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The formation mechanisms of pyrochlore-type Bi2Co1/2Cr1/2Nb2O9+Δ (space group Fd-3m, a = 10.4838(8) Å), in the temperature range from 400 to 1050 °C were studied by employing X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. An extensive reaction between the binary metal oxides was found to begin at temperatures above 550 °C, following the transition of monoclinic α-Bi2O3 into a tetragonal β-Bi2O3 polymorph. The synthesis process occurs in several stages when Bi-rich intermediate products (Bi6CrO12, Bi6Cr2O15, and Bi5Nb3O15) transform into bismuth-depleted BiNbO4 and a chromium–cobalt spinel is formed. The formation of a single pyrochlore phase occurs at the final reaction stage at 1050 °C via the doping of bismuth ortho-niobate, BiNbO4, by the transition metal cations. The observed mechanism is essentially similar to the mechanism of tantalate-based phases except for the formation of Bi5Nb3O15 at the intermediate reaction stages.

The effect of Zn-doping on the phase composition and optical properties of the Bi2ZnxFe1-xTa2O9.5-Δ (x = 0.3, 0.5, 0.7) was studied. XRD data showed that the samples crystallize in the structural type of pyrochlore (sp. gr.Fd-3m). For all the samples, an admixture of bismuth orthotantalate β-BiTaO4 triclinic modification up to 22.5 wt.% is observed. The content of β-BiTaO4 increases with zinc doping. The unit cell parameter of the pyrochlore phase rises from 10.4878 (x = 0.3) to 10.5154 Å (x = 0.7). The samples are characterized by a porous microstructure with indistinct grain boundaries. Zinc oxide has a sintering effect on ceramics. The charge state of the ions in Bi2ZnxFe1-xTa2O9.5-Δ was investigated by X-ray spectroscopy. NEXAFS and XPS data show that zinc doping does not change the oxidation degree of iron and bismuth ions in pyrochlore. The ions are in the charge states Bi(+3), Fe(+3), Zn(+2). In the Ta4fspectrum, an energy shift of the absorption band towards lower energies by ΔE = 0.5 eV is observed, which is typical for tantalum ions with an effective charge of (+5-δ). With the increase of x(Zn), the Bi 4f7/2 and Bi 4f5/2 bands are observed to shift to lower energies due to the distribution of some Zn(II) ions in the bismuth position.

The pyrochlore-type solid-solution formation in a Bi

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. Fd3m: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) A. 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.

The solid solutions based on the pyrochlore-type system Bi2MgNb2-xTaxO9 were formed in the compositional range х = 0–2.0 (Bi1·6Mg0·8Nb1.6-tTatO7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi2MgNb2-xTaxO9 (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) A for x = 0 and 10.54607 (3) A for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi1·6Mg0·8Ta1·6O7.2). The increase in the Ta5+ concentration led to the decrease in the dielectric permeability from 104 (Bi1·6Mg0·8Nb1·6O7.2) to 20 (Bi1·6Mg0·8Ta1·6O7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.

Two series of Bi2Mg1-xCoxNb2O9 and Bi2MgNb2-2xCo2xO9-δ preparations with pyrochlore structure were synthesized using solid phase method. It was found that solid solutions are formed in Bi2MgNb2-2xCo2xO9-δ series, foreign phases of MgNb2O6 and Bi5Nb3O15 are fixed in Bi2Mg1-xCoxNb2O9 samples, no impurity cobalt-containing phases were detected. The method of magnetic susceptibility and NEXAFS-spectroscopy investigated the electronic state and character of exchange interactions of cobalt atoms in solid solutions with the pyrochlore structure. According to X-ray spectroscopy and magnetic susceptibility of cobalt atoms in solid solutions in a dominant amount are in the charge state of Co(II) in the form of monomers and exchange-bound clusters mainly with antiferromagnetic type of exchange. Ceramics Co-doping of multicomponent bismuth niobate pyrochlore has at room temperature a high dielectric permeability of 84 and small dielectric losses of 0.001. The energy of activation of conductivity is equal to 0.75–1.03 eV. This type of ceramics with excellent dielectric properties can potentially be used as multilayer ceramic capacitors.

