Electrical Properties,Cation Distributions,and Thermal Expansion of Manganese Cobalt Chromite Spinel Oxides

As the oxidation and chromium volatilization of chromia‐forming alloy interconnects can cause Solid oxide fuel cells (SOFC) cathode poisoning and cell degradation, spinel coatings like Mn_1.5Co_1.5O₄ have been applied as a barrier to oxygen and chromium diffusion. To evaluate their long‐term stability, the properties of the reaction layer between the Mn_1.5Co_1.5O₄ coating and Cr₂O₃ scale formed on the alloy surface need to be characterized. Therefore, compositions of Mn_1.5?0.5xCo_1.5?0.5xCr_xO₄ (x = 0–2) were prepared to investigate their electrical properties, cation distributions, and thermal expansion behavior at high temperature. With increasing Cr content in manganese cobalt spinel oxides, the cubic crystal structure is stabilized and the electrical conductivity and coefficient of thermal expansion both decrease. The cation distributions determined from neutron diffraction show that Cr and Mn have stronger preference for octahedral sites in the spinel structure as compared with Co.     

Mullite: Crystal Structure and Related Properties

Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al₂(Al_2+2xSi_2‐2x)O_10‐x. Main members are sillimanite (x = 0), stoichiometric 3/2‐mullite (x = 0.25), 2/1‐mullite (x = 0.40), and the SiO₂‐free phase ι‐alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite‐type structural backbone of parallel chains consisting of edge‐sharing MO₆ octahedra and their specific cross‐linkage by TO₄ tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.     

First Principles Modeling of Pd‐doped (La,Sr)(Co,Fe)O3 Complex Perovskites

(La,Sr)(Co,Fe)O₃ (LSCF) perovskites are well known promising materials for cathodes of solid oxide fuel cells. In order to reduce cathode operational temperature, doping on B‐sublattice with different metals was suggested. Indeed, as it was shown recently experimentally, doping with low Pd content increases oxygen vacancy concentration which is one of factors controlling oxygen transport in fuel cells. In this Communication, we modeled this material using first principles DFT calculations combined with supercell model. The charge density redistribution, density of states, and local lattice distortion around palladium ions are analyzed and reduction of the vacancy formation energy confirmed.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1−x)Ba(Mg1/3Nb2/3)O3–xBaSnO3 Solid Solutions

Microwave dielectric ceramics of (1?x)Ba(Mg_1/3Nb_2/3)O₃‐xBaSnO₃ [(1?x)BMN‐xBS] with high quality factors was synthesized by the solid‐state reaction method. The effects of BaSnO₃ additions (x = 0–0.2) on the sinterability, crystal structures, microwave dielectric properties, and microwave dielectric loss mechanisms of BMN were investigated systematically. The degree of 1:2 cation ordering was decreased with increasing Sn content and eventually faded away as x ≥ 0.1, where the low‐temperature relaxations disappeared coincidently through the thermally stimulated depolarization current technique. It was supposed to be the short‐range misplacements of the B‐site cations within the long‐range ordered structure. Meanwhile, the high‐temperature relaxations associated with the in‐grain oxygen vacancies were found in all the title compounds. Though the concentrations of oxygen vacancies of 0.8BMN‐0.2BS were higher than BMN, high Q × f values could also be obtained even in the absence of 1:2 cation ordering. Specifically, the excellent characteristics like ε_r = 29.02, Q × f = 90 000 GHz and τ_f = 6.3 ppm/°C were achieved in the specimens of x = 0.2 sintered at 1450°C.     

Empirical Evidence for A‐Site Order in Perovskites

Models for composition–structure relationships are useful in both the lab and industry, yet few exist for perovskites‐containing extrinsic defects or cation ordering. In this work, an empirical model is used to predict the existence of A‐site cation ordering. Specifically, four compositions in the Na_(1?3x)/2La_(1+x)/2TiO₃ system (x = 0.0, 0.0533, 0.1733 and 0.225) were synthesized using a conventional solid‐state mixed‐oxide method. The structure of the x = 0 end‐member (Na_0.5La_0.5TiO₃) has been reported in various space groups, but always with a random distribution of Na⁺ and La³⁺ on the A site; however, empirical modeling suggests that it is not only ordered but also that a small volume increase accompanies the ordering process. While no evidence of long‐range A‐site ordering is observed in this composition via X‐ray or neutron diffraction, electron‐diffraction data indicate short‐range ordering of Na⁺ and La³⁺ ions, with the degree of cation ordering decreasing (but the scale of ordered domains and degree of vacancy ordering generally increasing) with increasing x. First‐principles calculations via density functional theory support both conclusions that short‐range ordering in Na_0.5La_0.5TiO₃ is stable and that it results in a volume increase with respect to the disordered analog. A similar analysis has been conducted for the Li_(1?3x)/2La_(1+x)/2TiO₃ and Na_(1?3x)/2La_(1+x)/2(Mg_0.5W_0.5)O₃ solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.     

