共查询到20条相似文献,搜索用时 15 毫秒
1.
V. V. Grebenev I. P. Makarova D. A. Ksenofontov V. A. Komornikov E. V. Dmitricheva 《Crystallography Reports》2013,58(6):894-898
The symmetry (sp. gr.I $\bar 4$ 3d) and lattice parameters have been determined for the first time for Cs5(H2SO4)2(H2PO4)3 crystals in the temperature range from 172 to 390 K. The thermal and optical properties of crystals, as well as their conductivity, have been investigated at elevated temperatures. It is shown that a crystal heated to T = 365 K undergoes a phase transition with symmetry lowering to the tetragonal phase (with the parameters a = 4.965(1) Å and c = 5.016(1) Å), while at T ≈ 390 K a phase transition to the cubic phase is presumably observed. With a decrease in temperature, a phase transition without a change in symmetry occurs at T = 240 K. 相似文献
2.
Phase transformations in titanium oxycarbide TiC0.545O0.08 have been studied by the methods of neutron diffraction and X-ray structure analysis. It was established that the ordered cubic structure δ′ (sp. gr. Fd3m) of the oxycarbide sample is the low-temperature ordered phase existing up to 990 K. The order-disorder phase transition (990 K) results in the formation of an ordered trigonal structure (sp. gr. or R3m or P3121). The α-Ti-phase is separated at the temperature 1020 K. The order-disorder phase transition was found at T = 1040 K. 相似文献
3.
Copper iron oxides, Cu1‐xFe2+xO4 (0 ≤ x ≤ 0.5), have been synthesized by thermal oxidation of copper ‐ iron mixtures. In this process, the phase formation and the phase stability were investigated as function of the temperature (800°C – 1200°C) and the oxygen partial pressure (1.013 x 101 – 1.013 x 105 Pa). The phase formation starts with the reaction of the metallic components to simple oxides (Fe3O4, Fe2O3, CuO). From these simple oxides, the formation of complex oxides requires a minimum temperature of 800°C. The synthesis of single phase spinel compounds Cu2+1‐2x Cu1+xFe2+xO4±δ is realized for 0.1 ≤ x ≤ 0.5, using specific temperature – p(O2) – conditions for a given value of x. Remarkably, to achieve our goal, we found that the increase of x implies that of the reaction temperature and/or a decrease of the p(O2) in the reaction gas stream. Besides, a single phase spinel CuFe2O4 does not exist in the temperature / p(O2)‐field investigated. Using the results of XRD ‐ phase analysis, T ‐ p(O2) – x – diagrams were constructed. These diagrams allow the prediction of phase compositions expected for different synthesis conditions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
4.
Yiming Zeng Yanqing Zheng Xiaoniu Tu Zhiping Lu Erwei Shi 《Journal of Crystal Growth》2012,343(1):17-20
A lead-free Ba(1?x)CaxTi(1?y)ZryO3 (BCZT) single crystal (x=0.08, y=0.26) was grown by the Czochralski (CZ) method in a mixed flux of TiO2 and ZrO2. The composition of as-grown BCZT was analyzed by electron probe micro-analysis. The structure, dielectric properties and phase transition were investigated at different temperatures. The X-ray diffraction results confirmed that the structure of the as-grown BCZT crystal was cubic both at 25 °C and 500 °C. The temperature dependence of the dielectric constant and Raman spectra characterization revealed that there was a phase transition from cubic to tetragonal, which happened between 200 K and 250 K. With increasing frequency, the Curie temperature shifted towards high temperature. 相似文献
5.
L. A. Shilkina A. V. Pavlenko L. A. Reznichenko S. I. Dudkina 《Crystallography Reports》2014,59(5):713-717
A high-temperature analysis of Pb(Ti0.485Zr0.515)0.98(Nb0.5Bi0.5)0.02O3 solid solution has been carried out in the temperature range 20 ≤ t ≤ 600°C by X-ray powder diffraction. Two different phase diagrams are plotted: one is based on the change in the diffraction pattern of main reflections, while the other is based on the diffraction pattern of real (defect) structure. A transition to the cubic phase can be observed at t ≥ 360°C (in the former diagram) and t ≥ 450°C (in the latter diagram). The nonmonotonic (with inflection points) behavior of the temperature dependence of the cell volume in the cubic phase is explained. 相似文献
6.
