The reaction of NaBiO3·nH2O with Ba2+ions in an alkaline medium has been studied using X-ray diffraction and chemicalanalysis methods. Dark green pseudocubic phases (а 4.271–4.272 Å) of sodium‒doped matrix barium‒bismuth(III,V)oxides, with a perovskite-like structure BamBim+nOy and a ratio of Ba and Bi atoms equal to 3 : 8,4 : 9, 1 : 2, 9 : 11, 7 : 8, and 1 : 1, are formed within 1 h at the boilingtemperature of 10 М NaOH solution (140°C). In sodium-doped phases, the amount ofsodium is variable and amounts to 1.4–4.7 at % of the sum of Σ(Ba, Bi). Theresulting samples are characterized by the bismuth average oxidation state $$\overline {Bi} $$ 4.49–4.84, which points to a partial reduction of Bi(V) toBi(III) during the reaction of NaBiO3·nH2O [ $$\overline {Bi} $$ 5.00(2)] with Ba2+ ions. Thereaction probably proceeds through the NaBiO3 hydrolysisstage.

The reliability of fundamental studies of superconductivity depends on the quality of the materials under study. Optical zone melting yields high-quality single crystals without impurities, which can be difficult for other technologies. The paper describes the growth procedure for single crystals of several families of superconductors: bismuth high-temperature superconductors Bi2Sr2CaCu2O8 + δ and Bi2Sr2–xLaxCuO6 + δ and a superconductor with an assumed p symmetry of the superconducting order parameter Sr2RuO4. We discuss the search criteria for synthesizing high-temperature yttrium superconductors YBa2Cu3O7 + δ by optical zone melting, which do not lead to the formation of single crystals. The procedure for obtaining single crystals includes several stages. The first is to anneal a mixture of powders of the required oxides and carbonates, taken in specific proportions, at temperatures up to 850°C. A solid-phase reaction takes place, resulting in the desired polycrystalline complex oxide; rods with a length of ~5–10 cm are obtained from this oxide using a hydraulic press. The second stage is annealing of the rods in air at temperatures up to 940°C and, if necessary, melting in an optical-zone-melting unit using lamps with a rated power of 500 W at an adjustable power from 20 to 95% with a drawing speed of 20–30 mm/h. The third stage is the growth of a single crystal at 20–95% power at a rate of 0.1–20 mm/h. The result is a mixture that disintegrates upon cracking into single crystals up to several millimeters in size. Measurements of the temperature dependence of the dynamic magnetic susceptibility of the synthesized single crystals at a frequency of 100 kHz are carried out, which makes it possible to determine the temperature of the superconducting transition and its width.

The role of oxygen in the synthesis and phase formation of perovskite-like oxides of the homologous series Bam + nBimOy (m = 1–10; n = 0–5, 7–9, 11, 13, 17) is discussed. X-ray diffraction and chemical analysis show that Bam + nBimOy oxides are formed in air in the presence of an oxidizing agent (oxygen) through the initial stage of formation of Bi(V) enriched phases. At this synthesis stage, the Ba : Bi 17 : 9–5 : 4 samples are of the two-phase type and consist of Ba2Bi+4.78O4.39 and BaBiO3 oxides. With an increase in the temperature, the samples first undergo solid-phase transformations into Ba : Bi 9 : 4, 5 : 2, 21 : 8 oxides, and in the liquidus–solidus region, liquid-phase transformations occur with the reduction of Bi(V) → Bi(III) and oxygen loss in the presence of oxygen-deficient phases. When cooling the oxygen-deficient Ba–Bi–O melt ( $$\overline {{\text{Bi}}} $$ = 3.00–3.06), oxygen is absorbed, as evidenced by an increase in the average degree of bismuth oxidation with a decrease in the annealing temperature due to the oxidation of Bi(III) → Bi(V). Cooling samples of Ba : Bi 25 : 8, 11 : 4, 21 : 8, 5 : 2, 9 : 4, and 2 : 1 to 20°C results in their complete saturation with oxygen; phases 25 : 8, 11 : 4, and 5 : 2 can be obtained almost completely oxidized ( $$\overline {{\text{Bi}}} $$ = 4.95–5.00).

The influence of the high valence forms Fe(IV), Cu(III), and Bi (V) forms on the processes of the phase formation of the Sr–Fe–O, Ba–Cu–O, Y(Eu)–Ba–Cu–O, Ba–Bi–O, and K–Ba–Bi–O systems is studied by powder X-ray diffraction, potentiometry, and chemical analysis. It is shown that, depending on the annealing conditions (temperature and oxygen partial pressure), complex oxides are synthesized, in which the oxygen content is determined by the “mixed-valence” state of iron, copper, or bismuth. Oxides in the systems under consideration are synthesized in the same way through the formation of oxygen-deficient phases during high-temperature annealing. During slow cooling or low-temperature treatment in an oxygen-containing atmosphere, these phases are saturated with oxygen with the oxidation Fe(III) → Fe(IV), Cu(I) → Cu(II) → Cu(III), and Bi(III) → Bi(V) and form phases with magnetic (Sr–Fe–O) and superconducting (Y(Eu)–Ba–Cu–O and K–Ba–Bi–O) properties. It is determined using chemical analysis that the oxidizing power of Bi(V) with respect to reducing agents is higher than the comparable oxidizing powers of Fe(IV) and Cu(III).

