Ag4B7O12X (X = Cl, Br, I) Heptaborate Family: Comprehensive Crystal Chemistry, Thermal Stability Trends, Topology, and Vibrational Anharmonicity.

Using glass crystallization and solid-state techniques, we were able to complete the family of salt-inclusion silver halide borates, Ag4B7O12X, by the X = Cl and I members. The new compounds are characterized by differential scanning calorimetry, single-crystal and high-temperature powder X-ray diffraction, optical spectroscopy, and density functional theory calculations. In all structures, the silver atoms exhibit strong anharmonicity of thermal vibrations, which could be modeled using Gram-Charlier expansion, and its asymmetry was characterized by the skewness vector. The topology of the silver halide and borate sublattices has been analyzed separately for the first time. Along the I → Br → Cl series, we observe a decrease of the melting point and configuration entropy and an increase of thermal expansion and its anisotropy and thermal vibration anharmonicity, which indicates decreasing stability.

Crystal Chemistry of the Copper Oxalate Biomineral Moolooite: The First Single-Crystal X-ray Diffraction Studies and Thermal Behavior.

Moolooite, Cu(C2O4)·nH2O, is a typical biomineral which forms due to Cu-bearing minerals coming into contact with oxalic acid sources such as bird guano deposits or lichens, and no single crystals of moolooite of either natural or synthetic origin have been found yet. This paper reports, for the first time, on the preparation of single crystals of a synthetic analog of the copper-oxalate biomineral moolooite, and on the refinement of its crystal structure from the single-crystal X-ray diffraction (SCXRD) data. Along with the structural model, the SCXRD experiment showed the significant contribution of diffuse scattering to the overall diffraction data, which comes from the nanostructural disorder caused by stacking faults of Cu oxalate chains as they lengthen. This type of disorder should result in the chains breaking, at which point the H2O molecules may be arranged. The amount of water in the studied samples did not exceed 0.15 H2O molecules per formula unit. Apparently, the mechanism of incorporation of H2O molecules governs the absence of good-quality single crystals in nature and a lack of them in synthetic experiments: the more H2O content in the structure, the stronger the disorder will be. A description of the crystal structure indicates that the ideal structure of the Cu oxalate biomineral moolooite should not contain H2O molecules and should be described by the Cu(C2O4) formula. However, it was shown that natural and synthetic moolooite crystals contain a significant portion of "structural" water, which cannot be ignored. Considering the substantially variable amount of water, which can be incorporated into the crystal structure, the formula Cu(C2O4)·nH2O for moolooite is justified.

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.

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.

Bridging the Salt-Inclusion and Open-Framework Structures: The Case of Acentric Ag4B4O7X2 (X = Br, I) Borate Halides.

An acentric borate family, Ag4B4O7X2 (X = Br, I), has been prepared by slow cooling stoichiometric melts in evacuated silica ampules. Their crystal structure is comprised of two porous interpenetrating frameworks and demonstrates a further development of the "salt-inclusion" architecture toward a "covalent-inclusion" structure. The (Ag2X)+ sublattice shows strong anharmonic vibrations. Thermal expansion is strongly anisotropic because of the presence of condensed rigid kernite boron-oxygen chains aligned perpendicular to the c axes.

High-Temperature Crystal Chemistry of α-, ß-, and γ-BiNbO4 Polymorphs.

Thermal behavior of the orthorhombic (α) and triclinic (ß) polymorphs of BiNbO4 was studied by the methods of high-temperature powder X-ray diffraction (HTPXRD) and differential scanning calorimetry (DCS) in the temperature range 25-1200 °C. The study revealed the sequence of thermal phase transformations and the new high-temperature modification, γ-BiNbO4, which was formed above 1001 °C and existed up to the melting temperature of BiNbO4. The incongruent melting of BiNbO4 was characterized by the formation of the cubic phase with the approximate composition Bi3NbO7. The HTPXRD method was used in this study to evaluate thermal deformations and to calculate thermal-expansion coefficients (TEC) of the three modifications of BiNbO4 (α, ß, and γ). The average volumetric TECs of these three modifications were in the range 19-36 × 10-6 °C-1. The triclinic phase ß-BiNbO4 demonstrated the highest anisotropy of thermal expansion. α-BiNbO4 was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of γ-BiNbO4 was determined at 1100 °C using powder data and was refined using the Rietveld method (the α-LaTaO4 structural type, the space group Cmc21, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) Å3, Rwp = 4.82, RBragg = 3.61%). The methods of thermal analysis and high-temperature powder X-ray diffraction revealed that, during the heating, bismuth orthoniobate underwent the following sequence of phase transitions: α-BiNbO4 → γ-BiNbO4 → ß-BiNbO4 and ß-BiNbO4 → γ-BiNbO4 → ß-BiNbO4 or, at slow heating, ß-BiNbO4 → (α-BiNbO4) → γ-BiNbO4 → ß-BiNbO4, where γ-BiNbO4 is the high-temperature phase of bismuth orthoniobate.

Perovskites with the Framework-Forming Xenon.

