Микроструктура и фазовый состав сплава дисилицидов железа и хрома

The phase composition, microstructure, and interphase interfaces of the disordered CrSi2-FeSi2 solid solution obtained by spontaneous crystallization (before and after annealing) have been investigated by scanning, transmission electron microscopy, electron diffraction, and X-ray energy dispersive spectrometry. The as-grown samples contained the phases of CrSi2 with the P6422 hexagonal structure and FeSi2 with the P4/mmm tetragonal structure. Annealing of the samples led to the phase transformation of tetragonal FeSi2 into the orthorhombic modification Cmca. Precipitates of cubic iron monosilicide FeSi with space group P213, nano-precipitates of Si and silicon silicide Cr5Si3 with a tetragonal structure I4/mcm were observed inside the FeSi2 grains. Impurities of interstitial Cr atoms with a concentration up to 2.0 at% are found in iron (di)silicides grains in all samples. The structure of the CrSi2 phase remains unchanged after annealing; the concentration of impurity iron atoms is about 0.7 at%. Orientation relationships between the crystal lattices of the phases are established and strains due to the mismatch of the crystal lattices are determined.

Mechanically-induced Shapiro steps: the effect of vibration on the dynamics of a charge density wave

Problem formulating. The quasi one-dimensional conductor TaS3 of the orthorhombic modification shows strong electromechanical coupling of the sliding charge density wave (CDW) with the lattice of crystals. However, until now the effect of the dynamic deformation of the sample on the CDW sliding has not been studied. Goal. Search and study of a new effect: the influence of sample vibration on CDW dynamics. Result. The conductivity of TaS3 samples is studied as a function of the electric field and the frequency of mechanical deformation of the sample. It is found that when the vibration frequency of the piezoelectric actuators deforming the sample coincides with one of the mechanical resonances of the sample, suppression of the threshold field and features in the form of Shapiro steps are observed on the I–V characteristics. The effect is similar (though not equivalent) to that of RF voltage applied directly to the sample at the same frequency. Evidence is provided that these features are not coupled with the effect of electrical interference. Practical meaning. The result opens up a new area of electromechanical effects in quasi one-dimensional conductors with CDW and can be used for detecting and processing of mechanical oscillations in the microwave range.

Synthesis and properties of nanocomposites based on zinc phosphate and fumed silica

The aim of the work was to synthesize nanocomposites based on pyrogenic silica and zinc phosphate by a simple method without using a large amount of solvent and to study the characteristics and properties of the obtained materials. The dual systems of zinc phosphate/pyrogenic silica with the different ratio of components were synthesized via mechanical grinding in a porcelain drum ball mill of fumed silica (Orysyl A-380), zinc acetate (Zn(CH3COO)2·2H2O) and phosphoric acid with distilled water, followed by air-drying in an oven at 125 °C (2 h) and calcination in a muffle oven at 450 °C for 2 h. The zinc phosphate content was 0.1, 0.2, and 0.3 mmol per 1 g of SiO2. The control sample (ZP-K) was synthesized by thermal treatment of the precipitate, formed after mixing on a magnetic stirrer an aqueous solution of zinc acetate with the addition dropwise of phosphoric acid, without the use of SiO2. X-ray diffraction studies of the nanocomposites confirmed the formation of the crystalline phase of Zn3(PO4)2·4H2O (orthorhombic modification) both in the silica-containing and control ZP-K samples after air drying at 125 °C, while heat treatment at 450 °C leaded to the formation of the anhydrous monoclinic Zn3(PO4)2 phase. The content of the zinc phosphate in the dual composites was 0.1, 0.2, and 0.3 mmol per 1 g of SiO2. The IR spectra of the nanocomposites indicated the presence of absorption bands in the range of 3760-3600 cm-1, which were attributed to the unequal structural ‒OH groups of silicon and phosphorus atoms. It was found that the presence of zinc phosphate on the SiO2 surface does not cause the chemical interaction with silica during heat treatment of composites in air even at 900-1000 °C. It was shown that the ability of Zn3(PO4)2/SiO2 composites to adsorb water vapor decreases with increasing amount of modifying compound. The effect of the obtained phosphorus-containing nanocomposite on the thermal stability of an alkyd polymer matrix was considered.

Ternary Aluminides of a New Homologous Series—CePt2Al2 and CePt3Al3: Crystal Structures and Thermal Properties

In the process of studying the Ce–Pt–Al system, we identified CePt2Al2 and CePt3Al3, two new ternary intermetallic compounds. CePt2Al2 aluminide undergoes a structural phase transition from a low-temperature orthorhombic modification (of its own structure type, Cmme, a = 5.84138(2) Å, b = 6.39099(3) Å, c = 10.11611(5) Å) to a high-temperature tetragonal modification (CaBe2Ge2 type, P4/nmm, a = 4.3637(9) Å, c = 10.0925(14) Å) at 280(1) °C. CePt3Al3 crystallizes with a new type of structure (Cmme, a = 6.36548(6) Å, b = 5.78301(6) Å, c = 13.36245(19) Å) built of structural units of low-temperature orthorhombic CePt2Al2-type and CsCl-type.

