In this paper, we studied substituted lanthanum orthoferrite La 0.67 Sr 0.33 FeO 3 – γ by Mössbauer spectroscopy using X-ray diffraction data. A series of vacuum annealings was performed in the temperature range of t ann = 200–650°C, after which no significant changes in the structure of the samples were detected. The Mössbauer measurements at room temperature show that the Fe ions are in an average valence state between Fe 3+ and Fe 4+ . Upon vacuum annealing, as the temperature t ann increased, the average hyperfine magnetic field on the 57 Fe nuclei and the isomer shift of the spectrum increased, which is associated with an increase in the number of vacancies and, accordingly, a decrease in the amount of Fe 4+ . Mössbauer measurements at 85 K showed that the average valence state of iron does not manifest itself. The hyperfine parameters of the low-temperature Mössbauer subspectra obtained from the model interpretation indicate that one of them belongs to Fe 4+ ions, and the rest belong to Fe 3+ . The presence in the spectra of several sextets related to Fe 3+ ions is due to the appearance of oxygen vacancies (breaking of the Fe 3+ –O 2– –Fe exchange bond) and Fe 4+ ions (weakening of the Fe 3+ –O 2– –Fe exchange bond) in the nearest ionic neighborhood of Fe atoms. Both factors cause a decrease in the hyperfine magnetic field and a change in the isomer shift of the spectrum. As a result of model interpretation of the Mössbauer spectra, the numbers of oxygen vacancies and Fe 4+ ions per formula unit depending on vacuum annealing temperature t ann were determined for all samples. It was shown that at t ann above 450°C, the process of oxygen leaving the lattice ends and only Fe 3+ ions are detected.

The article presents experimental studies of structural changes that occur during heating of nanosized powders of amorphous ice obtained by decay of a water-helium gel. Thermal annealing of the obtained samples was carried out by short exposures (about 15 minutes) at different temperatures in the range of 110-230K. The behavior of the amorphous phase during annealing was analyzed within the framework of its description by a mixture of amorphous ices of low and medium density (LDA and MDA, respectively). It was found that at the such description, the virgin sample was predominantly in the MDA state, while the proportion of the LDA phase was about 7 times less (MDA/LDA ≈ 7:1). It has been established that during annealing, a multistage process of structural transformations of the initial LDA + MDA sample takes place: from initial changes in the amorphous state at 110 K through crystallization of the cubic ice phase Ic with its intensive growth at a temperature of 130 K to the transformation of cubic ice into the hexagonal phase Ih in the temperature range T =135÷230K.

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.

The X-ray and Mossbauer studies of the Brownmillerite phase have been carried out in the given work and the well-known literature data on the single phase compounds with a perovskite structure in ferrite strontium SrFeO3-δ have been analyzed. It has been found out that all Fe valence states for any phase composition of ferrite strontium are determined by its local oxygen environment. It allows us to understand the behavior of Fe transition from one valence state to another when adding oxygen vacancies and to explain the Fe structural states in the SrFeO3-δ oxide including single and two-phase compositions. This approach is a more general case for description of the all known compounds and synthesized phase combinations in SrFeO3-δ and the formula considered in literature for the single-phase structures well agrees with it.

Metastable superconducting-like behavior of rhombohedral Bi samples exposed to surface oxidation under special conditions has been observed below the critical temperature range of 15-35 K. The ac magnetic susceptibility and electrical resistance measurements imply that the superconductivity appears in the Bi/Bi2O3 interfacial layer formed at the surface of the samples.

The brownmillerite phase has been studied by X-ray diffraction and Mössbauer methods. The available data on single-phase compounds with the perovskite structure in strontium ferrite SrFeO3 – δ are analyzed. All the valence states of iron for any phase state of strontium ferrite are found to be determined by its local oxygen environment. This fact enables us to understand the regularity of transitions of iron from one valence state to another state when adding oxygen vacancies and to explain the structural states of iron in strontium ferrite, such as the single-phase and two-phase compositions. This approach is a more general case for description of all known compounds and phase combinations synthesized in SrFeO3 – δ than the formula for single-phase structures that is considered in the literature and fit well into the scheme proposed in this work.

This article presents the results of SANS studies on the structure of nanocluster impurity gel samples, and X-ray observations of the structural transformations in amorphous powders obtained during the decay of heated gel samples. The possibilities of using these methods to determine the state and structural transitions of these substances are discussed.