Two series of iron-containing solid solutions Bi2Mg1−x Fe x Nb2O9+δ and Bi2MgNb2−2x Fe2x O9−δ of pyrochlore structure were obtained by the traditional solid phase synthesis method. The electronic state and character of exchange interactions of iron atoms in solid solutions were investigated by methods of magnetic dilution and NEXAFS-spectroscopy. According to X-ray spectroscopy and magnetic susceptibility data, iron(III) atoms are distributed mainly in octahedral positions of niobium(V) and in a dominant amount are in the charge state of Fe(III) in the form of monomers and exchange-bonded clusters mainly with antiferromagnetic type of exchange. Differences in magnetic behavior of iron-containing solid solutions of both series have been revealed. Antiferromagnetic and ferromagnetic exchange can be realized between iron atoms, which, with increasing concentration of paramagnetic atoms and averaging structure distortions, becomes less significant. The parameters of exchange interactions in clusters and distribution of iron paramagnetic atoms depending on the concentration of solid solutions have been calculated.

The electronic state and character of exchange interactions of nickel and chromium atoms in solid solutions of Bi5Nb3-3xМ3xO15-δ (М-Cr,Ni) was researched by methods of magnetic susceptibility and NEXAFS-spectroscopy. NEXAFS spectra of nickel and chromium oxides were obtained. According to X-ray spectroscopy in solid solutions, chromium atoms are mainly in the charge state of Cr(III), and nickel atoms in the high-spin state of Ni(II) in octahedral coordination. In solid solutions, paramagnetic chromium and nickel atoms are present in the form of monomers and clusters with a common antiferromagnetic type of exchange

The methods of NEXAFS, EPR spectroscopy and static magnetic susceptibility were used to analyze the condition and nature of exchange interaction of paramagnetic cobalt atoms within the bismuth niobate Bi3NbO7 matrix. The samples of Bi3Nb1-xCoxO7-δ with cubic fluorite-like structure were synthesized by the solid-phase method; their microstructure and magnetic properties were studied. The samples had monolithic, non-porous solid microstructure without clear crystal boundaries. The results of the magnetic susceptibility and NEXAFS studies showed that cobalt atoms in the solid solutions were predominantly represented by Co(II) and Co(III) high-spin atoms and their exchange-coupled aggregates with the antiferromagnetic type of exchange. The magnetic moment of cobalt atoms in the solid solutions exceeded the pure spin values for Co(II)s=3/2 and Co(III)s=2, and ranged from 6.12 to 7.15 μB. The analysis of the NEXAFS Co2p-spectra of the cobalt-doped solid solutions and cobalt oxides revealed that the studied Co atoms were mainly in the +3 and +2 oxidation states. Only one component was reliably established in the recorded EPR spectra. This was an asymmetric broad band with the g factor of about 4. This signal could be attributed to the high-spin state (S = 3/2) of Co(II).

In order to study the electronic state of Fe atoms and interatomic exchange interactions in solid solutions of Bi2CaNb2−2xFe2xO9−δ with perovskite structure, methods of magnetic dilution, NEXAFS, XPS and EPR spectroscopy were used. Increased values of the magnetic moment of iron atoms in solid solutions compared to pure spin values for Fe(III) are explained by the presence of clusters of Fe(III) atoms predominantly with antiferromagnetic type of exchange. Exchange parameters and cluster distribution in Bi2CaNb2−2xFe2xO9−δ solid solutions depending on the Fe content were calculated according to the Heisenberg–Dirac–van Vleck model. The effect of calcium atoms on the clusterization degree and intensity of Fe interatomic exchange interactions was displayed. XPS spectra of Bi2CaNb2-2xFe2xO9-δ, were identified to enable estimation of the effective charge of the atoms composing it. The analysis of the NEXAFS Fe2p-spectra of iron oxides and the iron-containing solid solutions revealed that Fe atoms were mainly in an oxidation state of +3. EPR spectra of Fe containing solid solution powder show an intense asymmetric line in the low-field part of the spectra at g = 4.27 with weakly manifested shoulder g ∼ 8–10 and a broad component in the g ∼ 2 range with a high-field wing.