Influence of polluted atmospheres on the natural aging of poly(vinyl chloride) stabilized with epoxidized sunflower oil

Semirigid and plasticized poly(vinyl chloride) (PVC) samples, stabilized with epoxidized sunflower oil (ESO), Zn, and Ca stearates, were exposed in Algiers (hot, Mediterranean climate) for 12 months in three sites where the concentrations of atmospheric pollutants (NO_x, O₃, hydrocarbons) are known. The evolution of the mechanical properties (tensile test and shore D hardness) was followed according to the aging time. The modification structure of polymer was investigated by Fourier transform infrared spectroscopy. The results have shown that all the considered properties were affected. Furthermore, it seems that the O₃ exerted the most deleterious effect, followed by hydrocarbons and then by NO_x. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Efficient red‐emitting phosphor of Eu3+‐activated (Na0.5Gd1.5)(TiSb)O7 derived via cation‐substitutions in Gd‐Pyrochlore

Rare‐earth (RE) titanate pyrochlore with perovskite‐layered structure is a well‐known engineering material in applied in many field. In this work, a red‐emitting phosphor of Gd_2?xNa_xTi_2?2xSb_2xO₇:Eu³⁺ (x = 0‐0.5) was developed via cation substitutions of (Sb⁵⁺→Ti⁴⁺) and (Na⁺→Gd³⁺) in Gd₂Ti₂O₇. The motivation is based on the fact that the introduction of cation‐disorders has been regarded to be an effective approach for improving the luminescent efficiency and thermal stability of RE‐activated materials. All the samples were synthesized via facile solid‐state reaction method. The morphology properties were measured via SEM and EDS measurements. The structural Rietveld refinement was performed to investigate the microstructure in pyrochlore lattices. The luminescence properties of Gd_2?xNa_xTi_2?2xSb_2xO₇:0.15Eu³⁺ (x = 0‐0.5) has a strict dependence on the cation substitution levels. The band energy of Gd₂Ti₂O₇ is 2.9 eV with a direct transition nature. The incorporation of Sb⁵⁺ and Na⁺ in the lattices moves the optical absorption to a longer wavelength. The cation disorder results in significant improvements of luminescence intensity, excitation efficiency in the blue region, longer emission lifetime and thermal stability.     

Thermodynamic Assessment of the Pr–O System

The Calphad method was used to perform a thermodynamic assessment of the Pr–O system. Compound energy formalism representations were developed for the fluorite α‐PrO_2–x and bixbyite σ‐Pr₃O_{5 ± x} solid solutions while the two‐sublattice liquid model was used to describe the binary melt. The series of phases between Pr₂O₃ and PrO₂ were taken to be stoichiometric. Equilibrium oxygen pressure, phase equilibria, and enthalpy data were used to optimize the adjustable parameters of the models for a self‐consistent representation of the thermodynamic behavior of the Pr–O system from 298 K to melting.     

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% YO_1.5) 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 (U_iso) 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.     

Structural and Mössbauer Investigation of Nanocrystalline SrFe1−xTixO3−δ

Crystal structure and cation distribution of nanocrystalline SrFe_1?xTi_xO_3?δ (0 ≤ x ≤ 0.3) synthesized by combined high‐energy ball milling and solid‐state reactions are investigated using Neutron powder diffraction and Mössbauer spectroscopy. Ti doping stabilizes the single phase tetragonal structure with I4/mmm space group up to x = 0.3. The neutron and Mössbauer data confirm that Fe exists in three different sites both crystallographically as well as magnetically in all the four compositions. The cation distribution at various sites is established through Rietveld refinement.     

Effect of Ordering on the Microwave Dielectric Properties of Spinel‐Structured (Zn1−x(Li2/3Ti1/3)x)2TiO4 Ceramics

Spinel‐structured (Zn_1?x(Li_2/3Ti_1/3)_x)₂TiO₄ (x = 0–1) microwave dielectric ceramics were manufactured via a conventional mixed‐oxide method. The X‐ray diffraction and Raman spectra revealed that a disordered face‐centered cubic phase was found in the composition range of x < 0.5, and an ordered primitive cubic spinel solid solution was achieved as x was beyond 0.5. Such a disorder–order transition near x = 0.5 was accompanied by the variation of composition‐induced cation occupancy. The Q × f value first kept increasing up to ~160 000 (GHz) in disordered ceramics, and then sharply decreased as an ordered structure appeared at x ≥ 0.5. An obvious decrease in τ_f value was also accompanied by the appearance of an ordered structure. The minimum τ_f value (~ ?20 ppm/°C) was obtained in the x = 0.75 sample with the highest structural order degree. These results demonstrated that microwave dielectric properties of current spinel ceramics could be successfully modified by adjusting their structural order degree, which could be appropriately adopted for the design of spinel‐structured materials with favorable properties.     

