Cu, Mg Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, XPS Spectra, Thermal Expansion, and Electrical Properties.

The pyrochlore-type solid-solution formation in a Bi1.6Mg0.8-xCuxTa1.6O7.2-Δ system, synthesized for the first time, is observed at x ≤ 0.56. High-temperature X-ray diffraction showed that the pyrochlore phase exists in air up to 1080 °C, where its thermal decomposition leads to the segregation of (Mg,Cu)Ta2O6. The thermal expansion coefficients of the end member, Bi1.6Mg0.24Cu0.56Ta1.6O7.2-Δ, increase from 3.3 × 10-6 °C-1 at room temperature up to 8.7 × 10-6 °C-1 at 930 °C. Rietveld refinement confirmed that the pyrochlore crystal structure is disordered with space group Fd3̅m:2 (Z = 8, no. 227). Doping with copper results in a modest expansion of the cubic unit cell, promotes sintering of the ceramic materials, and induces their red-brown color. X-ray photoelectron spectroscopy demonstrated that the states of Bi(III) and Mg(II) are not affected by doping, and the effective charge of tantalum cations is lower than +5, while the Cu(II) states coexist with Cu(I). The electron spin resonance spectra display a single line with g = 2.2, ascribed to the dipole-broadened Cu2+ signal. The dielectric permittivity of Bi1.6Mg0.8-xCuxTa1.6O7.2-Δ ceramics may achieve up to ∼105, with the dielectric loss tangent varying in the range from 0.2 up to 12. Multiple dielectric relaxations are found at room temperature and above for all samples.

Novel Ni-Doped Bismuth-Magnesium Tantalate Pyrochlores: Structural and Electrical Properties, Thermal Expansion, X-ray Photoelectron Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectra.

The samples of Ni-doped bismuth magnesium tantalate pyrochlores with the general formula Bi1.4(Mg1-x Ni x )0.7Ta1.4O6.3 (x = 0.3, 0.5, 0.7) were obtained by solid-phase synthesis. The crystal structure of the pyrochlore type (sp. gr. Fd3̅m:2) was clarified by the Rietveld method on the basis of X-ray powder diffraction data. The unit cell parameters increase with the decreasing nickel content in the range from 10.5319(1) to 10.5391(1) Å. The electronic state of atoms is established by the XPS method. According to XPS analysis, bismuth atoms have an effective charge of +3, nickel atoms +(2 + δ), and tantalum ions +(5 - δ). The coefficient of thermal expansion of the lattice of the samples was calculated from high-temperature X-ray structural measurements in the range of -180 to 1050 °C. The average values of linear TECs α in the temperature ranges of 30-570 and 600-1050 °C are 5.1 × 10-6 and 8.1 × 10-6 °C-1, respectively. The monotonicity of the change in the thermal expansion coefficient in the temperature range from -100 to 1050 °C indicates the absence of phase transformations. All samples are dielectric and exhibit high activation energies ∼2.0 eV, moderately high dielectric constants ∼24-28, and tangent dielectric losses ∼0.002 at 1 MHz and 21 °C. The electrical properties of the samples are described by a simple parallel equivalent scheme. The chemical composition of the materials has little effect on the polarizability of the medium or on the value of the activation energy of the conductivity. Ionic processes in investigated materials at frequencies 200-106 Hz and at temperatures 100-450 °C were not detected.

Thermal Expansion, XPS Spectra, and Structural and Electrical Properties of a New Bi2NiTa2O9 Pyrochlore.

