An observation of amorphous-crystalline phase transitions at severe plastic deformation of the Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The features of structural and phase transitions during severe plastic deformation (in Bridgman anvils) of the amorphous Ti₅₀Ni₂₅Cu₂₅ alloy have been studied by X-ray diffraction and transmission electron microscopy. Application of successively increasing deformation has revealed three cycles of successive phase transitions from amorphous to crystalline state and vice versa. The results obtained are explained in terms of the superposition of the different channels of elastic energy dissipation, which are activated during severe plastic deformation.     

Domain Structure and Orientational Ordering of NO<Subscript>2</Subscript> Groups in K<Subscript>2</Subscript>Ba(NO<Subscript>2</Subscript>)<Subscript>4</Subscript> Crystals

Polarization-optical studies of the domain structure of K₂Ba(NO₂)₄ crystals and differentialscanning calorimetric measurements have been performed in the vicinity of the high-temperature phase transition. The orientational ordering of NO₂ atomic groups is analyzed and the temperature dependence of the birefringence coefficient is theoretically described.     

Structure characterization of MHEMT heterostructure elements with In<Subscript>0.4</Subscript>Ga<Subscript>0.6</Subscript>As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In_0.4Ga_0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In_xGa_1–xAs ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.     

Diffractometric study of structural transformations in Cu<Subscript>1.70</Subscript>Cd<Subscript>0.05</Subscript>S crystals

The influence of the partial replacement of copper atoms by cadmium atoms on the real structure, phase formation, and temperatures of the structural transformations in Cu_1.75S are studied by high-temperature X-ray diffractometry. It was shown that Cu_1.70Cd_0.05S crystals at room temperature, unlike Cu_1.75S and Cu_1.70In_0.05S crystals, form not only orthorhombic anilite and monoclinic djurleite phases but also the respective metastable high-temperature fcc₁ and fcc₂ phases.     

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.     

Study of the Phase Transition in Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript> Crystals Using Anomalous X-Ray Transmission

The small-angle X-ray scattering in calomel (univalent mercury chloride Hg₂Cl₂) single crystals at low temperatures has been investigated. Data on the size of superstructural ferroelectric domains have been obtained for the first time. Lang measurements with anomalous X-ray transmission have made it possible to trace the temperature dynamics of crystal lattice of the high-temperature D_4h phase of Hg₂Cl₂ from 300 K to the Curie temperature (186 K) and the formation of ferroelastic domains in the temperature range of 186–100 K. The lowering of paraphase symmetry in the closest vicinity of the phase transition temperature is indicative of tricritical point transition.     

Synthesis and phase transitions of oxide-ion conducting compound La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> doped with alkaline metals

The specific features of synthesis, polymorthism, and electric conductivity of oxide-ion conducting compounds La_{2 − x} Me _x Mo₂O_{9 − y} , where Me = Na, K, Rb, or Cs, have been studied. Ceramic samples were obtained by solid-state synthesis in the temperature range of 960–1100°C. The regions where solid solutions exist have been found to depend on the temperature of the sample firing. According to the calorimetric and electrophysical data, the phase transition from the monoclinic phase (α) to the cubic phase (β) in samples doped with potassium and rubidium disappears at x = 0.02 and 0.04, respectively. In these cases the only transition from the cubic β _ms phase to the high-temperature cubic β phase is observed near 450°C. Doping with sodium and cesium does not suppress the α → β phase transition.     

Small-angle X-ray scattering study of the structure of powder fullerene C<Subscript>60</Subscript> and fullerene soot

Powder samples of fullerene C₆₀ and fullerene soot have been studied by the small-angle X-ray scattering method. The radii of gyration of scattering elements have been determined by constructing small-angle diffraction patterns in Guinier coordinates. The data obtained agree well with the results of wide-angle X-ray scattering study, the available data on the structure of the powder fullerene C₆₀ prepared by the Huffman-Krätschmer technique, and the structure of the C₆₀ molecules. Conglomerates of two C₆₀ molecules, along with crystallites ～20 nm in size that are distributed in an amorphous matrix, are present in fullerene powders. Fullerene soot contains C₆₀ crystallites 20–25 nm in size and graphite crystallites ～2–3 nm in size that are distributed in an amorphous matrix.     