The effect of synthesis conditions on the phase composition and valence stateof bismuth in perovskite-like oxides of the BamBim+nOy (m = 1–9;n = 0–3, 5, 7, 9) homologous series hasbeen studied by means of X-ray diffraction and chemical analysis. The oxidessynthesized at p(O2) =1 kPa almost have not contained Bi(V), have been characterized by averageoxidation number of bismuth $$\overline {{\rm{Bi}}} $$ = 3.00–3.01 and, consequently, have been stoichiometric inoxygen content. These phases have been thermodynamically stable from thecrystallization region to 20°C. Under cooling below the solidus line, theBamBim+nOy oxides have been oxidized, as shown byincrease in $$\overline {{\rm{Bi}}} $$ to above 3.06. The final oxidation products at ≈700–20°С havebeen ВаВiO3 andBa4Bi1+33.00O23.5oxides. Perovskite-like BamBim+nOy oxides with average oxidation number $$\overline {{\rm{Bi}}} $$ > 3.06 have not been found. At p(O2) = 21–100 kPa, individual oxides ofthis series which contain significant amount of Bi(V) or exclusively Bi(V)cannot be obtained.

The structure and the valence states of iron in substituted strontium ferrite Sr2/3La1/3FeO3 – δ or Sr3LaFe3O9 – δ at various condition synthesis and heat treatment have been studied using X-ray diffraction and the Mössbauer spectroscopy. A series of vacuum annealing (10–3 Torr, 400–650°C) allows the observation of the evolution of the structure from the rhombohedral (for the Sr2LaFe3O9) to the orthorhombic (Sr2LaFe3O8) via intermediate multiphase states and also the redistribution of valence iron states.

The structural features and Fe valence states in the substituted strontium ferrite Sr2/3La1/3FeO3-δ (or Sr2LaFe3O9-δ) have been studied for different synthesis and heat treatment conditions by X-ray diffraction analysis and Mossbauer spectroscopy. A series of annealing of Sr2LaFe3O9-δ in vacuum ((10-3 Torr) in the temperature range of 400 - 650оС allowed us to trace the structure evolution from rhombohedral phase (Sr2LaFe3O9) to orthorhombic one (Sr2LaFe3O8) over the formation of intermediate multiphase states and the redistribution of Fe valence states as well.

The structural features and valence states of Fe in substituted strontium ferrite Sr2LaFe3O9–δ after a series of annealing events in vacuum at different temperatures from 400°C to 650°C are studied by X-ray diffraction (XRD) and Mӧssbauer spectroscopy. According to X-ray data, the two samples with extreme (in terms of oxygen content) compositions (Sr2LaFe3О9 and Sr2LaFe3О8) are single phase and have a rhombohedral and orthorhombic structure, respectively. When a vacancy appears, the structural state changes, multiphase states are formed, and an intermediate orthorhombic phase close to the structure of non-substituted Sr4Fe4O11 arises. The ratio of phase contents in the mixture changes as the oxygen content decreases. When an oxygen-vacancy concentration reaches one vacancy per three perovskite unit cells, the final orthorhombic phase Sr2LaFe3О8 is formed. According to the obtained Mӧssbauer data, Fe ions in the sample Sr2LaFe3О9 with a rhombohedral structure have two valence states: Fe4+ with an octahedral symmetric oxygen environment and an averaged-valence state Fe3.5+. The sample Sr2LaFe3O8 with an orthorhombic structure, according to Mӧssbauer data, is magnetic; the iron ion has the Fe3+ valence state with two oxygen environments, octahedral and tetrahedral, as in the brownmillerite phase of unsubstituted Sr2Fe2O5. Analysis of the complete data set of Fe valence states, their redistribution as the oxygen concentration decreases, and transitions from the paramagnetic state to the magnetic-ordering state allows us to correlate the information on the local environment of Fe cations with structural data.

