Oxygen Sublattice Disorder Research Articles

The role of vanadium substitution in the oxygen sublattice disorder of Ba7Nb4MoO20-based hexagonal perovskite oxide-ion conductors.

Ba7Nb4MoO20-based hexagonal perovskite derivatives are promising oxygen-ion conductors for solid electrolytes in solid-oxide fuel cells and electrolysers. A thorough understanding of chemical substitution and its impact on structural features conducive to high ionic conductivity is fundamental for decreasing the operation temperature of such devices. Here, a new 7H polytype-based composition, namely Ba7Nb3.9-x V x Mo1.1O20.05, is investigated to assess the effect of vanadium substitution. Structural changes upon V incorporation are studied using X-ray and neutron diffraction, as well as 51V and 93Nb solid-state nuclear magnetic resonance spectroscopy. For the undoped composition at room temperature, two distinct oxygen sites (O1 and O5) are found along the palmierite-like layer, corresponding to a mix of four- and six-fold coordination for adjacent M2 cations. At high temperature (527 °C), reorganization of oxygen results in the major occupation of O1 and four-fold (tetrahedral) coordination of the M2 cations. The same rearrangement is observed upon V-substitution, but already at room temperature. From 51V NMR, we identified a tetrahedral coordination for V5+ cations, indicating their preferential occupation of the M2 site. This preferential occupation by V5+ cations is correlated with increasing tetrahedral coordination of Nb5+ cations as observed from 93Nb NMR. Altogether, these observations indicate that V-substitution impacts the oxygen sublattice so as to mimic the high-temperature structure. Additionally, BVSE calculations demonstrate a decreasing energy barrier for O2- migration associated with the presence of vanadium in the structure. This conclusion corroborates the hypothesis that vanadium's propensity for a lower coordination number is beneficial for promoting high O2- mobility in this promising class of oxide-ion conductors.

In-situ kinetics of modifications induced by swift heavy ions in Al2O3: Colour centre formation, structural modification and amorphization

This paper details in-situ studies of modifications induced by swift heavy ion irradiation in α-Al2O3. This complex behaviour is intermediary between the behaviour of amorphizable and non-amorphizable materials, respectively. A unique combination of irradiation experiments was performed at the IRRSUD beam line of the GANIL facility, with three different characterisation techniques: in-situ UV–Vis absorption, in-situ grazing incidence X-Ray diffraction and ex-situ transmission electron microscopy. This allows a complete study of point defects, and by depth profile of structural and microstructural modifications created on the trajectory of the incident ion. The α-Al2O3 crystals have been irradiated by 92 MeV Xenon and 74 MeV Krypton ions, the irradiation conditions have been chosen rather similar with an energy range where the ratio between electronic and nuclear stopping power changes dramatically as function of depth penetration. The main contribution of electronic excitation, above the threshold for track formation, is present beneath the surface to finally get almost only elastic collisions at the end of the projected range. Amorphization kinetics by the overlapping of multiple ion tracks is observed. In the crystalline matrix, long range strains, unit-cell swelling, local microstrain, domain size decrease, disordering of oxygen sublattice as well as colour centre formation are found. This study highlights the relationship between ion energy losses into a material and its response. While amorphization requires electronic stopping values above a certain threshold, point defects are predominantly induced by elastic collisions, while some structural modifications of the crystalline matrix, such as unit-cell swelling, are due to contribution of both electronic and nuclear processes.

Oxygen Disorder, a Way to Accommodate Large Epitaxial Strains in Oxides

Strain accommodation in oxide thin films: Density-functional calculations (total-energy comparisons) and checks for negative-frequency phonon modes are employed as a stability indicator to show that, in rutile- and fluorite-structure oxides, e.g., zirconia strained by a strontium titanate substrate, oxygen-sublattice disorder can be the energetically preferred way to accommodate strain.

Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

High‐temperature time‐of‐flight neutron diffraction experiments were performed on cubic yttria‐stabilized zirconia (YSZ, 10 mol% YO1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO2 laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10−6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10−6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10−6/K at 2350°C–2550°C with oxygen displacement parameters (Uiso) reaching 0.1 Å2, similar to behavior observed in UO2. Acquisition of powder‐like high‐temperature neutron diffraction data from solid‐levitated samples is feasible and possible improvements are outlined. This methodology should be applicable to a wide range of materials for high‐temperature applications.