The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe2 was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients αa and αc of the tetragonal lattice constant which are related to the linear thermal expansion coefficient αL were obtained, as were αa of the cubic lattice constant, also αu and αη. The transition temperature is clearly identified via a strong anomaly in αL. The temperature dependence of the anion position parameter was found to be rather weak, nearly αu∼0, whereas αη increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐Ttrans|0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu+ were found to occupy the Me1 site with a corresponding 67% of In3+ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐Ttrans|β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
A theory of the structural phase transition in LiZn0.5Mn1.5O4 cathode material is proposed. The symmetry of the order parameter, thermodynamics, and mechanisms of formation of the atomic structure of low-symmetry ordered cubic lithium-zinc-manganese oxide spinel have been studied. The critical order parameter inducing the phase transition has been found. It is shown that the calculated LiZn0.5Mn1.5O4 structure is formed due to displacements and orderings of lithium, zinc, manganese, and oxygen atoms. Within the Landau theory of phase transitions, it is shown that the phase states may change from high-symmetry cubic disordered Fd3m phase to the low-symmetry ordered cubic P213 phase as a result of first-order phase transitions. 相似文献
8.
N. B. Bolotina I. A. Verin N. Yu. Shitsevalova V. B. Filippov N. E. Sluchanko 《Crystallography Reports》2016,61(2):181-186
The unit-cell parameters of dodecaboride LuB12, which undergoes a transition to the cage-glass phase, have been determined for the first time in the temperature range of 50–75 K by X-ray diffraction, and the single-crystal structure of this compound is established at 50 K. Nonlinear changes in the unit-cell parameters correspond to anomalies in the physical properties near the glass-transition temperature T* ~ 50–70 K. This compound has cubic symmetry at room temperature, and it is reduced to tetragonal symmetry at lower temperatures. Based on the X-ray diffraction data and relying on the physical properties of the crystals, the structure model, in which a small part (~15%) of Lu atoms are displaced from the 2a sites at the centers of the B24 cuboctahedra to the 16n sites of sp. gr. I4/mmm, seems preferable. 相似文献
9.
A solid solution of the Cu1.95Ni0.05S composition has been synthesized for the first time due to the partial replacement of Cu with Ni atoms in Cu2S. The polymorphic transformations in the polycrystalline samples in the temperature range of 300–1400 K have been investigated by X-ray diffraction and differential thermal analysis. It is established that, at room temperature, the synthesized Cu1.95Ni0.05S samples have an orthorhombic lattice with unit-cell parameters a = 26.50 Å, b = 15.39 Å, and c = 13.85 Å (sp. gr. Abm2). Heating to T = 379 ± 2 K leads to its transformation into a hexagonal lattice with parameters a = 3.960 Å and c = 6.78 Å (sp. gr. P63/mmc). At 750 ± 2 K, the hexagonal modification is transformed into a cubic one with period a = 5.788 Å (sp. gr. Fm\(\bar 3\)m). The phase transition in this crystal is enantiotropic. 相似文献
10.
B. V. Mill B. A. Maksimov Yu. V. Pisarevskii N. P. Danilova A. Pavlovska S. Werner J. Schneider 《Crystallography Reports》2004,49(1):60-69
Possible structural changes described by the group-subgroup relationships in the Ca3Ga2Ge4O14-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La3SbZn3Ge2O14, whose crystal structure is refined as a derivative structure of the Ca3Ga2Ge4O14 structure type within the sp. gr. A2 (C2). At ~250°C, La3SbZn3Ge2O14 undergoes a reversible phase transition accompanied by symmetry rise, A2 ? P321. Similar phase transitions, P321 ? A2, are also observed in La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed. 相似文献
11.
