Magnetic susceptibility of quasicrystalline Al<Subscript>65</Subscript>Cu<Subscript>22</Subscript>Fe<Subscript>13</Subscript> powders

Magnetic properties of quasicrystalline Al₆₅Cu₂₂Fe₁₃ powders synthesized by the solid-phase diffusion method via thermal treatment in vacuum or a hydrogen atmosphere have been studied. The powders synthesized in vacuum are found to contain a ferromagnetic fraction. The formation of iron oxide Fe₃O₄ is shown to be the most probable cause of the existence of this fraction. It is possible to avoid the formation of the ferromagnetic fraction in the powders synthesized or annealed in hydrogen. Removal of the ferromagnetic fraction from the powder by repeated magnetic separation made it possible to obtain a quasicrystalline fraction, for which the behavior of the magnetic susceptibility can be explained by the formation of nanoclusters. Such behavior is a general and integral property of quasicrystalline powders synthesized in both vacuum and hydrogen.     

X-ray analysis of multilayer In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As/In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As HEMT heterostructures with InAs nanoinsert in quantum well

In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As HEMT heterostructures on InP substrates with elastically strained InAs insert in combined quantum well (QW) have been investigated using a combination of X-ray methods: double-crystal X-ray diffraction, X-ray reflectivity, and reciprocal space mapping. This approach has provided detailed complementary information about the layered and real crystal structures of the samples. The data obtained have made it possible to perform structural analysis of the multilayer systems and compare their characteristics with specified technological parameters, due to which the HEMT growth technology can be corrected and improved.     

Investigation of zirconium phosphate Zr<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>4</Subscript> during heating

Zirconium phosphate Zr₃(PO₄)₄ has been synthesized by the sol-gel technique and investigated using X-ray powder diffraction, IR spectroscopy, and differential scanning calorimetry. It has been established that the symmetry of the unit cell, R \(\bar 3\) c, which is characteristic of the NaZr₂(PO₄)₃ (NZP) family, is lowered to P \(\bar 3\) c. The behavior of the zirconium phosphate during heating has been examined using high-temperature X-ray diffraction at temperatures ranging from 25 to 575°C. It has been revealed that the structure of the zirconium phosphate is hardly subjected to expansion due to heating in the temperature ranges 25–125°C (α _a < 1 × 10^?6 K^?1, α _c < 1 × 10^?6 K^?1, Δα < 1 × 10^?6 K^?1) and 325–575°C (α _a = ?1.4 × 10^?6 K^?1, α _c < 1 × 10^?6 K^?1, Δα < ?2.4 × 10^?6 K^?1). In the temperature range 125–325°C, the synthesized compound undergoes a second-order phase transition (upon heating), which is accompanied by the contraction of the structure along all crystallographic directions. Upon cooling in the range from 75 to 25°C, the phase transition is accompanied by the expansion of the structure.     

Growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals by the low-thermal gradient Czochralski technique

Bi₁₂SiO₂₀ crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-quality polyhedral faceted front are found. The systematic features of shaping Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crystals is studied and their optical homogeneity is analyzed by interferometry.     

Microhardness and fracture toughness of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>-based nanocrystalline laser ceramics

The microhardness and fracture toughness of laser nanocrystalline ceramics based on the cubic oxides Y₂O₃ and Y₃Al₅O₁₂ are determined experimentally. It is shown by comparative measurements that the fracture toughness and microhardness of Y₂O₃ ceramics exceed the corresponding parameters of Y₂O₃ single crystals by factors of 2.5 and 1.3, respectively. The fine morphology of grains and grain boundaries in fractures is investigated. It is ascertained that changes in the mechanical properties of the nanocrystalline ceramics under study are related to both the sizes and structure of grains and the structure of grain boundaries. It is suggested that twinning processes determine the mechanisms of formation of nanocrystalline ceramics.     