Structural state of a radiation-modified Ti<Subscript>50</Subscript>Ni<Subscript>47</Subscript>Fe<Subscript>3</Subscript> single crystal

The structural state of a Ti₅₀Ni₄₇Fe₃ single crystal irradiated by fast neutrons (F = 2.5 × 10²⁰ cm⁻²) at 340 K was studied by thermal neutron diffraction at 78 and 295 K. The melt of this composition was chosen with the purpose of designing a radiation-resistant material exhibiting a shape-memory effect. It was found that the melt remains crystalline after irradiation, whereas the Ti₄₉Ni₅₁ crystal studied earlier becomes amorphous after an analogous irradiation. In spite of the fact that the main structural motif of the crystal remains unchanged after irradiation, martensitic transformations in the crystal do not occur and, consequently, the shape-memory effect is not retained. The radiation resistance of this class of crystals was estimated.     

Specific features of phase transitions and the conduction of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> oxide-ion conducting compound doped with vanadium

The X-ray powder analysis, calorimetric studies, and conductivity measurements of a series of ceramic La₂Mo_2−x V _x O _y specimens with different vanadium content are performed with the aim of following the dynamics of phase formation of the low-temperature α, high-temperature β, and metastable β _ms phases. At x ≥ 0.06, the cubic phase becomes stable and the monoclinic phase vanishes; therefore, the main α → β transition is suppressed. According to the data of differential thermal analyses, a weak thermal anomaly is observed in the range 450–470°C at x ≥ 0.06. This anomaly is indicative of the β _ms → β transition due to the conversion of the cubic phase with statically disordered oxygen atoms into the cubic phase with dynamic disorder. The conductivity of the high-temperature β phase obeys the Vogel-Tammann-Fulcher law.     

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.     

Refinement of the crystal structure of rhombohedral KU<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> as a representative of orthophosphates with the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structure

The crystal structure of the high-temperature β modification of synthetic orthophosphate KU₂(PO₄)₃ was refined from powder X-ray diffraction data by the Rietveld method: sp. gr. \(R\bar 3c\), the unit-cell parameters a= 9.113(1) Å and c= 24.997(1) Å. The isotropic refinement converged to R _wp = 6.15, R _B = 2.14, R _F = 3.52, and S = 0.42. It was confirmed that β-KU₂(PO₄)₃ belongs to the structure type of sodium zirconium phosphate containing an actinide atom in a sixfold (octahedral) coordination formed by oxygen atoms, which is unusual for orthophosphates. The principal interatomic distances and bond angles in the structure are reported.     

Crystal structure of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals doped with bismuth

Precision X-ray diffraction studies of La_{2 − x} Bi _x Mo₂O₉ (x = 0.04, 0.06, and 0.18) single crystals are performed. It is found that in the compounds doped with bismuth, analogously with the structure of the metastable β_ms phase of pure La₂Mo₂O₉ (LM), the La, Mo1, and O1 atoms deviate from the threefold axis on which they are located in the high-temperature β phase. It is shown that bismuth atoms substitute for part of lanthanum atoms and occupy a position at the threefold axis in the neighborhood of the split lanthanum position. The implantation of bismuth atoms in the LM structure results in the return of a part of the molybdenum atoms to the position at the threefold axis. The occupancy of this position is equal to the occupancy of the bismuth atomic position.     

Structure and properties of antimony-doped lanthanum molybdate La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>

Polycrystalline samples of the composition La₂Mo_{2 − x} Sb _x O_{9 − y} , where 0 ≤ x ≤ 0.05, were prepared by solid-phase synthesis. Single crystals of La₂Mo_1.96Sb_0.04O_8.17 were obtained by spontaneous crystallization from flux. The structure of the metastable β _ms phase of this compound was determined at room temperature by X-ray diffraction. It was found that the La, Mo, and O1 atoms are displaced from the threefold axis on which they are located in the high-temperature β phase. It was shown that molybdenum atoms in the crystal structure are partially replaced by antimony atoms, which are located on the threefold axis. In antimony-doped crystals, lanthanum atoms partially return to the site on the threefold axis and the coordination environment of molybdenum cations becomes more ordered, thus facilitating the stabilization of the cubic phase at room temperature. Calorimetric measurements (DSC) showed that the introduction of Sb as the dopant into the La₂Mo₂O₉ structure leads to a decrease in the temperature of the α → β phase transition from 570 to 520°C and to the partial suppression of this transition. The temperature behavior of the conductivity confirms the DSC data. Thus, doping with Sb contributes to the stabilization of the cubic phase at room temperature.     

Analysis of diffuse background on the X-ray diffraction pattern of fullerite C<Subscript>60</Subscript>

Diffuse scattering on the X-ray diffraction pattern of fullerite C₆₀ is analyzed. The characteristics of the short-range order of the amorphous component of fullerite are determined by the Finbak-Warren method. It is established that the short-range order of the amorphous component is similar to the short-range order of lonsdaleite.     