Structural and phase states of titanium borides produced by the self-propagating high-temperature synthesis method in the field of ultrasound oscillations

The effect of ultrasound oscillations (USO) on the velocity and temperature of combustion during self-propagating high-temperature synthesis (SHS) in the Ti-B system and structural and phase states of the produced titanium borides is studied using the earlier developed experimental setup. The effect of USO on SHS is subdivided into thermal and physical (non-thermal). The thermal influence is connected with cooling of the specimen surface because of the occurrence of forced convection of the ambient gas, and the physical effect is due to the action of USO on complex interaction processes in the SHS wave such as melt spreading, heterogeneous reactions and mass transfer in the liquid phase. Imposition of USO on the SHS process brings about changes in the phase composition of the synthesis products. For charge composition Ti–1.0В the content of orthorhombic modification of phase TiB increases from 78.2 % without USO to 82.9 % at the USO amplitude ξ = 10 mm, while the content of the cubic modification of this phase decreases from 9.2 % at ξ= 0 to 6.8 % at ξ = 10 mm. For all the examined compositions, the amount of residual titanium and Ti3B4decreases and the content of TiB2increases. It is determined that carrying out SHS in the field of USO results in a change of the specific heat capacity of the target synthesis products: with raising the USO amplitude it increases by 4–5 %. Thereby it is shown that imposition of USO on SHS is an efficient physical method for purposeful regulation of structural and phase states and therefore properties of the synthesis products and can be used as a means for controlling the synthesis process.

Phase transition of BiNb1−xMnxO4−δ: Thermal analyse, NEXAFS, XPS and ESR-study

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.

Lithium tetrachloridoaluminate, LiAlCl4: a new polymorph (oP12,Pmn21) with Li+in tetrahedral interstices

Dissolving lithium chloride and aluminium chloride in boilingpara- ormeta-xylene and keeping the colourless solution at room temperature led to crystal growth of a new modification of lithium tetrachloridoaluminate, LiAlCl4, which represents a second modification (oP12,Pmn21) of the ternary salt besides the long known monoclinic form [LiAlCl4(mP24,P21/c); Mairesseet al.(1977).Cryst. Struct. Commun.6, 15–18]. The crystal structures of both modifications can be described as slightly distorted hexagonal closest packings of chloride anions. While the lithium cations in LiAlCl4(mP24) are in octahedral coordination and the aluminium and lithium ions in the solid of orthorhombic LiAlCl4occupy tetrahedral interstices with site symmetriesmand 1, respectively, the lithium cation site being half-occupied (defect wurtz-stannite-type structure). From differential scanning calorimetry (DSC) measurements, no evidence for a phase transition of the orthorhombic modification is found until the material melts at 148 °C (Tpeak= 152 °C). The melting point is nearly identical to the literature data for LiAlCl4(mP24) [146 °C; Weppner & Huggins (1976).J. Electrochem. Soc.124, 35–38]. From the melts of both polymorphs, the monoclinic modification recrystallizes.

Syntheses, single-crystal structures, vibrational spectra and DSC/TG analyses of orthorhombic and trigonal Ag[N(CN)2]

Colorless powders of orthorhombic Ag[N(CN)2] were synthesized by blending aqueous solutions Ag[NO3] with stoichiometric amounts of Na[N(CN)2] equally dissolved in water. Single crystals of both modifications of Ag[N(CN)2] could be obtained by recrystallizing the powder with aqueous ammonia. Non-isothermal evaporation of the solvent at room temperature within a few hours yielded the orthorhombic modification as the main product crystallizing in the space group Pnma (no. 62) with the unit-cell parameters a = 1612.45(12), b = 361.58(3) and c = 599.02(4) pm (Z = 4). Upon evaporating the solvent much more slowly, isothermally within a few days, mainly thin hexagonal platelets of the trigonal modification of Ag[N(CN)2] were obtained, crystallizing in the space group P3121 (no. 152) with the lattice constants a = 359.86(3) and c = 2285.91(17) pm (Z = 3). Both results corroborate earlier structure determinations, but show higher precision. The vibrational spectra confirm the presence of the dicyanamide anion [N(CN)2]−, but exhibit slight differences to literature data. Differential scanning calorimetry/thermogravimetry (DSC/TG) analyses were performed to further characterize the two modifications.

Low temperature phase transition and structure of CsMgPO4

CsMgPO4 doped in radioisotopes is a promising compound for usage as a radioactive medical source. However, a low temperature phase transition at the temperatures close to ambient conditions (-370C) was observed. Information about structural changes is important in order to understand whether it can cause any problem for medical use of this compound. Structural changes have been investigated in detail using synchrotron powder diffraction methods, Raman spectroscopy and DFT calculations. The structure undergoes transformation from orthorhombic modification, sp. gr. Pnma (RT phase) to monoclinic modification, sp.gr P21/n (LT phase). New LT modification adopts similar to RT but slightly distorted unit cell: a=9.58199(2)Å, b=8.95501(1) Å, c=5.50344(2)Å, β=90.68583(1)0, V=472.198(3) Å3. The framework is made up of alternating magnesia and phosphate tetrahedra sharing vertices with caesium counter cations located in the channels formed. Upon the transformation a combined rotation of PO4 and MgO4 tetrahedral takes place. A comparison with other phase transition in ABW-type framework class compounds is given.