The Group 18 elements (noble gases) were the last ones in the periodic system to have not been encountered in perovskite structures. We herein report the synthesis of a new group of double perovskites KM(XeNaO6) (M = Ca, Sr, Ba) containing framework-forming xenon. The structures of the new compounds, like other double perovskites, are built up of the alternating sequence of corner-sharing (XeO6) and (NaO6) octahedra arranged in a three-dimensional rocksalt order. The fact that xenon can be incorporated into the perovskite structure provides new insights into the problem of Xe depletion in the atmosphere. Since octahedrally coordinated Xe(VIII) and Si(IV) exhibit close values of ionic radii (0.48 and 0.40 Å, respectively), one could assume that Xe(VIII) can be incorporated into hyperbaric frameworks such as MgSiO3 perovskite. The ability of Xe to form stable inorganic frameworks can further extend the rich and still enigmatic chemistry of this noble gas.

Features of Phase Formation of Pyrochlore-type Ceramics Bi2Mg(Zn)1-x Ni x Ta2O9.

The phase formation of complex pyrochlores (space group Fd-3m) Bi2Mg(Zn)1-x Ni x Ta2O9 was investigated during solid-phase synthesis. It was found that the pyrochlore phase precursor in all cases was α-BiTaO4. The pyrochlore phase synthesis reaction proceeds mainly at temperatures above 850-900 °C and consists in the interaction of bismuth orthotantalate with a transition element oxide. The influence of magnesium and zinc on the course of pyrochlore synthesis was revealed. The reaction temperatures of magnesium and nickel (800 and 750 °C, respectively) were determined. The change in the pyrochlore unit cell parameter depending on the synthesis temperature was analyzed for both systems. Nickel-magnesium pyrochlores are characterized by a porous dendrite-like microstructure with a grain size of 0.5-1.0 microns, and the porosity of the samples reaches 20 percent. The calcination temperature does not significantly affect the microstructure of the samples. Prolonged calcination of the preparations leads to the coalescence of grains with the formation of larger particles. Nickel oxide has a sintering effect on ceramics. The studied nickel-zinc pyrochlores are characterized by a low-porous dense microstructure. The porosity of the samples does not exceed 10%. The optimal conditions for obtaining phase-pure pyrochlores (1050 °C and 15 h) were determined.

Effect of Fe-Doping on Thermal Expansion and Stability of Bismuth Magnesium Tantalate Pyrochlorere.

A continuous series of solid solutions (Bi1.5Mg0.75-xFexTa1.5O7±Δ (x = 0-0.75)) with the pyrochlore structure were synthesized with the solid-phase method. It was shown that iron, like magnesium, is concentrated in the structure in the octahedral position of tantalum. Doping with iron atoms led to an increase in the upper limit of the thermal stability interval of magnesium-containing pyrochlore from 1050 °C (x = 0) up to a temperature of 1140 °C (x = 1). The unit cell constant a and thermal expansion coefficient (TEC) increase uniformly slightly from 10.5018 Å up to 10.5761 Å and from 3.6 up to 9.3 × 10-6 °C-1 in the temperature range 30-1100 °C. The effect of iron(III) ions on the thermal stability and thermal expansion of solid solutions was revealed. It has been established that the thermal stability of iron-containing solid solutions correlates with the unit cell parameter, and the lower the parameter, the more stable the compound. The TEC value, on the contrary, is inversely proportional to the cell constant.

One of Nature's Puzzles Is Assembled: Analog of the Earth's Most Complex Mineral, Ewingite, Synthesized in a Laboratory.

Through the combination of low-temperature hydrothermal synthesis and room-temperature evaporation, a synthetic phase similar in composition and crystal structure to the Earth's most complex mineral, ewingite, was obtained. The crystal structures of both natural and synthetic compounds are based on supertetrahedral uranyl-carbonate nanoclusters that are arranged according to the cubic body-centered lattice principle. The structure and composition of the uranyl carbonate nanocluster were refined using the data on synthetic material. Although the stability of natural ewingite is higher (according to visual observation and experimental studies), the synthetic phase can be regarded as a primary and/or metastable reaction product which further re-crystallizes into a more stable form under environmental conditions.

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.

Allabogdanite, the high-pressure polymorph of (Fe,Ni)2P, a stishovite-grade indicator of impact processes in the Fe-Ni-P system.

Allabogdanite, (Fe,Ni)2P, is the only known natural high-pressure phase reported in the Fe-Ni-P system. The mineral, which was previously described from a single meteorite, the Onello iron, is now discovered in the Santa Catharina and Barbianello nickel-rich ataxites. The occurrence of allabogdanite in Santa Catharina, one of the largest and well-studied meteorites, suggests that this mineral is more common than was believed. The formation of allabogdanite-bearing phosphide assemblages in a given meteorite provides evidence that it experienced peak pressure of at least 8 GPa at a temperature above 800 °C. Since the pressure-temperature stability parameters of allabogdanite fall within the margins of the stishovite (rutile-type SiO2) stability area, the former can be employed as a convenient stishovite-grade indicator of significant impact events experienced by iron and stony-iron meteorites and their parent bodies.