The specific features of the structure in a polycrystalline anion-deficient strontium ferrite SrFeO3 – δ have been studied at different oxygen contents by the Mössbauer spectroscopy, X-ray diffraction, and scanning electron microscopy. Three compounds with different compositions have been prepared in the dependence on the condition of heat treatment. Within each of the structures, there are several nonequivalent positions of Fe corresponding to different valence states of Fe and different local oxygen environments, the relation and the degree of distortion of which are changed in the dependence on the oxygen content. Based on the Mössbauer data, the oxygen content in each of the structures is estimated. Yet another ideal Sr16Fe16O45 composition of the SrFeO3 – δ compound is proposed for an intermediate composition in addition to those available in the literature.

The structure features of polycrystalline anion-deficient ferrite strontium SrFeO3-δ have been investigated for different oxygen content by Mossbauer spectroscopy, X-ray diffraction analysis and scanning electron microscopy. Three structures with a different composition have been prepared depending on heat treatment conditions. Several non-equivalent Fe positions exist within each structure that correspond to different local oxygen environments the relation and distortion degree of which change depending on oxygen quantity. Based on the Mossbauer data obtained an oxygen content has been estimated for each structure. One more the model intermediate composition Sr16Fe16O45 of the SrFeO3-δ compound is proposed.

AbstractWe studied the structure, IR absorption spectra, the spectral characteristics of photoluminescence and morphology of cerium- and terbium-doped orthoborates of gadolinium and yttrium obtained by hydrothermal synthesis at 200°C, as well as solid solutions of orthoborates on the basis of yttrium, gadolinium, and lutetium with composition RECe_0.01Tb_0.1BO_3 (RE = Lu_0.5Gd_0.39, Lu_0.5Y_0.39, and Y_0.5Gd_0.39). The X-ray diffraction spectrum of yttrium orthoborate Y_1 – _ x _– _ y Ce_ x Tb_ y BO_3 is described by a hexagonal lattice with space group P 6_3/ m , which, after annealing at 970°C, transforms into a monoclinic lattice with space group C 2/ c . High-temperature annealing of the studied orthoborates leads to a multiple, more than two orders of magnitude, increase in the luminescence intensity of Tb^3+ ions when the samples are excited in the absorption band of cerium ions. This effect is the result of a significant increase in the concentration of Ce^3+ ions in the orthoborates at high temperatures. It is shown that the luminescence of terbium ions is due to energy transfer from Ce^3+ to Tb^3+, which proceeds with high efficiency (∼85%) by the mechanism of dipole-dipole interaction between cerium and terbium.

We studied the structure, IR absorption spectra, the spectral characteristics of photoluminescence and morphology of cerium- and terbium-doped orthoborates of gadolinium and yttrium obtained by hydrothermal synthesis at 200°C, as well as solid solutions of orthoborates on the basis of yttrium, gadolinium, and lutetium with composition RECe0.01Tb0.1BO3 (RE = Lu0.5Gd0.39, Lu0.5Y0.39, and Y0.5Gd0.39). The X-ray diffraction spectrum of yttrium orthoborate Y1 – x – yCexTbyBO3 is described by a hexagonal lattice with space group P63/m, which, after annealing at 970°C, transforms into a monoclinic lattice with space group C2/c. High-temperature annealing of the studied orthoborates leads to a multiple, more than two orders of magnitude, increase in the luminescence intensity of Tb3+ ions when the samples are excited in the absorption band of cerium ions. This effect is the result of a significant increase in the concentration of Ce3+ ions in the orthoborates at high temperatures. It is shown that the luminescence of terbium ions is due to energy transfer from Ce3+ to Tb3+, which proceeds with high efficiency (∼85%) by the mechanism of dipole-dipole interaction between cerium and terbium.

Experimental studies of the kinetics of phase transitions of C2H5OH ethyl alcohol samples from amorphous to crystalline phase demonstrated that their transition rate and energy characteristics strongly depend on the size and composition of the samples (rectified alcohol, absolute alcohol). Almost all bulk amorphous samples of rectified alcohol that were several millimeters in size crystallized into the monoclinic phase within few hours at a temperature of T ∼ 125 K, while in amorphous nanocluster samples consisting of clusters of the order of tens nanometers in size, a similar transition was observed at T ∼ 110 K. Characteristic transition times from one crystalline phase to another also differed greatly. Temperatures at which the transition from the plastic to the monoclinic phase occurred for both bulk and nanoclusters samples of absolute alcohol were lower (100–105 K), and crystallization of nanocluster samples occurred much more rapidly. The exponent moduli in time dependence of x ∼ tn in the JMAK model (crystallization of an amorphous substance) strongly depended on temperature, and for nanocrystallites were of the order of one or less. However, for massive samples, the exponent modulus n turned out to be closer to two for both rectified and absolute alcohol.

