of Bi(2212) was observed at this stage. Direct formation of the Bi,Pb(2223) phase from the melt was observed, opening up the possibility of processing Ag-sheathed tapes from a partial melting route. The possibility of reforming the Bi,Pb(2223) phase from the melt proved to be extremely sensitive to temperature and strongly dependent on the Pb-losses. The study of the mass losses due to Pb-evaporation was complemented by thermogravimetric analysis which proved that the Pb-losses are responsible for moving away from the equilibrium and therefore hinder the Bi,Pb(2223) to reversibly form from the melt. Hot isostatic pressure has proved to be an effective remedy for preventing volatile elements from evaporating and for increasing the density of the tapes. The first annealing stage of the Ag-Bi,Pb(2223) processing was performed under isostatic pressure after making the Ag-sheath airtight. Positive effects of pressure on filament density, formation kinetics and critical current were observed at 10 MPa. We have extended our search for an alternative processing route to the Pb-free Bi(2223) phase. High purity Bi(2223) bulk samples and Ag-sheathed tapes were successfully prepared from pre-reacted powders. Very large Bi(2223) grains (up to /spl sim/500 /spl mu/m) were observed to grow at the Ag-ceramic interface.

Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series

120.334(3)°, and V ) 620.46(3) A 3 ) and the fits were satisfactory for such complex pseudohexagonal structure. This structure contains 72 variable positional parameters and 24 thermal factors. High-temperature neutron powder diffraction (NPD) data were also collected at 773 and 1173 K for hexagonal La9.60(GeO4)6O2.40. The electrical results suggest that the samples are bulk oxide ion conductors. The plots of the imaginary parts of the impedance, Z′′, and the electric modulus, M′′, vs log(frequency), possess maxima for both curves separated by less than a half decade in frequency with associated capacities of 2 pF. The curvatures observed in the Arrhenius plots are not due to a phase transition. The conductivities are almost independent of the oxygen partial pressure under oxidizing conditions, which suggests pure oxide-ion conduction with negligible electronic contribution.

High-temperature neutron powder diffraction study of the structural phase transition between α and α′ phases in tricalcium phosphate Ca 3(PO 4) 2

The α– α ′ phase transition of tricalcium phosphate (TCP) has been investigated using high-temperature neutron powder diffraction and the Rietveld method. This transition occurs between 1458 and 1508 °C. Hysteresis is not observed between heating and cooling within the observed temperature interval of 25 °C. Structural refinement of the α ′-TCP was successfully performed by the trigonal structure with space group P 3 ̄ m . The temperature dependence of unit-cell parameters of the TCP was determined around the α– α ′ phase transition point for the first time. At the α– α ′ phase transition point, the unit-cell parameters a, c, γ angle, and unit-cell volume discontinuously changed, indicating that the transition is of first order.

High-temperature neutron powder diffraction study of cerium dioxide [formula omitted] up to 1770 K

We have investigated the temperature dependence of unit-cell and structural parameters of ceria (cerium dioxide, CeO 2 ) from room temperature to 1770 K by neutron powder diffraction and the Rietveld method. The unit-cell parameter increases continuously with temperature. It was confirmed that the Debye–Waller factor of oxygen B(O) is larger than that of cerium B(Ce) at any temperature from room temperature to 1770 K. Both B(O) and B(Ce) increase with an increase of temperature.