The electron state and the nature of the exchange interactions of cobalt atoms in BiNb1-xСoxO4-δ solid solutions of triclinic and orthorhombic modifications were studied using the magnetic susceptibility and NEXAFS-spectroscopy. In order to determine the electronic state of cobalt atoms, the solid solutions and oxides of cobalt CoO, Co3O4 were studied by NEXAFS-spectroscopy. The X-ray spectroscopy and the study of the magnetic susceptibility of the solid solutions revealed the presence of the monomers and exchange-related clusters of cobalt with the Co(II) and Co(III) charge states characterized mostly by the antiferromagnetic type of exchange. The cobalt containing solid solutions allowed us to confirm the reversibility of the α↔β-BiNbO4 phase transformation

Results of geochemical studies of suevites of the Kara astrobleme (Pay-Khoy) produced by a new approach based on square microprobe analysis of suevite matrix and consolidated impact melts with multivariate statistics have been described for the first time. At list three suevite varieties have been divided which are essentially differ by geomorphological, mineralogical, petrological and geochemical criteria. The protoliths of the initial rocks of the target for the suevite varieties have been supposed on the basis of a complex analysis.

The results of geochemical studies of the diamondiferous suevites of the Kara astrobleme (Pay-Khoy) using a new approach based on “area” microprobe analysis of suevite matrix and consolidated impact melt aggregates with subsequent data processing by multivariate statistic methods are described for the first time. At least three suevite varieties that differ essentially in geomorphology, mineralogy, petrography, and geochemical features have been recognized. The predominant protoliths of the rocks of the target are proposed for these suevite varieties on the basis of integrated data analysis.

Thermal stability, magnetic properties, ESR (Electron Spin Resonance) and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of the orthorhombic and triclinic modifications of solid solutions BiNb1-xMnxO4-δ have been studied. X-ray diffraction determined the reversibility of the phase transition from the high-temperature triclinic modification to the orthorhombic modification at 950 °C in the BiNb1-xMnxO4-δ solid solutions. The ESR spectra of the triclinic BiNb1-xMnxO4-δ revealed the sextet structure of Mn(II) ions with 8.4 mT splitting and some features at g = 3.80 and 1.47, and a broad diffuse band with g ∼ 2.2 having a sextet with 8–9 mT splitting and g = 2.0 against its background. The measurements of magnetic susceptibility allowed us to calculate the parameters of exchange interactions in dimers and the distribution of manganese atoms (II), (III) and (IV) in dependence on concentrations of the solid solutions in the orthorhombic and triclinic modifications. It was established that in the manganese-doped solid solutions of triclinic modification, the degree of aggregation and the nuclearity of the clusters were higher than in the solid solutions of orthorhombic modification. The clusters were presented in the forms of dimers and trimers of Mn(II), Mn(III) and Mn(IV) atoms with different valence atoms with antiferro- and ferromagnetic types of exchange. The solid solutions BiNb1-xMnxO4-δ as well as manganese oxides MnO, Mn2O3 and MnO2 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of manganese atoms. The DSC (Differential Scanning Calorimetry) curves of the samples BiNb1-xMnxO4-δ revealed the endothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at 1050 °C (x = 0.03) and 1048 °C (x = 0.06). The melting peaks were observed at 1231 °C (x = 0.03) and 1220 °C (x = 0.06), respectively. The differences in the magnetic characteristics and in the results of ESR and NEXAFs spectroscopy of the BiNb1-xMnxO4-δ of orthorhombic and triclinic modifications were due to the relative content of Mn(II), Mn(III), Mn(IV) ions and the degree of distortion of their local surroundings, composition and nature of exchange interactions in the clusters of manganese atoms.

Phase transformations and thermal stability of nickel-containing solid solutions of bismuth orthoniobate BiNbO4 were studied by methods of thermal and X-ray phase analysis. The DSC curve of the BiNb1-xNixO4-δ samples exhibited endothermic effects near the temperatures of 1049 °C and 1227 °C during the heating, and exothermic effects at ˜990 °C during the cooling, which were associated with the phase transition from the orthorhombic to triclinic modification and with congruent melting. In the compacted polycrystalline samples of the solid solutions, the phase transition from the high-temperature triclinic modification to the orthorhombic modification was revealed after calcination at 750 °C for 150 h, as well as after 1 h calcination at 950 °C.

