V. Sh. Shekhtman

Diffraction analysis of incommensurate phases in crystals of quasi-binary system Sn2P2(S1−xSex)6

Abstract X-ray analysis of Sn2P2(S1 − xSex)6 single crystals with 0 < x < 1 is performed in the temperature interval 70 to 400 K in which there exist the incommensurate modulated structures. Crystallographic characteristics of the modulation wave are obtained: the direction of a wave vector q at an angle 9° with respect to the [100] direction in the (010) plane; the polarization plane (010); the transversal wave of tin atoms displacement. A description of (3 + l)-dimensional space group for incommensurate phase is given. Temperature and concentrational dependencies for crystallographic characteristics are obtained. It is shown that the Lifshitz point on the concentrational phase diagram corresponds to a composition with X = 0.28 at T = 280 K. We have analyzed the evolution of thermal expansion tensor in the range of phase transition. The temperature boundaries for incommensurate phase range are shown to be fixed by the knees of thermal expansion and inside the temperature range the “C” parameter remains u...

Structural changes in Bi2Sr2CaCu2O8+x single crystals between 20° and 875°C

Abstract Structure changes in Bi 2 Sr 2 CaCu 2 O 8+ x . (2212) single crystals were investigated by X-ray methods between 20°– 875°C. It was shown that the crystal is not orthorhombic as was believed. It has a monoclinic cell that changes from a primitive to a side-centered one above 450°C. It was also shown that some of the crystals have a monoclinic twin structure. The annealing of the single phase samples above 500°C led to the emergence of new phases which had lattice parameters c=9.4 A and c=24.6 A , were coherent with the initial matrix and reconstructed into each other under subsequent heating.

Specifics of the formation of alloys and their hydrides in Ti-Zr-H system

Studies of the combustion processes in the Ti-H, Zr-H, and Ti-Zr-H systems made it possible to develop a principally new method for producing refractory metal alloys through the compaction of a titanium hydride-zirconium hydride powder mixture followed by dehydrogenation. The procedure is briefly described. Experimental data on the formation of titanium-zirconium alloys with different structures, including an ω-phase alloy obtained at atmospheric pressure, are discussed. The experimental results clearly show that the structure of the alloy depends on the composition of the initial charge and the hydrogen content in the hydrides used. The interaction of the alloys obtained with hydrogen under conditions of self-propagating high-temperature synthesis yielded Ti0.9Zr0.1H1.87 (Ti9ZrH18.7), Ti0.67Zr0.33H1.81 (Ti2ZrH5.42), Ti0.5Zr0.5H1.37 (TiZrH2.74), and Ti0.3Zr0.7H1.96 (TiZr2.3H6.53) hydrides with face-centered cubic (fcc) and body-centered tetragonal (bct) structures. The removal of hydrogen from these hydrides (by annealing in vacuum at 700–1050°C) results in the recovery of the initial α and ω phases.

Incommensurate phase in proper ferroelectric Sn2P2Se6

Using X-ray diffraction methods it has been ascertained that the intermediate phase in Sn2P2Se6 crystals existing in the range 193 K to 220 K and preceding the ferroelectric transition at 193 K is an incommensurate phase. Modulated structure is characterized by the wave vector q(T) = (-1.146 δ(T)a*, 0, δ(T)c*), where δ(T) changes smoothly within 0.085 to 0.065 as temperature decreases and jumps to zero value at the ferroelectric transition. The transition from para- to ferroelectric phase is accompanied by a symmetry decrease from P21/c to Pc, the translational symmetry being unchanged. The incommensurate phase existence range manifests itself in the distinctive behaviour of both thermal expansion coefficients and reflection integrated intensities. In view of the results obtained some features of the behaviour of the mixed crystals like Sn2P2(S1-x Sex)6 and (Pb1-Y SnY)2P2Se61-6 are also discussed. It is concluded that the triple point in the Sn2P2S6-Sn2P2Se6 system phase diagram is a Lifshitz point. Furth...

structural aspects of low-temperature phase transitions in proustite (Ag3AsS3)

Proustite crystals have been studied using X-ray diffraction methods over the temperature range 4.2 to 300 K. The results confirm the existence of structural phase transitions at 26, 48, and 60 K. At 60 K, there is a second-order transition to an incommensurate (I) phase which is characterized by a star of six wave vectors like {-1/3(1 - δa), 0,., 1/3(1 - δc)} (in hexagonal axes), where δ (T) ≠ δc (T). Significant is the fact that in proustite a 3K-state is realized resulting in three-dimensional incommensurability. At 48 K, a continuous transition to a commensurate (C) phase takes place. The C-phase has the same symmetry as that of prototypic phase (T > 60 K), i.e. R3c (C63V). At 26 K, there is a first-order transition to ferroelectric phase, in the result of which the evidence of the I- and C-phases (extra reflections) disappears and structural domains are formed. The low-temperature phase structure can be treated as a result of the monoclinic or triclinic distortion of the prototypic phase structure.