Comparative analysis of the heat transfer processes during growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystals by the low-thermal-gradient Czochralski technique

The heat transfer processes occurring in the solid and liquid phases during growth of Bi₁₂GeO₂₀ and Bi₄Ge₃O₁₂ crystals by the low-thermal gradient Czochralski method are analyzed and compared. It is experimentally found that, under similar growth conditions, the deflection of the crystallization front for the Bi₁₂GeO₂₀ crystal is considerably smaller than the deflection of the crystallization front for the Bi₄Ge₃O₁₂ crystal and the faceting of the former front is observed at the earlier stage of pulling. The results of the numerical simulation demonstrate that the different behavior of the crystallization fronts is associated with the difference between the coefficients of thermal absorption in the crystals.     

Structure of amorphous oxide Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Results of a molecular-dynamics experiment

The structure of amorphous aluminum oxide was simulated by the molecular-dynamics method. A random distribution of Al³⁺ and O^2? ions over the volumes of cubes with sides of 21 and 24 Å was used as a starting configuration. The character of the distribution of cations in the anion subsystem was analyzed. It was shown that formation of voids in model clusters is possible when the average electron density is underestimated     

Crystal structure of new decaborate Na<Subscript>2</Subscript>Ba<Subscript>2</Subscript>[B<Subscript>10</Subscript>O<Subscript>17</Subscript>(OH)<Subscript>2</Subscript>]

The crystal structure of a newly synthesized compound Na₂Ba₂[B₁₀O₁₇(OH)₂] has been determined (Syntex \(P\bar 1\) diffractometer, MoK_α radiation, 1784 crystallographically nonequivalent reflections, anisotropic approximation, R = 1.7%). The parameters of the monoclinic unit cell are a = 11.455(7), b = 6.675(4), c = 9.360(7) Å, β = 93.68(5)°, Z = 2, sp. gr. C2. The structure consists of double pseudohexagonal layers built by BO₄-tetrahedra and BO₃-triangles forming three-membered rings of two mutually orthogonal orientations. The neighboring layers along the [001] direction are bound by Na-polyhedra and hydrogen bonds with participation of OH groups. The interlayer tunnels along the [100] direction are filled with columns of Ba-polyhedra. The crystallochemical characteristics of a number of synthetic Ba-borates (to which the structure of new decaborate is related) are considered in terms of borate building blocks singled out in the structure.     

Crystal structure of the rhombohedral phase of Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br

The crystal structure of the rhombohedral phase of Fe₃B₇O₁₃Br was determined and compared with the structures of Fe₃B₇O₁₃Cl and Fe₃B₇O₁₃I. The influence of the ionic radii of the halogen atom on the structural features of the crystals of the boracite family is discussed.     

Structure of the metastable cubic B<Subscript>1</Subscript> phase of the La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystal

The structure of the metastable B₁ phase of the La₂Mo₂O₉ single crystal is investigated using X-ray diffraction. It is established that the crystal structure of the compound under investigation is described by the cubic unit cell with the parameter a = 7.158(5) Å, which makes it possible to index approximately 84% of the reflections measured for this single crystal. The structure of the metastable cubic B₁ phase is characterized by a local lowering of the symmetry for the La and Mo atoms, which are displaced from their positions on the threefold axis, thus forming three sites around it with an occupancy of 0.333(2). The O(1) atom in the structure of the metastable cubic B₁ phase remains in the 4a position on the threefold axis but occupies it by only 86%. The O(2) and O(3) atoms located in a general position occupy their own sites with occupancies of 0.77(2) and 0.35(2), respectively. The final R factor of the refinement of this structural model is 2.52%.     

The behavior of thermal conductivity in the chaotic phase of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystal

A thermal investigation of the modulated structure dynamics at the transition between metastable states in the incommensurate phase of [N(CH₃)₄]₂ZnCl₄ crystal have been performed. It is established that the anomalous behavior of the thermal conductivity in the absence of a defect-density wave is due to the inelastic scattering of thermal phonons from critical phonons of the soft optical branch. In the presence of a defect-density wave, the anomalous increase in thermal conductivity is caused by the existence of a chaotic phase and is related to the contribution that soft optical phonons make to the heat transfer due to the increase in their group velocity.     