Phase transformations of bismuth orthoniobate BiNbO4 were studied by the methods of differential scanning calorimetry and high-temperature X-ray diffraction. The DSC curve in the heating–cooling mode of the orthorhombic samples (α-BiNbO4) showed endothermic and exothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at the temperatures of ~ 1328 K and ~ 1270 K. This phase transition passed through a high-temperature phase of γ-BiNbO4, in which reflections were indicated in the temperature range of 1323 K → 1353 K → 1263 K. The X-ray diffraction pattern of the sample quenched in air (77 K) and calcined at the temperature from this range, 1323 K, corresponded to the triclinic modification. During the heating, the triclinic modification (β-BiNbO4) was transformed into the high-temperature phase of γ-BiNbO4, which was stable up to ~ 1270 K. Then, it was transformed into the triclinic modification again. The DSC curve of the sample of triclinic BiNbO4 showed endo- and exothermic effects at temperatures of ~ 1274 K and ~ 1270 K associated with the transition into the high-temperature phase of γ-BiNbO4. The cooling of the sample leads to the phase transition from the triclinic to the orthorhombic modification of BiNbO4. Thermostating at 1223 K for 1 h and quenching in air the compact sample or dispersed powder of bismuth orthoniobate of the triclinic modification led to the formation of the orthorhombic modification.

Magnetic dilution and NEXAFS spectroscopy were used in the study of iron-containing solid solutions Bi2MNb2O9 (M – Ba, Sr) with layered perovskite-like structure. The analysis of the NEXAFS Fe2p-spectra of iron-containing solid solutions and iron oxides revealed that the studied Fe atoms were mainly in the +3 oxidation state. The study of magnetic susceptibility of the Bi2Ba(Sr)Nb2−2xFe2xO9−δ solid solutions showed the influence of atoms of the second coordination sphere on the distribution, degree of clustering, and intensity of exchange interactions in clusters of iron atoms. It was established that iron (III) atoms exhibited selectivity in filling the cation positions, preferring less distorted octahedral positions. The substitution of barium with strontium atoms led to intensification of the exchange of the ferromagnetic type and to a decrease in the degree of clustering of iron atoms in the solid solutions Bi2SrNb2−2xFe2xO9−δ.

The manganese-doped polymorphs of bismuth orthoniobate were studied using thermal analysis, ESR, XPS and NEXAFS spectroscopy. It was shown that the phase transition from the orthorhombic (α) to the triclinic (β) modification can be reversible and it can be achieved by prolonged calcination of samples at 750°C. The DSC curves revealed no thermal effects associated with the phase transformation from the triclinic to the orthorhombic modification near 750°C. In the ESR spectra, the absorption band with g = 3.8 characterized by sextet structure disappeared during the phase transformation from the - to the -form and only the broad component with average intensity was observed at g = 2.2-2.0. According to NEXAFS spectroscopy data, the content of oxidized Mn(III) atoms increased and the content of Mn(II) decreased in the orthorhombic polymorph obtained at 750°C from the tri-clinic modification.

Thermal stability of calcium copper titanate was studied by differential scanning calorimetry, thermogravimetry and high-temperature mass spectrometry. Calcium copper titanate (CCTO) had no thermal effects and mass losses caused by thermal dissociation or any phase transitions, besides melting, in the temperature range of 298–1423 K. The melting point of calcium copper titanate is 1398 K. The endothermic effect at 1250 K was associated with the decomposition of copper (II) oxide segregated in the intergrain space of the CaCu3Ti4O12-CuO ceramics. In this connection, we proposed a simple method for estimating the content of copper oxide in the CaCu3Ti4O12-CuO composite. The processes of evaporation of CaCu3Ti4O12 in the temperature range of 1500–2100 K were studied by high-temperature mass spectrometry. In the temperature range of 1500–1750 K, easily volatilized copper oxide was evaporated selectively from the calcium copper titanate. At the temperature of 2100 K, atomic calcium and titanium oxides, TiO and TiO2, were present in the vapor.

In nature extreme PT-conditions result in numerous glass-like solids, the study of which allows specifying various structural and chemical features of extreme materials, which have a great application potential in various fields of technology. We studied features of nanostructure of natural impact glasses, including recently discovered ultrahigh-pressure high-temperature impact glasses, compared to low-pressure natural and synthetic standard silica glasses. In this paper we presented complex data of atomic force microscopy, X-ray diffraction, X-ray energy-dispersive spectrometry, infrared and Raman spectroscopy. We described nanostructural characteristics of impact glasses and showed influence of chemical composition on the features of their structure. We discovered that the elemental composition was the most important factor determining the glasses nano-heterogeneity. We noted an essential role of influence of Na impurity on glass nanostructure. The impact glasses with pure SiO2 composition have the smallest sizes of nanostructural elements.

