Phase Transformations, Magnetic Susceptibility and NEXAFS-Spectra of Cobalt-Doped Solid Solutions of Bismuth Orthoniobate

Методами магнитной восприимчивости и NEXAFS-спектроскопии исследовано электронное состояние и характер обменных взаимодействий атомов кобальта в твердых растворах BiNb1-xСoxO4-δ триклинной и орторомбической модификаций. Для определения электронного состояния атомов кобальта исследованы твердые растворы и оксиды кобальта СoO, Co3O4 методом NEXAFS-спектроскопии. По данным рентгеновской спектроскопии и магнитной восприимчивости, атомы кобальта в твердых растворах находятся в зарядовом состоянии Co(II) и Co(III) в форме мономеров и обменносвязанных кластеров преимущественно с антиферромагнитным типом обмена. На примере кобальтсодержащих твердых растворов подтверждена обратимость фазового превращения α↔β-BiNbO4The 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 transformatio

NEXAFS and XPS Studies of Co Doped Bismuth Magnesium Tantalate Pyrochlores

Co doped bismuth magnesium tantalate with a pyrochlore structure (sp. gr. Fd-3m) was synthesized for the first time using the standard ceramic method. Single phase Bi2Mg1−xCoxTa2O9 samples were found to be formed when x 4 (sp. gr. P-1) is detected, and its amount is proportional to the degree of cobalt doping. The formation of solid solutions is evidenced by a uniform increase in the unit cell parameter of the Co,Mg co doped bismuth tantalate phase with an increase in the content of cobalt ions in the samples from 10.5412(8) (x = 0.3) to 10.5499(8) Å (x = 0.7). The samples exhibit a porous microstructure consisting of chaotically oriented and partially fused elongated grains measuring 1–2 μm. The dependence of the ceramic grain size on the n(Mg)/n(Co) ratio was not determined. X-ray spectroscopy (ear dge X-ray bsorption ine tructure (NEXAFS) and X-ray photoelectron spectroscopy (XPS)) was used to study the charge state of ions in Bi2Mg1−xCoxTa2O9. The NEXAFS and XPS data showed that doping with cobalt and magnesium did not change the bismuth and tantalum oxidation states in pyrochlore; in particular, the ions maintained their oxidation states of Bi(+3), Mg(+2) and Ta(+5). The energy position of the peaks of the Ta4f-, Ta5p-, Ta4d spectra had a characteristic shift towards lower energies compared to the binding energy in pentavalent tantalum oxide Ta2O5. A shift towards lower energies is characteristic of a decrease in the effective positive charge; in particular, for the Ta4f and Ta4d spectra we presented, this energy shift was ΔE = 0.65 eV, and in the region of the Ta4d edge—0.55 eV. This in turn allowed for us to assume that tantalum atoms have the same effective charge +(5-δ). The oxidation state of cobalt ions was predominantly 2+ and partially 3+, according to NEXAFS spectroscopy data

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

Features of the Phase Formation of Cr/Mn/Fe/Co/Ni/Cu Codoped Bismuth Niobate Pyrochlore

The phase formation process of Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Nb2O9+Δ containing 3d-ions of transition elements in equimolar quantities was studied in a wide temperature range (400–1050 °C). The complex oxide crystallizes in the structural type of pyrochlore (sp. gr. Fd-3m:2, a = 10.4937(2) Å). The investigation of the multi-element pyrochlore phase formation process showed that the synthesis goes through a series of successive stages, during which the transition from Bi-rich to Bi-depleted compounds takes place. The predecessor of the pyrochlore phase is bismuth orthorhombic modification orthoniobate (α-BiNbO4) with an equimolar ratio of Bi(III)/Nb(V) ions. The pyrochlore phase is formed as a result of bismuth orthoniobate doping with transition element ions. The complex oxides Bi14CrO24, Bi25FeO40, BiNbO4, and Bi5Nb3O15 appeared as intermediate phases during the synthesis. The interaction between the initial oxide precursors is fixed at temperatures above 500 °C. The phase transition of α-Bi2O3 into β-Bi2O3 near 500 °C is observed. Varying the heat treatment duration at each synthesis step did not qualitatively change the phase composition of the sample but had an effect on the quantitative phase ratio. Phase-pure pyrochlore of the given composition by solid-phase synthesis method can be obtained at a temperature no lower than 1050 °C. Ceramics are characterized by low-porous dense microstructure with blurred outlines of grain boundaries

NEXAFS-спектры и магнитная восприимчивость ниобата висмута, допированного никелем и хромом

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 exchangeМетодами магнитной восприимчивости и NEXAFS-спектроскопии исследовано электронное состояние и характер обменных взаимодействий атомов никеля и хрома в составе твердых растворов Bi5Nb3-3xМ3xO15-δ (М-Cr,Ni). Получены NEXAFS-спектры оксидов никеля и хрома. По данным рентгеновской спектроскопии, в твердых растворах атомы хрома находятся преимущественно в зарядовом состоянии Cr(III), а атомы никеля – в высокоспиновом состоянии Ni(II) в октаэдрической координации. В твердых растворах парамагнитные атомы хрома и никеля присутствуют в форме мономеров и кластеров с общим антиферромагнитным типом обмен