The equilibrium in the (NH4)3ScF6-NH4HF2-H2O system in the range of hydrodifluoride ammonium solution concentrations of 0.05–3.5 mol/L at temperatures of 18 and 90°C is studied. The composition of solid phases in equilibrium is analyzed using elemental analysis, X-ray diffraction, and neutron diffraction. A new phase, (NH4)5Sc3F14 with a tetragonal lattice (sp. gr. I41/a, unit-cell parameters a = 0.80843 nm and c = 2.5177 nm), is found at fluoride ion concentrations in the solution of ≤1.06 mol/L at 18°C and 2.92–3.01 mol/L at 90°C; the crystal structure of this phase is determined. 相似文献
12.
The thermal decomposition of freeze‐dried Li‐Mn(II)‐Fe(III)‐formate precursors was investigated by means of DTA, TG and mass spectroscopy. By the thermal treatment of the prefired precursors between 400 and 1000°C, single phase solid solutions LixMn1+xFe2–2xO4 (0 ≤ x ≤ 1) with cubic spinel structure were obtained. To get single phase spinels, special conditions concerning the temperature T and the oxygen partial pressure p(O2) during the synthesis are required. Because of the high reactivity of the freeze‐dried precursors, in comparison with the conventional solid state reaction, the reaction temperature can be lowered by 200°C. The cation distribution and the properties of the Li‐Mn‐ferrites were studied by chemical analysis, X‐ray powder diffraction and magnetization measurements. It was found that for high substitution rates, almost all lithium occupies the tetrahedral coordinated A‐sites of the spinel lattice AB2O4, while at small x‐values, lithium ions are distributed over the tetrahedral and octahedral sites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
《Journal of Non》2007,353(47-51):4384-4389
Lithium manganese spinels Li1+xMn2−xO4, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn2O4 exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn2O4, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons. 相似文献
14.
Tetrathioureacopper(I) chloride, hereafter abbreviated as TCC, was synthesised and single crystals were obtained from saturated aqueous solution by slow evaporation (solution growth) method at room temperature. The crystals obtained are bright, colourless and transparent having well defined external faces. The grown crystals were characterized through elemental analysis, single crystal X‐ray diffraction study, thermal analysis, electron spin resonance spectroscopy and Fourier Transform infrared spectroscopy. The elemental analysis confirms the stoichiometry of the compound. The single crystal diffraction studies indicate that TCC crystallises in the tetragonal lattice and the unit cell parameters are a = b = 13.4082 Å, c = 13.8074 Å, V = 2482.29 Å3, α = β = γ = 90°. Space group and the number of molecules per unit cell (Z) are found to be P41212 and 8 respectively. The TG curve of the sample shows a prolonged decomposition from 210 to 628.3 °C, from which the decomposition pattern has been formulated. The endothermic peaks in the DTA curve indicate melting and decomposition of the compound at 165.2 and 633.8 °C respectively. An exothermic peak in high temperature DSC indicates a phase transition in the compound at 274.8 °C. Thermal anomalies observed in the low temperature DSC at –163.3, –152.0, –141.5, –108.3, 1.0 and 12.1 °C in the heating run and –157.1 and –153.9 °C in the cooling run reveal first order phase transitions in the crystal. The peaks observed at –146.2 °C in both the heating and cooling runs suggest occurrence of a second order phase transition in this compound. The IR spectroscopic data were used to assign the characteristic vibrational frequencies of various groups present in the compound. The ESR study confirms that the copper is in the +1 oxidation state in the complex. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
I. E. Animitsa S. G. Titova A. Ya. Neiman N. A. Kochetova D. I. Bronin E. V. Isaeva 《Crystallography Reports》2002,47(6):991-995
The structural transformations of Sr6Ta2O11 and Sr5.92Ta2.08O11.12 oxygen-deficient cryolites with variations in the partial pressures of oxygen p(O2) and water vapor p(H2O) are investigated using X-ray diffraction, thermogravimetry, and electrical conductivity measurements. It is found that a change in the oxygen partial pressure leads to a phase transition accompanied by the transformation of the cubic cell into the tetragonal cell, most probably, due to ordering of oxygen vacancies. The intercalation of water into the matrix of the complex oxides under investigation results in a structural-chemical transformation during which the solid solution undergoes a transition to an oxyhydrate phase of variable composition and the cubic cell transforms into the orthorhombic cell. 相似文献
16.