Twin structure and structure of twin boundaries in 1–2–3-O7-δ crystals

Abstract The twin and twin boundary structures of the high temperature superconductors (HTSC) 1–2–3-O7-δ and La2-x Srx CuO4 systems were investigated by X-ray and electron microscopy methods. The twin parameters determined for 1–2–3-O 7-δ and La2-x Srx CuO4 crystals respectively are {110}/⟨110⟩, {100}/⟨100⟩. It was also shown that the twin boundaries in 1–2–3-O7-δ crystals are not monoatomic planes. They are the transitional regions inside which the “a” parameter of one twin orientation continuously transforms into the “b” parameter of the other. For HoBa2Cu3O7-δ crystals obtained by special technology of growth, we discovered a new type of the regular macrostructure referred to as the quasitwin structure. The quasitwins are analogous to the original twins wherein the orthorhombic parameter continuously changes from a/b at one end of the quasitwin into b/a at the other. The quasitwinned structure is shown to be caused by the oxygen ordering in “a”-“b” planes rather than by its deficit.

Crystal structures and thermal properties of titanium carbo-deuterides as prepared by combustion synthesis

Titanium carbo-deuterides of various compositions have been prepared by thermal radiation and cold syntheses, and by self-propagating ¯ high-temperature synthesis. While the former methods favored the formation of a cubic structure of composition TiC D (Fm3m, 0.48(1) 0.60(2) ¯ ˚ ˚ a54.30963(3) A) the latter yielded a trigonal structure of composition TiC D (P3m1, a53.08208(2), c55.04052(6) A). 0.50(2) 0.707(4) Differential thermal analysis and thermogravimetry data suggest the cubic structure to be thermally less stable (desorption onset temperature 823 K) than the trigonal structure (1093 K). Joint Rietveld structure refinements based on neutron and X-ray powder diffraction data show that carbon occupies octahedral interstices in the f.c.c. and h.c.p. metal atom networks of the cubic and trigonal phase, respectively, while deuterium occupies both octahedral and tetrahedral interstices in the cubic but tetrahedral interstices only in the trigonal phase. The different thermal stability of the compounds is attributed to these structural differences.  2003 Elsevier B.V. All rights reserved. able. In this paper we present the first structure refinements of, and thermal stability data for the Ti-C-H system. The Hydrogen reacts with transition metal carbides to form samples investigated were obtained by combustion syn- so-called carbo-hydrides. Representative examples occur in thesis (11-13). This method is of particular interest in the systems based on titanium (1-6), zirconium (3-5,7-10), context of hydrogen storage materials because it is capable hafnium (2,3,5), vanadium (9,10), niobium (5) and others. of yielding large quantities of materials at a relatively low The compounds crystallise mostly with face-centred cubic cost. Consequently, the structural and thermal properties ( f.c.c.) and / or hexagonal close-packed (h.c.p.) metal atom reported in this paper refer to relatively extreme conditions arrangements in which carbon and hydrogen atoms occupy of synthesis.

Effect of pressure on structural properties of intermetallic LnM lanthanide compounds

Abstract Energy dispersive powder X-ray diffraction measurements with diamond anvil high pressure cells at ambient temperature and pressure up to 50 GPa on the LnM compounds GdCu, LaAg, NdAg, NdZn, CeZn and LaZn show systematically high pressure phase transformations from the cP2 (CsCl-type) structure to lower symmetry phases, however, with only minor effects from valence instabilities.

X-ray diffraction studies of high-Tc Bi2Sr2CaCu2O8−x superconductor

Abstract The present work reports new features of an X-ray diffraction pattern from BiSrCaCuO (2212). The crystal had the orthorhombic cell with the lattice parameters: a=5.407 A , b=5.410 A , c=30.762 A , wave vectors of modulation q ∗ =±δ b ∗ + c ∗ , where δ =0.21. A new observation is diffuse spots located between satelite reflections and whose position can be described by the vectors, analogous to those of satelite reflections with the origin on the position of forbidden reflections. A possible mechanism for the occurence of these diffuse spots is proposed.

Synthesis of transition metal hydrides and a new process for production of refractory metal alloys: An autoreview

Briefly overviewed are the SHS-based processes developed at our lab for synthesis of metal hydrides and their alloys. More than 200 binary and complex hydrides have been synthesized by SHS reactions of metals in hydrogen. Radiation-induced synthesis of transition metal hydrides, termed Thermal-Radiation Synthesis (TRS), was suggested and used to prepare more than 50 different binary and complex hydrides. A phenomenon of Cold Synthesis (CS) of hydrides at room temperature from irradiated metal powders has been observed. Advance in these studies has led to elaboration of a new technique, termed hydride-cycle process, for fabrication of compact blocks of hydrides and refractory metal alloys. A wide range of binary and multicomponent alloys and their hydrides have been synthesized by this route. The developed processes are offered for industrial-scale implementation.