Influence of the Lanthanide Oxides on the Catalytic Behavior of Au/Al<SUB>2</SUB>O<SUB>3</SUB> Catalysts for Total and Preferential CO Oxidation

In this work the influence of the lanthanide series oxide addition to gold supported alumina catalyst is discussed. A clear promoting effect was observed no matter the employed reaction. Nevertheless, the presence of hydrogen in the oxidation mixture reveals interesting dissimilarities within the series of studied oxides. The differences in the catalytic behaviour of the samples are correlated to the crystal structure variations, oxygen sub-lattice disorder, gold presence and oxide's ability to undergo hydration/dehydration reactions.

Molecular dynamics simulation of dislocations in uranium dioxide

The plasticity of the fluorite structure in UO2 is investigated with molecular dynamics simulation and empirical potential. The stacking fault energies and the dislocation core structures with Burgers vector a2〈110〉 are systematically calculated. All dislocation core structures show a significant increase of the oxygen sub-lattice disorder at temperatures higher than 1500K. The threshold stress for dislocation glide is found to decrease with increasing temperature but its values is always very high, several GPa at 0K and several hundred of MPa at 2000K. A relation between the dislocation mobility dependence with temperature and the increase of the oxygen sub-lattice disorder in the dislocation cores is established.

Synthesis, structure and photoluminescence properties of europium-, terbium-, and thulium-doped Ca3Bi(PO4)3 phosphors

A series of Eu(3+), Tb(3+), Tm(3+) singly and triply doped Ca3Bi(PO4)3 (CBP) phosphors were synthesized by solid-state reaction. Their structure and photoluminescence (PL) properties were first investigated in detail. Rietveld refinement analysis confirmed that the CBP has a cubic unit cell. Its space group was determined to be I4[combining macron]3d with cell parameters a = b = c = 9.941 Å and Z = 4. In addition, the Ca3Bi(PO4)3 shows both cation disorder and oxygen sublattice disorder. For the CBP:Eu(3+) phosphor, the charge transfer bands in the PLE spectra were different at 293 K and 4.3 K, and a model was presented to give a possible explanation for this phenomenon. CBP:Eu(3+) shows intense red emission due to (5)D0-(7)F2 transition and its integral emission intensity and quantum efficiency are higher than the commercial Y2O3:Eu(3+) phosphor under irradiation of 397 nm, indicating that the CBP:Eu(3+) phosphor might have potential application in the NUV range for solid-state lighting. The CBP:Tb(3+) and CBP:Tm(3+) phosphors show intense green and blue emission due to (5)D4-(7)F2 transition of the Tb(3+) ions and the (1)D2-(3)F4 transition of the Tm(3+) ions, respectively. The energy transfer from the Tb(3+) to Eu(3+) ions was also validated by the spectra and decay curves of the Tb(3+) ions. Tunable emission colors were obtained by triply doping Eu(3+), Tb(3+) and Tm(3+) activators in a single host and adjusting their relative ratio.

Melting and superionic transition of Gd-doped ceria nanocrystals: Molecular dynamics study

Molecular dynamics program for simulation of nanocrystals in vacuum, with ionic model approximation, was developed. The use of graphics processor allows us to speed up calculation by two orders of magnitude in comparison with CPU realization. For Ce1−XGdXO2−X/2 system by fitting to experimental data pair interaction potentials were obtained, comparison with existing potential set was carried out. Melting temperature of CeO2 nanocrystal depends on its octahedron diagonal size as Тm(L,nm)=Тm(∞)−(5534±153)⋅L−2.Structural disorder of Ce1−XGdXO2−X/2 nanocrystals (X=0–0.3) in superionic transition and melting regimes was investigated. Dependences of melting temperature and specific volume change at melting from gadolinium dopant quantity have downward trend. It is shown, that oxygen sublattice disorder (“melting”) in crystallites takes place at about 0.7Tm. The contribution of intrinsic disorder to anion diffusion coefficient in systems with gadolinium dopant was extracted and the presence of superionic transition is shown. Comparison with high-temperature ionic conductivity data was carried out.

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al 2O 3 substrate to form an intermediate ZnAl 2O 4 spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm −1 is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.

