High-temperature Neutron Diffraction Research Articles

Structure of Fast Ion Conductors Li3xLa2/3-xTiO3 Deduced from Powder Neutron Diffraction Experiments

High-temperature neutron diffraction (ND) experiments have been carried out in two representative members of the Li3xLa2/3-xTiO3 series (x = 0.067 and 0.167), to analyze the influence of temperature on the perovskite structure. In the orthorhombic Li-poor ordered member Li0.2La0.6TiO3 (Cmmm space group), prepared by slowly cooling from 1673 K, the heating of the sample produced the elimination of the octahedral tilting along the b-axis (a0b-c0 scheme) at 873 K, requiring higher temperatures to induce the vacancies disordering. In the rhombohedral Li-rich disordered member Li0.5La0.5TiO3 (R3c space group), prepared by quenching from 1673 K, the sample heating produced the elimination of the octahedral tilting along the [111] direction (a-a-a- scheme) at 1073 K. The elimination of octahedral tilting drived the orthorhombic−tetragonal transformation in the first perovskite and the rhombohedral−cubic transformation in the second perovskite. A detailed analysis of four identified phases showed that distortion...

Crystal Structure of Tricalcium Phosphate investigated by High-Temperature Neutron Diffraction Method

Crystal Structure of Tricalcium Phosphate investigated by High-Temperature Neutron Diffraction Method

Deuterium site energy difference in ZrTi 2D 4.3 as studied by high-temperature neutron diffraction

The deuterium atom distribution in the deuterium stabilized C15-type Laves phase derivative ZrTi 2D 4.3 has been studied by neutron powder diffraction in the temperature range 300 K < T < 1000 K. As the temperature is increased the occupancy of the tetrahedral ZrTi 3-type interstices decreases from 89% at 300 K to 56% at 800 K while that of the tetrahedral Zr 2Ti 2-type interstices increases from 7% to 17%. The deuterium site energy difference derived from these data is ∼127 meV. Above 800 K the compound loo\\ses deuterium mainly from the ZrTi 3-type interstices while its metal atom substructure becomes partially disordered. This disorder persists after cooling to room temperature.

Why does uranium oxide phosphate contract on heating?

(U2O)(PO4)2 is related to ultra-low expansion β-(Zr2O)(PO4)2 ceramics, but shows a continuous thermal contraction. High-temperature neutron diffraction has allowed to follow accurately the thermal variations of its cell edges and to give a structural explanation to the phenomenon: like in β-(Zr2O)(PO4)2, the dilatometric anomaly arises simultaneously from a contractive push–pull effect due to Coulombic repulsions and from a libration of the PO4 and UO7 polyhedra, but in the present case, the second mechanism predominates. The size of the tetravalent cation appears as a key parameter in monitoring the thermal expansion of ceramics of this family.

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.

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.

Neutron Diffraction Studies of Si-Mn Trip Steel In Situ upon Thermomechanical Processing

An excellent combination of high strength and formability can be obtained in a Si-Mn TRIP steels when processed by thermomechanical treatment consisting of high temperature deformation followed by isothermal holding in the bainite region and cooling to room temperature. Microstructure of these steels consists of ferrite, bainite and retained austenite. The combination of high strength and ductility is attributed to the transformation-induced plasticity (TRIP) effect resulting from the strain induced transformation of the retained austenite to martensite. These steels exhibit tensile strengths ranging 800-1000 MPa and elongation up to 30%. In the present paper, the influence of transformation conditions on volume fraction of ferrite and retained austenite in selected 0.2C1.9Si1.45Mn TRIP steel was investigated. Based on results of in situ high-temperature neutron diffraction experiments, the set of specimens was prepared and their transformation characteristics at room temperature were studied again by neu...

Nitrogen ordering in ζ-manganese nitrides with hcp arrangement of Mn – MnN y with 0.39 < y < 0.48 – determined by neutron diffraction

The crystal structures occurring in the ζ-region of the Mn–N phase diagram were investigated by X-ray and high temperature neutron diffraction within a range of compositions MnN y with 0.39< y<0.48. The ζ-region consists of several phases all based on an hcp arrangement of Mn. They differ by their arrangements of N atoms on the octahedral sites. A high temperature ‘γ-type’ phase of defect anti-NiAs type, a hcp× a hcp× c hcp, space group P6 3/mmc, is disordered with respect to nitrogen. Three long-range nitrogen ordered superstructure phases exist: (1) an ‘ε-type’ superstructure, observed below 570 K for MnN 0.388 and MnN 0.421 with 3 1/2 a hcp×3 1/2 a hcp× c hcp with respect to an hcp unit cell. Powder diffraction data can be equally well described in P6 322, P312 or P 3 ̄ 1 m ; (2) a ‘ζ-type’ superstructure, observed below about 870 K for MnN 0.457 and MnN 0.472 with anti-α-PbO 2 type structure with 2 a hcp×3 1/2 a hcp× c hcp, space group Pbna; (3) a ‘β-type’ superstructure observed above 870 K for MnN 0.484 with anti-CaCl 2 type superstructure, a hcp×3 1/2 a hcp× c hcp, space group Pmnn. These findings require a revision of the Mn–N phase diagram that is, up to now, presented with only one single ζ-phase in the literature.