Magnetic susceptibility and EPR of iron-containing solid solutions with layered perovskite-like structure Bi2BaNb2-2xFe2xO9-δ and Bi5Nb3-3xFe3xO15-δ have been studied. The solid solutions Bi2BaNb2-2xFe2xO9-δ as well as iron oxides FeO, Fe2O3, and Fe3O4 were studied by the NEXAFS spectroscopy. The formation of exchange-bound aggregates of Fe(III) atoms with antiferroand ferromagnetic exchange types has been found in the solid solutions. In the ESR spectra of the solid solution samples, the lines with g ≈ 2.0 and g = 4.27 having a weak shoulder at g ~ 8 were attributed to Fe(III) atoms in the octahedral field with strong rhombic distortion.

Liquation structures were described in ultrahigh-pressure impact glasses of the Kara astrobleme (Pay-Khoy) with differentiation into the bisilica, aluminosilicate, and ore components for the first time. The sequence of differentiation of mineral phases upon solidification of an ultrahigh-pressure impact melt was established: coesite, silicate glass, augite, aluminosilicate glass of albite composition, and pyrite. The discovered impact glasses are highly resistant to postimpact alterations.

The state of iron atoms and their interatomic interactions were investigated by the method of magnetic dilution in Bi2SrNb2−2xFe2xO9−δ with a layered perovskite-like structure. It was found that the solid solutions are characterized by increased values of the magnetic moment of iron atoms, compared with purely spin values of Fe(III). This was explained by the presence of exchange-bound aggregates of Fe(III) atoms with antiferro- and ferromagnetic types of exchange. The dependencies of the exchange parameters and cluster distribution on the iron content of Bi2SrNb2−2xFe2xO9−δ solid solutions were calculated.

On the basis of the data of ESR and magnetic susceptibility of Bi2BaNb2–2xMn2xO9–δ solid solutions with perovskite-like structure ferro- and aniferromagnetic bonded dimers, trimers, and tetramers of Mn(II), Mn(III), and Mn(IV) atoms in various valence states were found. The distribution of aggregates and exchange parameters in the clusters were calculated as a function of fraction x of manganese atoms in the solid solutions. In the ESR spectra of diluted solid solutions there are absorption bands with g-factors ~4.3 and 2.1 assigned to Mn(II) and Mn(IV) atoms.

A monoclinic distortion of tetragonal cell was found in the solid solutions with layered perovskite-like structure [Bi5Nb3–3xMn3xO15–δ (0.005 < х ≤ 0.06)]. In the ESR spectra absorption bands with g-factors ~4.3 and 2.1 assigned to Mn(II) and Mn(IV) atoms are recorded. The studies of magnetic susceptibility of the Bi5Nb3–3xMn3xO15–δ solid solutions revealed the formation of ferro- and antiferromagnetically bonded dimers of Mn(II), Mn(III), and Mn(IV) atoms. The exchange parameters in dimers and the distribution of paramagnetic manganese atoms in dependence to solid solution concentrations were calculated.

Magnetic susceptibility and ESR properties of manganese-containing solid solutions of bismuth orthoniobate BiNb1–x MnxO4–δ have been studied. Mn(II), Mn(III), and Mn(IV) atoms as well as their dimers exhibiting antiferro- and ferromagnetic types of exchange have been found in the solid solutions. Parameters of the exchange interactions in the clusters and the distribution of monomers and dimers of manganese atoms in the solid solutions have been calculated as functions of the paramagnetic atom fraction.

Magnetic susceptibility and ESR properties of iron-containing Bi2BaNb2–2xFe2xO9–δ solid solutions with layered perovskite-like structure have been studied. In the diluted solid solutions, iron atoms are combined into Fe(III) dimers and tetramers with antiferro- and ferromagnetic exchange types. The exchange parameters and the distribution of aggregates in the solid solutions have been calculated as functions of the paramagnetic atoms fractions.

From the data of magnetochemical study and ESR spectra of samples of the Bi3Nb1–xFexO7–δ (х ≤ 0.06) solid solutions iron atoms in the solid solutions of cubic modification of bismuth niobate were found to be located as Fe(III) monomers and exchange-bonded Fe(III)–O–Fe(III) dimers with anti- and ferromagnetic exchange types. The exchange parameters and the distribution of monomers and dimers over the solid solution were calculated as functions of the paramagnetic atom fraction.