A phase-pure nickel bismuth tantalate with pyrochlore structure was synthesized by a solid-phase synthesis method for the first time. The crystal structure of pyrochlore (refined formula Bi1.58Ni0.60Ta1.40O7, sp. gr. Fd-3m, a = 10.5343 Å, Z = 8) was clarified by the Rietveld method on the basis of X-ray powder diffraction (XRD) data. The crystallite size determined by the Scherrer method is ∼46 nm. The sample has an atypical pink-purple color. The electronic state of the atoms was investigated by XPS. According to XPS analysis, bismuth atoms have an effective charge of +3; nickel atoms, +(2 + δ); tantalum ions, +(5 - δ). The thermal expansion coefficient of the cell is calculated from high-temperature X-ray measurements in the range of 30-1200 °C. The thermal expansion coefficient (TEC) increases monotonically from 3.79 × 10-6 °C-1 (30 °C) to 8.32 × 10-6 °C-1 (990 °C). Above 1080 °C, the TEC decreases due to the thermal dissociation of pyrochlore with new NiTa2O6 phase formation. Ni-doped bismuth tantalate refers to dielectrics and exhibits a moderately high dielectric constant, ∼32, and low dielectric losses, ∼2 × 10-3 at 1 MHz and ∼30 °C. Above 300 °C, the dielectric losses and dielectric permittivity increase in the low-frequency region due to the activation of oxygen anions. It is found that the electrical characteristics of the sample are significantly affected by the ambient air humidity. An equivalent scheme which satisfactorily describes the electrical properties of the sample has been proposed.

Elucidating the formation of Al-NBO bonds, Al-O-Al linkages and clusters in alkaline-earth aluminosilicate glasses based on molecular dynamics simulations.

Exploring the reasons for the initiation of Al-O-Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community. Evidence for the formation of Al-O-Al and Al-NBO bonds in the glass composition 38.7CaO-9.7MgO-12.9Al2O3-38.7SiO2 (CMAS, mol%) has been provided based on Molecular Dynamics (MD) simulations. Analyses in the short-range order confirm that silicon and the majority of aluminium cations form regular tetrahedra. Well-separated homonuclear (Si-O-Si) and heteronuclear (Si-O-Al) cluster regions have been identified. In addition, a channel region (C-Region), separated from the network region, enriched with both NBO and non-framework modifier cations, has also been identified. These findings are in support of the previously proposed extended modified random network (EMRN) model for aluminosilicate glasses. A detailed analysis of the structural distributions revealed that a majority of Al, 51.6%, is found in Si-O-Al links. Although the formation of Al-O-Al and Al-NBO bonds is energetically less favourable, a significant amount of Al is found in Al-O-Al links (33.5%), violating Lowenstein's rule, and the remainder is bonded with non-bridging oxygen (NBO) in the form of Al-NBO (Al-O-(Ca, Mg)). The conditions necessary for the formation of less favourable bonds are attributed to the presence of a high amount of modifier cations in current CMAS glass and their preferable coordination.

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle.

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO2 for Al2O3 to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO4 and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of 29Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO4 and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al[4]-O-Al[4] bonds; this region coexists with a predominantly SiO4-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO4 and AlO4 structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO4-rich region at higher temperatures, leading to the formation of additional crystalline phases.

Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies.

An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO-9.7MgO-12.9Al2O3-38.7SiO2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, 29Si MAS-NMR, and 27Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29Si MAS-NMR spectra for the CMAS-GC suggest the presence of structural units in the remaining glassy phase with a polymerization degree higher than dimer units, whereas the 27Al MAS-NMR spectra revealed that most Al3+ cations exhibit a 4-fold coordination. In situ Raman spectroscopy data indicate that the formation of anorthite crystalline phase initiated after 240 h of heat treatment at 850 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

A (3 + 3)-dimensional "hypercubic" oxide-ionic conductor: type II Bi2O3-Nb2O5.

The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated "hypercubic" structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an "inflated" pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system.

Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O(7+δ).

Mössbauer spectroscopy and magnetization studies of YBaCo(4-x)Fe(x)O(7+δ) (x = 0-0.8) oxidized at 0.21 and 100 atm O(2), indicate an increasing role of penta-coordinated Co(3+) states when the oxygen content approaches 8-8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo(4-x)Fe(x)O(8.5) from 2 K up to 55-70 K, whilst the average magnetic moment of Co(3+) is lower than that for δ ≤ 0.2, in correlation with the lower (57)Fe(3+) isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo(4)O(8.5). The iron solubility limit in YBaCo(4-x)Fe(x)O(7+δ) corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo(4)O(7+δ) at 470-700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10(-5) to 1 atm, the linear strain in YBaCo(4)O(7+δ) ceramics at 598 K is as high as 0.33%.