In situ Observations of Phase Transition Using High‐Temperature Neutron and Synchrotron X‐Ray Powder Diffractometry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Characterisation of La 2NiO 4+ δ using in-situ high temperature neutron powder diffraction

La 2NiO 4+ δ , has been studied using in-situ high temperature neutron diffraction over a temperature range of 25–800 °C in vacuum. The behaviour of this material, and in particular the oxygen interstitial content, is discussed and quite remarkable bond length changes observed. It is observed that at temperatures above 150 °C La 2NiO 4+ δ transforms to the tetragonal I4/ mmm structure and maintains this over the entire temperature range on both heating and cooling. The loss of the interstitial oxygen was observed over the low temperature region of the study and significant changes in both lattice constant and bond lengths found to mirror these changes, indicating the structural importance of the interstitial oxygen.

In Situ Observations of Phase Transition Using High‐Temperature Neutron and Synchrotron X‐ray Powder Diffractometry

It is of vital importance for the study of phase diagrams to investigate in situ diffusionless phase transition points at high temperatures. We have developed high‐temperature techniques to measure in situ neutron and synchrotron X‐ray powder diffractometry profiles. This paper reviews our recent studies obtained using these methods. The new furnace for high‐temperature neutron diffraction study yields no extra peaks caused by heaters and refractory; thus, we have been able to measure very small neutron signals from the sample. Quality neutron diffraction data at high temperatures up to ∼1590°C have been analyzed using the Rietveld method for various materials, such as zirconia solid solutions and calcium titanate. High‐resolution synchrotron X‐ray diffractometry enables exact determination of the phase transition temperatures of perovskite‐related materials.

Reversible melting and equilibrium phase formation of (Bi, Pb)2Sr2Ca2Cu3O10+δ

The decomposition and the re-formation of the (Bi, Pb)2Sr2Ca2Cu3O10+δ (Bi, Pb(2223)) phase have been investigated in situ by means of high-temperature neutron diffraction, both in sintered bulk samples and in Ag-sheathed monofilamentary tapes. Several decomposition experiments were performed at various temperatures and under various annealing atmospheres, under flowing gas as well as in sealed tubes, in order to study the appropriate conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was found to melt incongruently into (Ca, Sr)2CuO3, (Sr, Ca)14Cu24O41 and a Pb,Bi-rich liquid phase. Phase re-formation after melting was successfully obtained both in bulk samples and in Ag-sheathed tapes. The possibility of crystallizing the Bi,Pb(2223) phase from the melt was found to be extremely sensitive to the 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 Pb losses are responsible for moving away from equilibrium and therefore hinder the re-formation of the Bi,Pb(2223) phase from the melt. Thanks to the full pattern profile refinement, a quantitative phase analysis was carried out as a function of time and temperature and the role of the secondary phases was investigated. Lattice distortions and/or transitions were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212), (Ca, Sr)2CuO3 and (Sr, Ca)14Cu24O41, due to cation diffusion and stoichiometry changes. The results indicate that it is possible to form the Bi,Pb(2223) phase from a liquid close to equilibrium conditions, such as Bi(2212) and Bi(2201), and open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape processing.

Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO3 Perovskites at High Temperatures by Neutron Diffraction

The influence of small amounts of Fe3+ on the phase transitions of CaTiO3 perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO3 is shown by both CaTi0.9Fe0.1O2.95 and CaTi0.8Fe0.2O2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3m at high temperature, with an intermediate I4/mcm tetragonal phase which exists over a temperature range of about 100°C. The two phase boundaries in the temperature vs composition phase diagram of the system CaFexTi1−xO3−x/2 (0≤x≤0.4) decrease in a quasi-linear manner with increasing Fe content up to x=0.2 and then they both drop abruptly to RT. The existence of a second orthorhombic phase (Cmcm), which has been postulated for CaTiO3, is ruled out in the Fe-doped CaTiO3 perovskites in view of the behavior of specific diffraction peaks. Strain analysis shows first-order thermodynamic character for the Pnma→I4/mcm transition, while the character of the Pm3m→I4/mcm transition could be second order or tricritical. Shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models. The volume strain has an anomalous coupling with the order parameter components, which appears to be temperature-dependent.

Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO 3 Perovskites at High Temperatures by Neutron Diffraction

The influence of small amounts of Fe 3+ on the phase transitions of CaTiO 3 perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO 3 is shown by both CaTi 0.9Fe 0.1O 2.95 and CaTi 0.8Fe 0.2O 2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3 m at high temperature, with an intermediate I4/ mcm tetragonal phase which exists over a temperature range of about 100°C. The two phase boundaries in the temperature vs composition phase diagram of the system CaFe x Ti 1− x O 3− x/2 (0≤ x≤0.4) decrease in a quasi-linear manner with increasing Fe content up to x=0.2 and then they both drop abruptly to RT. The existence of a second orthorhombic phase ( Cmcm), which has been postulated for CaTiO 3, is ruled out in the Fe-doped CaTiO 3 perovskites in view of the behavior of specific diffraction peaks. Strain analysis shows first-order thermodynamic character for the Pnma→ I4/ mcm transition, while the character of the Pm3 m→ I4/ mcm transition could be second order or tricritical. Shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models. The volume strain has an anomalous coupling with the order parameter components, which appears to be temperature-dependent.

In situstructural study of ceramic materials by high-temperature neutron and high-resolution synchrotron X-ray powder diffraction techniques

<i>In situ</i>structural study of ceramic materials by high-temperature neutron and high-resolution synchrotron X-ray powder diffraction techniques

Jahn-Teller transition inLa1−xSrxMnO3in the low-doping region(0&lt;x&lt;~0.1)

We have investigated the Jahn-Teller transition in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ by high-temperature powder neutron diffraction in the low-doping region $(0lxl~0.1).$ The Jahn-Teller transition temperature which is about ${T}_{\mathrm{JT}}\ensuremath{\approx}750\mathrm{K}$ for stoichiometric ${\mathrm{LaMnO}}_{3}$ is drastically reduced on doping with Sr, becoming ${T}_{\mathrm{JT}}\ensuremath{\approx}475\mathrm{K}$ for $x=0.10.$ The previously identified metrically cubic orthorhombic O phase (Pmnb) for $x=0$ becomes clearly (also metrically) orthorhombic for small doping $x=0.05$ and remains so in the doping range investigated. The Jahn-Teller transition is accompanied only by the partial reduction of the distortion of the ${\mathrm{MnO}}_{6}$ octahedra. From the distortion of the ${\mathrm{MnO}}_{6}$ octahedra we have determined the orbital mixing coefficients.

Large low-field magnetoresistance and TC in polycrystalline (Ba0.8Sr0.2)2−xLaxFeMoO6 double perovskites

Large low-field magnetoresistance (LFMR) together with high Curie temperatures (TC) are requirements for some applications in magnetoelectronics. In order to optimize both parameters, we have investigated double perovskites (Ba0.8Sr0.2)2−xLaxFeMoO6 (0⩽x⩽0.4). High-temperature neutron diffraction measurements indicate a strong increase in TC with La doping (from ≈345 K for x=0 to ≈405 K for x=0.4). The LFMR is very large for x=0 (at 10 KOe≈27% at 10 K and ≈7% at 290 K) and decreases with La doping. This decrease cannot be attributed to a substantial enhancement of Fe/Mo antisite disorder, which is small as tracked by means of x-ray and high-resolution neutron diffraction, but to grain boundaries modifications.

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba2In2O5 and perovskite structured Ba(InxZr1−x)O3−x/2. The ambient temperature structure of Ba2In2O5 is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In3+) ions of the type observed previously in Sr2Fe2O5. Ba2In2O5 undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(InxZr1−x)O3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.

Thermal expansion anisotropy in a Ti–6Al–4V/SiC composite

We studied the thermal expansion behavior of a Ti–6Al–4V/35 vol.% continuous SiC fiber composite using in situ high temperature neutron diffraction (ND). The lattice expansion of constituent phases within the composite was monitored from axial (parallel to the unidirectionally aligned fibers) and transverse (perpendicular to the fibers) directions during heating from room temperature (RT) to 1170 K. The phase-specific thermal expansion of the Ti–6Al–4V matrix and SiC fibers in the composite is discussed in the context of thermal load partitioning between the matrix and fibers. In the axial direction, the matrix and the fiber share the thermal load and co-expand up to about 800–900 K, above which the thermal load transfer becomes ineffective. In the transverse direction, the matrix and fibers expand independently over the whole temperature range. Using the Schapery model (J. Comp. Mater. 2 (1968) 380) and the rule-of-mixtures (ROM), the macroscopic thermal expansion of the composite is predicted and compared with the experimental results.