Bulk nanocrystalline superconducting YBa2Cu3O7-x

A method to fabricate high density nanocrystalline superconducting YBa2Cu3O7-x(YBCO) is presented in which commercially available YBCO powder is ball milled and then hot isostatically pressed (HIP'ed) at 0.2 GPa and 400 C. Ball milling decreased the average particle size from 2 μm to ~ 20 nm within 2 hours and to ~ 4 nm after milling for 30 hours with no noticeable strain. Ball milling increases the oxygen sublattice disorder and the Y/Ba anti-site disorder and drives the orthorhombic YBCO phase towards a disordered metastable cubic phase Y1/3Ba2/3CuO3-x consistent with the work by Simoneau et al. [14, 19]. Powders milled for 30 hours were HIPed at temperatures from 400 C to 550 C. At 450 C and above, the powder decomposed into the parent oxides. At 400 C, the metastable cubic phase reordered to form bulk YBCO with an average grain size of 5 - 18 nm and a relative mass density of ~ 96%. The bulk YBCO material has an onset melting temperature and associated enthalpy of 970 °C and 157 J.g-1 respectively which can be compared to 965 °C and 156 J.g-1 for the starting commercial powder. These results are consistent with XRD data and they suggest a predominantly single-phase nanocrystalline YBCO has been fabricated. The superconducting transition temperature (TC) of the nanocrystalline YBCO determined from ac. susceptibility measurements is 88.8 K, with a slope of upper critical field near TC of ~ 0.12 T.K-1.

Heat capacity anomaly in UO 2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO 2 were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO 2 and the temperature dependence of Debye–Waller factors for oxygen and uranium atoms were determined. The dependence of Debye–Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300–1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.

The structural chemistry and oxide ion conducting properties of the new bismuth oxide sulfate, Bi8O11(SO4)

A new compound Bi8O11(SO4), synthesised using ceramic techniques, has been studied using variable temperature X-ray powder diffraction, neutron powder diffraction and impedance spectroscopy. The material was found to undergo a phase change, on heating, from a complex structure stable at ambient temperatures to a high temperature form with tetragonal symmetry (P41212; a = 11.78840(4) A, c = 22.7642(1) A). A gradual phase transition was determined to occur between ∼545 K and ∼600 K. High-resolution neutron powder diffraction data collected at 623 K were used to provide structural information on the high temperature phase. Disorder of the sulfate groups, presumably due to dynamic rotation, renders it impossible to locate the oxygen atoms bonded to S, but the distribution of the sulfate groups within the structure has been established. The bismuth oxide framework also displays oxygen sublattice disorder, and the material exhibits relatively high oxide ion conductivity.

Disordering of oxygen sublattice in Cr-doped YSZ

Abstract The effect of chromium addition (0.05–0.3 wt.% Cr2O3) on the rearrangement and the ordering of oxygen sublattice in yttria-stabilized zirconia consists in: 1) the formation of substitutional solid solution, 2) the smaller degree of ordering and association, 3) the higher number of defined impurity states, stable over the temperature ranges of inhomogeneities (plateaus) on the temperature dependence of microhardness, 4) the small decrease of intensities in defect-induced Raman modes and the broadening of Restrahlen bands and the increase of shoulder at 615 cm−1 in IR spectra and 5) Kramers-Kronig analysis of reflectance data allows to determine the frequency dependence of dielectric function e(ω) and optical parameters n(ω) and k(ω) including the transverse and longitudinal polar mode frequencies.

Growth mechanism and transport properties of a-axis oriented YBa 2Cu 3O 7 thin films without PrBa 2Cu 3O 7 interface

The growth of a-axis YBa 2Cu 3O 7-δ films deposited by inverted cylindrical magnetron sputtering by the low temperature and the template layer processes, without a PrBa 2Cu 3O 7-δ interface, was investigated. It was found that the template process with a YBa 2Cu 3O 7-δ seed layer removes the oxygen sublattice disorder present in the low temperature films, this improving the transition temperature and the critical current density of the film.

Oxygen-sublattice ordering and intercalation mechanism of chlorine in YBa2Cu3O6+ delta.