The structural and spectroscopic features of the EuAI(3)(BO3)(4) individual skeletal microcrystals synthesized by a melt solution method have been studied. Their infrared spectra taken from the as-grown microcrystal surfaces mainly contain the lines of the rhombohedral modification of EuAl3(BO3)(4) and additional peaks of its monoclinic modification. TEM and X-ray diffraction studies confirm that these additional peaks in the IR spectra belong to the monoclinic C2/c polytype of the EuAl3(BO3)(4) compound. We are the first to demonstrate the presence of coherent monoclinic domains in rhombohedral EtuAl(3)(BO3)(4) crystals by TEM. Cathodoluminance spectroscopy shows that the microcrystals generate strong emission lines in the range 580-630 nm, and their intensities are strongly influenced by the crystal orientation.

Europium alumoborate EuAl 3 (BO 3 ) 4 microcrystals have been synthesized by the flux method at a temperature of 1050°C. The obtained crystals have different morphologies: both plane-faced and skeletal microcrystals have been observed. Infrared spectroscopy, cathodeluminescence, and transmission electron microscopy investigations of individual microcrystals showed that the spectral and structural characteristics of these morphological forms coincide. The obtained crystals are characterized by the rhombohedral symmetry (sp. gr. R 32) with the inclusions of C 2/ c monoclinic phase domains.

Микрокристаллы алюмобората европия EuAl3(BO3)4 были получены из раствора-расплава при температуре 1050oC. Синтезированные кристаллы имели различную морфологию: наблюдались как плоскогранные, так и скелетные микрокристаллы. Исследования отдельных микрокристаллов с помощью инфракрасной спектроскопии, катодолюминесценции и просвечивающей электронной микроскопии показали, что спектральные и структурные характеристики этих морфологических форм совпадают. Полученные кристаллы имели ромбоэдрическую симметрию с пространственной группой R32 с включенными в структуру доменами моноклинной фазы C2/c. Работа выполнена при частичном финансовом содействии Министерства образования и науки Российской федерации (грант 14.B25.31.0018). DOI: 10.21883/FTT.2017.12.45238.141

The structure, IR absorption spectra, morphology, and photoluminescence spectral characteristics of Lu 1– x – y Ce x Tb y BO 3 solid solutions prepared by the hydrothermal synthesis method have been investigated. According to the X-ray powder diffraction data, the Lu 1– x – y Ce x Tb y BO 3 samples hydrothermally synthesized at a temperature T = 200°C have the vaterite structure, and all the observed diffraction peaks correspond to the hexagonal phase with the space group P 6 3 / mmc , which is isostructural to pure lutetium borate LuBO 3 . Annealing of these samples at T = 970°C leads to a change in the structural modification and to the transition of the samples to the monoclinic phase with the space group C 2/ c . It has been found that, after annealing of these samples at temperatures T = 800–970°C, the luminescence intensity of Tb 3+ ions upon excitation in the absorption band of Ce 3+ ions increases by more than two orders of magnitude and becomes much higher than that in compounds of the same composition, but prepared by high-temperature synthesis. At the same time, annealing of lutetium borate doped only with terbium does not lead to a significant change in the luminescence intensity of Tb 3+ ions. The possible reasons for a multiple increase in the luminescence intensity of terbium ions due to the annealing of the hydrothermally synthesized lutetium orthoborate samples doped with cerium and terbium ions have been discussed.

Проведены исследования структуры, ИК-спектров поглощения, морфологии и спектральных характеристик фотолюминесценции полученных методом гидротермального синтеза твердых растворов Lu1-x-yCexTbyBO3. Согласно данным рентгенофазового анализа, полученные в результате гидротермального синтеза при T=200oC образцы Lu1-x-yCexTbyBO3 имеют структуру ватерита, и все дифракционные пики соответствуют гексагональной фазе с пр.гр. P63/mmc, изоструктурной чистому LuBO3. Отжиг этих образцов при T= 970oC приводит к изменению структурной модификации и переходу образцов в моноклинную фазу с пространственной группой C2/c. Обнаружено, что после отжига этих образцов при T=800-970oC интенсивность свечения ионов Tb3+ при возбуждении в полосе поглощения ионов Cе3+ увеличивается более чем на два порядка и становится значительно выше, чем в соединениях такого же состава, полученных при высокотемпературном синтезе. В то же время отжиг бората лютеция, легированного только тербием, не приводит к заметному изменению интенсивности свечения ионов Tb3+. Обсуждаются возможные причины многократного усиления свечения ионов тербия при отжиге полученного методом гидротермального синтеза ортобората лютеция, легированного церием и тербием. DOI: 10.21883/FTT.2017.06.44487.426