Flux crystallization of trigonal GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> competing with the crystallization of α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

The formation of trigonal GdFe₃(BO₃)₄ crystals in the Bi₂Mo₃O₁₂-B₂O₃-Li₂MoO₄-Gd₂O₃-Fe₂O₃ system was studied. The flux compositions for which GdFe₃(BO₃)₄ is the high-temperature phase with a wide range of crystallization were determined. The features of nucleation of these crystals and their growth near the phase boundary with α-Fe₂O₃ were analyzed. The growth of GdFe₃(BO₃)₄ single crystals involving preliminary nonequilibrium crystallization of α-Fe₂O₃ is described.     

Synthesis,Spectrum Properties,and Crystal Structure of a New Pentaborate,Na<Subscript>2.18</Subscript>K<Subscript>0.82</Subscript>SrB<Subscript>5</Subscript>O<Subscript>10</Subscript>

Abstract  

A novel quaternary borate, Na_2.18K_0.82SrB₅O₁₀, has been prepared by high-temperature solution reaction below 800 °C. Single-crystal XRD analyses showed that it crystallizes in the triclinic P[`1] P\bar{1} group with a = 7.3900(15) ?, b = 7.6490(15) ?, c = 9.773(2) ?, α = 79.31(2)°, β = 70.85(2)°, γ = 62.09(1)°, Z = 2. The basic structural unit in Na_2.18K_0.82SrB₅O₁₀ is a double ring [B₅O₁₀]⁵⁻ composed of one BO₄ tetrahedron and four BO₃ triangles. The [B₅O₁₀]⁵⁻ groups are arranged around crystallographic centers of symmetry to form [B₁₀O₂₀]¹⁰⁻ columns that are held together by Na⁺, K⁺/Na⁺, and Sr²⁺ cations via electrostatic interactions. The IR spectrum further confirmed the presence of both BO₃ and BO₄ groups. UV–vis diffuse reflectance spectrum showed a band gap of about 3.80 eV. Solid-state fluorescence spectrum exhibited the maximum emission peak at around 337.6 nm.     

Synthesis and Structure of Ba<Subscript>5</Subscript>(V<Subscript>2</Subscript>O<Subscript>7</Subscript>)<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

Abstract  A new barium chlorovanadate, Ba₅(V₂O₇)₂Cl₂, was isolated by a high-temperature (850 °C) reaction employing a CsCl/RbCl flux. The structure was determined by single crystal X-ray diffraction methods. This compound crystallizes in an orthorhombic crystal system, Pmmn (No. 59), with a = 11.558(2) ?, b = 15.164(3) ?, c = 10.023(2) ?, Z = 4 and V = 1756.7(6) ?³. The structure of Ba₅(V₂O₇)₂Cl₂ was determined by full-matrix, least-squares methods with R ₁ = 0.0398, wR ₂ = 0.1069 and GOF = 1.048 for all data. This new structure can be described as a composite lattice made up of mixed covalent and ionic moities. The extended framework is orchestrated by stacked [Ba(V₂O₇)Cl]³⁻ slabs that are interconnected by Ba²⁺ cations through Ba–O bonds to the [V₂O₇] units. The Ba²⁺ and Cl^- ions form BN-type “[BaCl]” sheets with pseudo-hexagonal windows that are centered by [V₂O₇]⁴⁻ pyrovanadate units. Graphical Abstract  The structure of a new chlorovanadate, Ba₅(V₂O₇)₂Cl₂, exhibits an interesting BN-type salt lattice that consists of an extended [BaCl] sheet containing pseudo-hexagonal windows that are centered by [V₂O₇] pyrovanadate units.      

Investigation of the structural conditionality for changes in physical properties of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

With the aim of elucidating the nature of anomalies in the physical properties of K₃H(SO₄)₂ crystals that arise as the temperature grows, the dielectric and optical properties of the crystals are studied, an X-ray diffraction analysis of single-crystal and polycrystalline specimens are performed, and the morphology and chemical composition are studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. As a result of the studies performed, a phase transition from the phase with the monoclinic symmetry (space group C2/c) to the phase with the trigonal symmetry (space group R $ \bar 3 $ \bar 3 m) is found in a number of K₃H(SO₄)₂ specimens at a temperature of ≈457 K, the responsibility of the dynamically disordered hydrogen-bond system for the rise of high proton conductivity in the high-temperature phases of the crystals of this family is confirmed, and data on the solid-phase reactions proceeding at high temperatures are obtained.