Raman spectroscopy of the ferroelectric-antiferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.2</Subscript>Nb<Subscript>0.8</Subscript>O<Subscript>3</Subscript> ceramic solid solution

The phase transition in the Li_0.12Na_0.88Ta_0.2Nb_0.8O₃ ceramic solid solution has been studied by Raman scattering spectroscopy at 350°C. A considerable broadening of the lines due to translational vibrations of cations in octahedral and cuboctahedral voids and to vibrations of the oxygen framework, as well as the decrease to zero of the intensity of the line due to bridge stretching vibrations of oxygen atoms of BO₆ octahedral anions, was revealed as the temperature of the solution approached the transition point from below. It was found that the solution loses its ferroelectric properties due to the preferential increase in the anharmonicity of vibrations of cations in octahedral voids.     

Monotonic and jumpwise deformation of bulk amorphous Zr<Subscript>46.8</Subscript>Ti<Subscript>8</Subscript>Cu<Subscript>7.5</Subscript>Ni<Subscript>10</Subscript>Be<Subscript>27.5</Subscript> alloy in nanoindentation

The study is aimed at the establishment of regions of stable (monotonic) and unstable (jumpwise) plastic flow in bulk amorphous Zr_46.8Ti₈Cu_7.5Ni₁₀Be_27.5 alloy under the conditions of local deformation. The characteristics of the jumpwise plastic flow in bulk amorphous Zr_46.8Ti₈Cu_7.5Ni₁₀Be_27.5 alloy are studied by the method of continuous nanoindentation (depth-sensing testing) in the range of strain rates \(\dot \varepsilon \) from 10^?2 to 10⁴ s^?1. The boundaries between the regions of homogeneous and localized plastic strain are established and the fractions of monotonic and jumpwise plastic strain under the indenter are determined.     

Evolution of the vanadium pentoxide V<Subscript>2</Subscript>O<Subscript>5</Subscript> crystal surface after vacuum annealing

A complex study of the (001) cleavage surface of a V₂O₅ single crystal annealed in vacuum at 450 and 550°C has been performed. Tunnel microscopy of the sample surface annealed in vacuum at 550°C showed the formation of a plane with a corundum structure, reconstructed according to the V₂O₃ (0001)-(1/√3 × 1/√3)R30° type, on the surface. X-ray photoelectron spectroscopy revealed a significant modification of interatomic bonds in the surface layers of V₂O₅ single crystal after vacuum annealing at 550°C, which is related to the partial reduction of V⁵⁺ ions and formation of lower vanadium oxides. These modifications lead to a decrease in the electrical resistivity of V₂O₅ and the occurrence of bending at a temperature of 61.5°С in the temperature dependence of the sample resistance, which is indicative of phase transition.     

X-Ray Structure Investigation of Sr<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> Langasite Family Crystal

An accurate structure analysis of Sr₃NbGa₃Si₂O₁₄ single crystals, belonging to the langasite family, has been performed. Two datasets are obtained on an Xcalibur S diffractometer equipped with a CCD detector. The structure is been refined using an averaged dataset: sp. gr. P 321, Z = 1, 295 K, sin θ/λ ≤ 1.35 Å^–1, a = 8.2797(3) Å, c = 5.0774(5) Å; the agreement factors between the model and experiment are found to be R/wR = 0.76/0.64% and Δρ_min/Δρ_max =–0.21/0.17 e/ų for 3820 independent ref lections. The Sr₃NbGa₃Si₂O₁₄ and Sr₃NbFe₃Si₂O₁₄ structures are compared, and the role of magnetic ions in the predicted P321 → P3 phase transition is analyzed.     