A comparative analysis of the behavior of oxides containing Fe (III) and Fe (IV) in redox reactions with organic and inorganic reagents has been carried out. SrFeO3-x oxide obtained by solid-phase synthesis from SrCO3 and Fe2O3 has been an object to develop test methods. It has been shown that Fe (IV) exhibits the properties of a stronger oxidizer than Fe (III), thus providing a set of reagents which enable identification of Fe (IV): hydrobromic acid, Fe2+ complex with V (IV), and Mn (II) salts, and organic reagents, namely, amines (diphenylamine, o-tolidine, benzidine) and dyes (methyl red). Potentiometric method proved stronger oxidative properties of Fe (IV) compared to Fe (III) revealed in different character of changes in EMF during dissolution of the corresponding oxides in HCl. SrFeO3–x oxide does not oxidize Mn (II) to Mn (VII), Cr (III) to Cr (VI), Ce (III) to Ce (IV) in acidic media, and Cu (II) to Cu (III) in alkaline media. Since the oxide under study oxidize Cl– = 1.3583 V) and Br– = 1.087 V) ions to the corresponding halogens, but does not oxidize Mn2+ to = 1.51 V), an estimated value of the standard redox potential is ~1.4 V. We have developed for the first time a system of analytical tests for differentiation of Fe (IV) and Fe (III) which can provide monitoring of the synthesis of complex oxides and phase formation in the systems containing iron, alkali and alkaline earth metals.

The reaction of KBiO3–δ withBa2+ ions at a Ba : Bi ratio of 0.5‒1.6 (mol.) ina 10 M. KOH solution under reflux (~140°C) has furnished oxides, whichcomposition and structure have been studied by X-ray diffraction, energydispersive X-Ray, and chemical analysis. Pseudo cubic (а = 4.271‒4.285 Å) perovskite-like phases ofbarium‒bismuth(III,V)‒potassium oxides with insignificant impurity of sodiumhave been formed in theKBiO3–δ‒Ba2+‒OH–‒H2Osystem during 1 h. The products have been characterized by an average oxidationstate of bismuth –Bi = 4.36‒4.59. Barium content inthe resulting oxides has increased with an increase in its concentration in thecharge. According to the Ba–Bi ratio, the resulting phases can be classified asnon-superconducting oxides of homologous series МxBamBim+nOy (x <n), МxBamBim+nOy, and (М, Ba)m+nBimOy(m = 1, 2, ...; n = 0, 1, 2, ...), M = K, Na.

The physico-chemical basis of optimal conditions for synthesis of BaBiO 3 oxide, free from impurities of BaBiO 3 and BaBiO 3−x oxygen-deficient phases, is given. The reduction of time of the melt contact with a crucible makes it possible to obtain BaBiO 3 samples minimum polluted by a crucible material. It was found that an abnormal behavior of BaBiO 3 in the course of long-time annealing in air at 450°C is caused by the interaction with CO 2 .

A radically different behavior of oxides — BaBiO 3 and superconducting K n Ba m Bi m+n O y (n = 1, 2, 3; m = 1, 3, 4, 5) — and BaO 2 peroxide is observed in reactions of hydrolysis and acid dissolution. Chemical tests and potentiometric analysis did not reveal peroxide-ions present in BaBiO 3 and K n Ba m Bi m+n O y , as well as in the products obtained by oxide treatment with water, alkali and acids. Most probably this indicates to the absence of unusual valence states (UVS) of oxygen in the form of peroxide- and superoxide-ions in the structure of oxides under study. Data of potentiometric titration (PT) also proved the absence of oxygen UVS and bismuth in the oxidation state of < + 3 in superconducting K n Ba m Bi m+n O y oxides. A number of agents (Mn (II), Cr (III), Cu (II), Ce (III) salts, Arnold’s base, methyl red, chromazurol S, arsenazo III, indigo carmine, and thoron) are revealed which provide Bi (V) detection in BaBiO 3 and K n Ba m Bi m+n O y . An experimentally observed behavior of the oxides in chemical reactions regarding agents-reductants is attributed to Bi (V) present in their structure which is much stronger oxidizer compared to peroxide dioxygen.

The composition and the structure of ceramic EuBa2Cu3O6 + δ (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu n Ba m Cum + nO y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu n Ba m Cum + nO y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature Tc of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu n Ba m Cum + nO y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with Tc = 82–90 K.

The solubility of hydrogen in amorphous Mg 0.6 SiO 2.6 at a temperature of 250°C and pressures up to 75 kbar is studied using a quenching technique. The molar ratio H 2 /formula unit is found to nonlinearly increase with pressure from x = 0.12 at P = 10 kbar to x = 0.303 at P = 75 kbar. An investigation of the quenched samples by Raman spectroscopy demonstrated that hydrogen dissolves in amorphous Mg 0.6 SiO 2.6 in the form of H 2 molecules. X-ray diffraction and Raman studies showed that the hydrogenation of the samples is likely to be accompanied by a phase transition in the amorphous lattice of Mg 0.6 SiO 2.6 at P ≈ 52.5 kbar to a denser amorphous modification.