Structural,Raman, and Dielectric Studies on Multiferroic Mn‐doped Bi1−xLaxFeO3 Ceramics

Multiferroic Bi_1?xLa_xFeO₃ [BLFO (x)] ceramics with x = 0.10–0.50 and Mn‐doped BLFO (x = 0.30) ceramics with different doping contents (0.1–1.0 mol%) were prepared by solid‐state reaction method. They were crystallized in a perovskite phase with rhombohedral symmetry. In the BLFO (x) system, a composition (x)‐driven structural transformation (R3c→C222) was observed at x = 0.30. The formation of Bi₂Fe₄O₉ impure phase was effectively suppressed with increasing the x value, and the rhombohedral distortion in the BLFO ceramics was decreased, leading to some Raman active modes disappeared. A significant red frequency shift (~13 cm^?1) of the Raman mode of 232 cm^?1 in the BLFO ceramics was observed, which strongly perceived a significant destabilization in the octahedral oxygen chains, and in turn affected the local FeO₆ octahedral environment. In the Mn‐doped BLFO (x = 0.30) ceramics, the intensity of the Raman mode near 628 cm^?1 was increased with increasing the Mn‐doping content, which was resulted from an enhanced local Jahn–Teller distortions of the (Mn,Fe)O₆ octahedra. Electron microscopy images revealed some changes in the ceramic grain sizes and their morphologies in the Mn‐doped samples at different contents. Wedge‐shaped 71° ferroelectric domains with domain walls lying on the {110} planes were observed in the BLFO (x = 0.30) ceramics, whereas in the 1.0 mol% Mn‐doped BLFO (x = 0.30) samples, 71° ferroelectric domains exhibited a parallel band‐shaped morphology with average domain width of 95 nm. Dielectric studies revealed that high dielectric loss of the BLFO (x = 0.30) ceramics was drastically reduced from 0.8 to 0.01 (measured @ 10⁴ Hz) via 1.0 mol% Mn‐doping. The underlying mechanisms can be understood by a charge disproportion between the Mn⁴⁺ and Fe²⁺ in the Mn‐doped samples, where a reaction of Mn⁴⁺ + Fe²⁺→Mn³⁺ + Fe³⁺ is taken place, resulting in the reduction in the oxygen vacancies and a suppression of the electron hopping from Fe³⁺ to Fe²⁺ ions effectively.     

掺杂的阳离子缺陷型磷灰石 La9.33－2x/3MxSi6O26 (M=Mg,Sr,Ca)的合成及离子导电特性

Apatite-type lanthanum silicate with special conduction mechanism via interstitial oxygen has attracted considerable interest in recent years. In this work, pure powder of La9.33－2x/3MxSi6O26 (M＝Mg, Ca, Sr) is prepared by the sol-gel method with sintering at 1000°C. The powder is characterized by X-ray diffraction (XRD) and scan-ning electron micrograph (SEM). The apatite can be obtained at relatively low temperature as compared to the con-ventional solid-state reaction method. The measurements of conductivity of a series of doped samples La9.33－2x/3MxSi6O26 (M＝Ca, Mg, Sr) indicate that the type of dopant and the amount have a significant effect on the conductivity. The greatest decrease in conductivity is observed for Mg doping, following the Ca and the Sr doped apatites. The effect is ultimately attributed to the amount of oxygen interstitials, which is affected by the crystal lat-tice distortion arising from cation vacancies.     

Identification of the Zn Substitution Sites in La–Zn Substituted SrAl12O19 from 27Al Solid‐State NMR Studies

The La–Zn substituted hexagonal strontium aluminate, Sr_1?xLa_xAl_12?xZn_xO₁₉, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using ²⁷Al solid‐state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr_1?xLa_xFe_12?xCo_xO₁₉, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f₂ AlO₆ octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta‐coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr_1?xLa_xFe_12?xCo_xO₁₉, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.     

The complexity of structural phase transitions in Pb(Hf0.92Sn0.08)O3 single crystals

Pb(Hf_1−xSn_x)O₃ single crystals with x = 0.08 were characterized using single-crystal X-ray diffraction and Raman scattering, in a wide temperature range. The information concerning the structure of two intermediate phases (IMs), situated between low-temperature antiferroelectric A1 and high-temperature paraelectric PE phases, was obtained. The lower temperature IM, A2, is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature IM, is characterized by tilting of oxygen octahedra, and serious disorder coming from lead ions represented by X-ray diffuse scattering. Optical phonons and phase transitions in Pb(Hf_1−xSn_x)O₃ single crystals were investigated by temperature-dependent Raman spectra. It was found that several soft modes control the phase transition between two antiferroelectric phases indicating its displacive character, whereas, in the paraelectric phase, both soft modes and Rayleigh scattering were observed.     