Study of Marine Sponges Graphitization during Heat Treatment up to 1200 °C

The results of studies of marine sponge carbonization processes during thermal treatment in an argon atmosphere in the temperature range from room temperature to 1200 °C are presented. The spatial structure, atomic composition of native and carbonized sponges, and their changes during pyrolysis were characterized using a set of methods that are informative at the macro- (thermogravimetric analysis, derivative thermogravimetric analysis, differential scanning calorimetry), micro- (Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy), and nanoscales (X-ray absorption and photoelectron spectroscopy using synchrotron radiation and a sample charge compensation system). Preservation of the 3D architecture at the macro- and microlevels and graphitization of the interfibril medium with the formation of turbostratic graphite at the nanolevel were demonstrated. It was shown that the atomic contents of nitrogen, carbon, and oxygen in the spongin were ~2–3 at.%, ~5 at.%, and ~4 at.%, respectively. The matter concentrated in the space between the spongin fibrils included ~70 at.% carbon and ~11 at.% oxygen, with a large proportion of carbon (~63 at.%) involved in the formation of aromatic and C–C bonds and the remainder in carbon monoxide compounds. After the decomposition of spongin at 400 °C, this substance transformed into turbostratic graphite, preserving the 3D architecture of the original marine sponge as the temperature rose

XPS and NEXAFS Characterization of Mg/Zn and Mn Codoped Bismuth Tantalate Pyrochlores

Two series of the bismuth tantalate pyrochlore samples, codoped with Mg,Mn and Zn,Mn, were synthesized via solid-phase reaction. It was established that the Bi2Mg(Zn)xMn1−xTa2O9.5−Δ (x = 0.3; 0.5; 0.7) samples contain the main phase of cubic pyrochlore (sp. gr. Fd-3m) and an admixture of triclinic BiTaO4 (sp. gr. P-1). In both sets, the amount of BiTaO4 is proportional to the amount of manganese doping, however, zinc-containing samples have a higher level of impurities than magnesium-containing ones. The unit cell parameter of the Zn,Mn codoped bismuth tantalate phase increases with an increasing content of zinc ions in the samples from 10.4895(5) (x = 0.3) to 10.5325(5) Å (x = 0.7). The unit cell parameter of Mg,Mn codoped bismuth tantalate pyrochlores increases uniformly with an increasing index x(Mg) from 10.4970(8) at x = 0.3 to 10.5248(8) Å at x = 0.7, according to the Vegard rule. The NEXAFS and XPS data showed that the ions were found to have oxidation states of Bi(+3), Ta(+5), Zn(+2) and Mg(+2). In the Ta 4f XPS spectrum of both series of samples, a low energy shift of the absorption band characteristic of tantalum ions with an effective charge of (+5-δ) was observed. The XPS spectra of Bi4f7/2 and Bi4f5/2 also show a shift of bands towards lower energies which is attributed to the presence of some low-charge ions of transition elements in the bismuth position. The NEXAFS spectroscopy data showed that manganese ions in both series of samples have predominantly 2+ and 3+ oxidation states. XPS data indicate that in zinc-containing preparations the proportion of oxidized manganese ions is higher than in magnesium-containing ones

The Formation of Nanoscale Closed Graphene Surfaces during Fullerite C₆₀ Hot Isostatic Pressing

The fullerite C60 modified by hot isostatic pressing (HIP) at 0.1 GPa in argon near and beyond its thermal stability region (920–1270 K temperature interval) was studied by X-ray diffractometry, Raman spectroscopy, ultra soft X-ray photoelectron and near edge X-ray absorption fine structure spectroscopy. It was found that the C60 molecules merge into closed nanocapsules with a graphene surface during the thermal treatment. The conducted studies showed that using HIP treatment of the fullerite C60, it is possible to obtain a chemically resistant material with a high hardness and elasticity, as well as a density lower than that of the graphite. This new material, consisting of closed graphene nanocapsules 2–5 nm in size, formed by sp2 covalent bonds between carbon atoms is promising for various applications, and as a basis for the synthesis of new composite materials

The Formation of Nanoscale Closed Graphene Surfaces during Fullerite C60 Hot Isostatic Pressing

The fullerite C60 modified by hot isostatic pressing (HIP) at 0.1 GPa in argon near and beyond its thermal stability region (920–1270 K temperature interval) was studied by X-ray diffractometry, Raman spectroscopy, ultra soft X-ray photoelectron and near edge X-ray absorption fine structure spectroscopy. It was found that the C60 molecules merge into closed nanocapsules with a graphene surface during the thermal treatment. The conducted studies showed that using HIP treatment of the fullerite C60, it is possible to obtain a chemically resistant material with a high hardness and elasticity, as well as a density lower than that of the graphite. This new material, consisting of closed graphene nanocapsules 2–5 nm in size, formed by sp2 covalent bonds between carbon atoms is promising for various applications, and as a basis for the synthesis of new composite materials

The Identification of Cu–O–C Bond in Cu/MWCNTs Hybrid Nanocomposite by XPS and NEXAFS Spectroscopy

The results of the research of a composite based on multi-walled carbon nanotubes (MWCNTs) decorated with CuO/Cu2O/Cu nanoparticles deposited by the cupric formate pyrolysis are discussed. The study used a complementary set of methods, including scanning and transmission electron microscopy, X-ray diffractometry, Raman, and ultrasoft X-ray spectroscopy. The investigation results show the good adhesion between the copper nanoparticles coating and the MWCNT surface through the oxygen atom bridge formation between the carbon atoms of the MWCNT outer graphene layer and the oxygen atoms of CuO and Cu2O oxides. The formation of the Cu–O–C bond between the coating layer and the outer nanotube surface is clearly confirmed by the results of the O 1s near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) of the Cu/MWCNTs nanocomposite. The XPS measurements were performed using a laboratory spectrometer with sample charge compensation, and the NEXAFS studies were carried out using the synchrotron radiation of the Russian–German dipole beamline at BESSY-II (Berlin, Germany) and the NanoPES station at the Kurchatov Center for Synchrotron Radiation and Nanotechnology (Moscow, Russia)