We have investigated the modifications occurring in the oxygen sublattice and in the phonon spectra of chlorinated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (YBCO) compounds. Susceptibility measurements suggest that the diffusion of chlorine is different in oxygen-deficient and in oxygen-rich materials. We find occurrence of superconductivity after chlorine treatment at 200 \ifmmode^\circ\else\textdegree\fi{}C in an initially nonsuperconducting reduced sample. X-ray photoelectron spectra and x-ray-diffraction patterns of this sample show that the oxidation number of chlorine is between -1 and 0 in the lattice. To explain this result we assume that when an oxygen-deficient sample of approximate composition ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.20}$ is substantially chlorinated, the chlorine enters the structure at vacancies of BaO planes and promotes the diffusion of oxygen towards empty O(1) sites of copper planes. This conclusion is supported by a comparison of vibrational results from chlorinated YBCO with those recently obtained on YBCO pristine thin films presenting oxygen-sublattice disorder and by the different magnetic behavior of chlorinated reduced and oxidized samples. A Raman resonant feature is found at 72 meV (580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and is ascribed to defect-induced modes of the oxygen sublattice rather than to electronic excitations. Indeed at 10 K, the line shapes of this band are clearly asymmetric and can be fitted using different Fano functions indicating phononic interaction with an electronic spectrum. The unusual broadening of this line on going from normal to superconducting state suggests the presence of charge-transfer excitons traveling from ${\mathrm{CuO}}_{2}$ planes to CuO chains.

TEM STUDY OF MICROSTRUCTURAL CHANGES INDUCED BY AR ION IMPLANTATION IN YBa2Cu3O7-x SUPERCONDUCTING FILMS

Ar ion implantation induced superconductivity change and structural change in YBa2Cu3O7-x epitaxial films have been studied. After implantation, not only Jc and Tc of the sa-mples decreased with the increasing of Ar ion fluence, but also a metal-to-semiconducfor transition occurred. The TEM photographs showed that the lattice frame of the implanted sa-mples was nearly unchanged under low fluence. Only at high fluence, the structure was comple-tely destroyed. According to the experimental results, we suggest that the degradation of Jc and Tc may mainly result from the disordering of oxygen sublattice.

Interpretation of anomalies in the Raman spectrum ofK2SeO4in terms of oxygen sublattice disorder

Raman scattering from ${\mathrm{K}}_{2}$Se${\mathrm{O}}_{4}$ crystals has been studied in the (20-800)-K temperature range. Three portions of the spectrum are discussed: defect-induced scattering, primarily below 100 ${\mathrm{cm}}^{\ensuremath{-}1}$, the external mode spectrum below 200 ${\mathrm{cm}}^{\ensuremath{-}1}$, and the internal mode spectra in two regions, 300-500 and 800-950 ${\mathrm{cm}}^{\ensuremath{-}1}$. The temperature dependence of the low-frequency, defect-induced scattering has been correlated (in previous studies) with the temperature dependence of certain nonzero-wave-vector phonons that have been observed by others using inelastic neutron scattering. Close to the incommensurate transition temperature, ${T}_{i}=129$ K, a large enhancement and line narrowing develops below 10 ${\mathrm{cm}}^{\ensuremath{-}1}$; no satisfactory interpretation of this effect has yet emerged. Above 129 K, in the paraelectric phase, the multiplicity of the external modes agrees with the predictions of the Raman-scattering selection rules. The internal mode spectra, however, consistently contain more lines than prescribed by the selection rules. These observed internal mode spectra would have the correct multiplicity if the crystal did not possess the center of symmetry of the presumed Pnam space group, thus suggesting a distortion of the selenate sublattice. Small frequency shifts above 375 K in the external mode spectra are similar to those observed in the incommensurate phase and, also, differential thermal analyses of powders and crystals show small, diffuse, reproducible peaks in the (373-473)-K range. Both effects may be associated with glasslike phase changes in the selenate sublattice. A qualitative model of orientational disorder in the selenate sublattice is offered to account for all these observations.

Anomalies in the Raman Spectrum of K2SeO4and their relation to possible oxygen sublattice disorder

Abstract Raman spectra of the internal modes of K2SeO4 in the 300–450 cm−1 and 800–950 cm−1 ranges, contain the same nine lines for each of the four crystallographically-allowed irreducible representations of the Raman tensor and except for thermal broadening, remain essentially the same from 20 degK to above 400 degK. From symmetry, 6A1g, 6B1g, 3B2g and 3B3g lines are expected in the 135K<T<745K (paraelectric) phase (D2h 16 space group). In the T<93K (ferroelectric) phase, because of cell-tripling and loss of inversion symmetry, 108 internal modes are predieted (C2v 9 space group). The lattice modes in the 0–200 cm−1 range appear to satisfy the selection rule predictions. A qualitative model based on the existence of small, orientational disorder in the oxygen sublattice is outlined to account for these observations.