New type of borophosphate anionic radical in the crystal structure of CsAl<Subscript>2</Subscript>BP<Subscript>6</Subscript>O<Subscript>20</Subscript>

The crystal structure of a new borophosphate CsAl₂BP₆O₂₀ obtained by spontaneous crystallization in a multicomponent Cs–Cu–B–P–O system is determined by X-ray diffraction (a = 11.815(2), b = 10.042(2), and c = 26.630(4) Å; space group Pbca, Z = 8, V = 3159.5(10) ų; R₁ = 0.043). A new type of borophosphate anionic 2D radical characterized by the lowest B: P = 1: 6 ratio and containing P₃O₁₀ phosphate groups is found in the compound. A mixed-type anionic framework consisting of vertex-sharing BO₄ and PO₄ tetrahedra and AlO₆ octahedra is distinguished in the structure. Large cesium atoms are located in the channels of the framework. Topological relationships are revealed between the structures of the CsAl₃(P₃O₁₀)₂ and CsAl₂BP₆O₂₀ phases having different cationic compositions. These compounds can be considered quasi-polytypic phases.     

Polyamorphous transition in amorphous fullerites C<Subscript>70</Subscript>

Samples of amorphous fullerites C₇₀ have been obtained by mechanical activation (grinding in a ball mill). The structure of the samples has been investigated by neutron and X-ray diffraction. The high-temperature (up to 1200°C) annealing of amorphous fullerites revealed a polyamorphous transition from molecular to atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles. Possible structural versions of the high-temperature amorphous phase are discussed.     

Growth kinetics and adsorption trapping of impurity during crystallization of NH<Subscript>4</Subscript>Cl in the NH<Subscript>4</Subscript>Cl-CuCl<Subscript>2</Subscript>-H<Subscript>2</Subscript>O-CONH<Subscript>3</Subscript> system

The growth of NH₄Cl crystals and their trapping of copper impurity in the NH₄Cl-CuCl₂-H₂O-CONH₃ quaternary system have been experimentally studied. The epitaxial adsorption of copper complexes on (100) faces leads to a sharp decrease in the growth rate in good correspondence with the Bliznakov-Chernov equation. The copper impurity enters the crystal composition in amounts up to 6.5 mol %. The impurity distribution coefficient nonlinearly changes with the copper concentration in the solution: it is much larger than unity at low concentrations and sharply decreases with an increase in supersaturation. Such behavior is indicative of the adsorption mechanism of copper trapping by NH₄Cl crystals. Single-crystal X-ray study shows that the impurity is incorporated in NH₄Cl crystals in the form of oriented intergrowths of different complex coppercontaining compounds. The concentration and variety of impurity phases increase with an increase in the copper content in the solution and decrease with an increase in supersaturation. Heterogeneous 2D isomorphous trapping of copper impurity by NH₄Cl crystals induces high (up to 60 MPa) internal stresses in them, as a result of which anomalous birefringence and splitting of crystals occur.     

Synthesis and structure of fullerene halides C<Subscript>70</Subscript><Emphasis Type="Italic">X</Emphasis><Subscript>10</Subscript> (<Emphasis Type="Italic">X</Emphasis> = Br,Cl) and C<Subscript>78</Subscript>Cl<Subscript>18</Subscript>

The crystal and molecular structures of the fullerene halides C₇₀Br₁₀ · 3C₆H₂Cl₄, C₇₀Cl_8.4Br_1.6, and C₇₈Cl₁₈ have been determined and refined using single-crystal X-ray diffraction with synchrotron radiation. In the molecular structure of C₇₀ X ₁₀ (X = Br, Cl), ten X halogen atoms are located in the equatorial region of the molecule and form one 1,2 and nine 1,4 contacts in C₆ X ₂ hexagons. The structure of the higher fullerene chloride C₇₈Cl₁₈ contains two isomers with symmetry C _2v , which results in the hexagonal symmetry of the crystal due to disordering.