Phase Structure Evolution and Thermo‐Physical Properties of Nonstoichiometry Nd2−xZr2+xO7+x/2 Pyrochlore Ceramics

Nonstoichiometry pyrochlore composites of Nd_2?xZr_2+xO_7+x/2 (x = 0, 0.1, 0.2) were synthesized by chemical‐coprecipitation and calcination method. The phase structure evolution and thermo‐physical properties of Nd_2?xZr_2+xO_7+x/2 were investigated. Structural analysis by Raman spectroscopy showed that Nd_2?xZr_2+xO_7+x/2 underwent an ordering degree decrease and a lattice distortion increase with increasing x value. Nd_1.9Zr_2.1O_7.05 and Nd_1.8Zr_2.2O_7.1 exhibited lower thermal conductivities than Nd₂Zr₂O₇, which might be related to the lower ordering degree and the enhanced phonon scattering due to lattice distortion. As ZrO₂ content increasing, the thermal expansion coefficients of Nd_2?xZr_2+xO_7+x/2 increased, which possibly arised from the decreased crystal energy due to reduced ordering degree.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

The NbO6 octahedral distortion and phase structural transition of Eu3+‐doped K0.5Na0.5NbO3–xLiNbO3 ferroelectric ceramics

A small quantity of Eu³⁺ ions were doped in the lead‐free ferroelectric K_0.5Na_0.5NbO₃–xLiNbO₃ (KNN–xLN, 0 ≤ x ≤ 0.08) ceramics to investigate the NbO₆ octahedral distortion induced by the increasing LN content. In addition, the phase structure, ferroelectric, and photoluminescence properties of K_0.5Na_0.5NbO₃–xLiNbO₃:0.006Eu³⁺ (KNN–xLN:0.006Eu³⁺) lead‐free piezoelectric ceramics were characterized. All the X‐ray diffraction, Raman spectra, dielectric constant vs temperature measurements and the photoluminescence of Eu³⁺ ions demonstrated that the prepared ceramics undergo a polymorphic phase transition (PPT, from orthorhombic to tetragonal phase transformation) with the rising LN content, and the PPT region locates at 0.05 ≤ x ≤ 0.06. The ferroelectric properties, Raman intensity ratios and photoluminescence intensity ratios show similar variations with the increasing LN content, all with a maximum value achieved at the PPT region. We believe that the close relationship among the ferroelectric properties, Raman intensity ratios, and photoluminescence intensity ratios is caused by the NbO₆ octahedral distortion. The photoluminescence of Eu³⁺ ion was discussed basing on the crystal‐symmetry principle and Judd‐Ofelt theory.     

Probing the Local Structure and Phase Transitions of Bi4V2O11‐Based Fast Ionic Conductors by Combined Raman and XRD Studies

In this article, we report the structural phase transitions in Bi₄V₂O₁₁ as observed from temperature‐dependent Raman scattering and X‐ray diffraction measurements. Four different types of highly disordered coordination polyhedra around the vanadium atoms with large dispersion of V–O bond lengths are observed in Bi₄V₂O₁₁ at ambient temperature. The observed V–O bond lengths could be grouped into two categories, viz. shorter <1.7 Å and longer >1.7 Å. The Raman modes of Bi₄V₂O₁₁ could be assigned to vibration of these bonds and V–O–V linkages. We could correlate the difference in degree of anharmonicity of the phonon modes with temperature to differences in V–O bond strength. The local structure of vanadium–oxygen network in Bi₄V_1.8Cu_0.2O_10.7 was also obtained by similar studies. The effect of highly disordered anion sublattice in the doped compound is reflected in the broadening of the Raman modes.     

A Combined Experimental and Computational Thermodynamic Investigation of the U–Th–O System

The thermodynamics of the U–Th–O system have been assessed using the Calphad method. The compound energy formalism (CEF) and a partially ionic two‐sublattice liquid model (TSLM) were used for the fluorite U_1–yTh_yO_2±x, γ‐(U,Th)₄O₉, and the U–Th–O melt. The O₂ activity of fluorite U_1–yTh_yO_2±x with temperature and composition was determined by thermogravimetric analysis. Thermodynamic studies for the Th–O binary and U–Th–O ternary available in the open literature were critically reviewed. A self‐consistent data set was selected and compiled with the equilibrium oxygen pressures determined by thermogravimetry in order to optimize the adjustable parameters of models selected to represent the phases in the Th–O and U–Th–O systems.