The crystal structure of an aluminum-rich variety of the mineral rondorfite with the composition Ca16[Mg2(Si7Al)(O31OH)]Cl4 from the skarns of the Verkhne-Chegemskoe plateau (the Kabardino-Balkarian Republic, the Northern Caucasus Region, Russia) was solved in the triclinic space group with the unit-cell parameters a = 15.100(2) Å, b = 15.110(2) Å, c = 15.092(2) Å, α = 90.06(1)°, β = 90.01(1)°, γ = 89.93(1)°, Z = 4, sp. gr. P1. The structural model consisting of 248 independent atoms was determined by the phase-correction method and refined to R = 3.8% with anisotropic displacement parameters based on all 7156 independent reflections with 7156 F > 3σ(F). The crystal structure is based on pentamers consisting of four Si tetrahedra linked by the central Mg tetrahedron. The structure can formally be refined in the cubic space group (a = 15.105 Å, sp. gr. Fd \(\overline 3 \), seven independent positions) with anisotropic displacement parameters to R = 2.74% based on 579 reflections with F > 3σ(F) without accounting for more than 1000 observed reflections, which are inconsistent with the cubic symmetry of the crystal structure. 相似文献
17.
The specific features of synthesis, polymorthism, and electric conductivity of oxide-ion conducting compounds La2 − x
Me
x
Mo2O9 − 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. 相似文献
18.
E. S. Gagarina E. I. Éknadiosyants L. A. Reznichenko L. A. Shilkina I. P. Raevskii V. P. Sakhnenko V. G. Smotrakov V. V. Eremkin 《Crystallography Reports》2002,47(6):979-990
The domain structure of single-crystal and ceramic samples of Na1 ? x LixNbO3 solid solutions (at x ≤ 0.14) in the orthorhombic ferroelectric and antiferroelectric phases at room temperature is investigated by optical and electron microscopies and X-ray diffraction. The characteristic feature of the domain structure is the formation of 90° complexes consisting of laminar domains with a specific orientation relative to the lattice of the initial cubic phase. Consideration is given to the specific features in packing of these complexes and typical configurations of domains in the crystals. Observations revealed that the domain structure can involve 90°, 60°, 120°, and 180° boundaries, as well as (hhl) boundaries of the S type whose orientation depends on the cell distortion and changes with a variation in x. The indices of these boundaries are determined. The density of 180° boundaries in the ferroelectric phase is very low compared to that of non-90° boundaries. 相似文献
19.
M. S. Molokeev G. A. Petrakovskii A. M. Vorotinov D. A. Velikanov L. V. Udod 《Crystallography Reports》2013,58(2):234-236
Li8FeSm22O38 single crystals have been grown by spontaneous flux crystallization, and their structure has been identified by X-ray diffraction analysis as follows: cubic crystals, a = 11.9078(5) Å, sp.gr. $Im\bar 3m$ , Z = 2. Fe atoms occupy two mixed positions. Magnetic ions have different ligand environments: a regular octahedron (Sm/Fe)O6, a three-cap trigonal prism SmO9, and a square antiprism SmO8. The rather low Neel temperature (~3 K) can be explained by the considerable variation in the angles and lengths of exchange couplings between magnetic ions. 相似文献
20.
The ferroelastic phase transitions of KMgCl3 crystal grown by the Czochralski method were investigated by direct observation using a polarizing microscope with varying temperature. The results showed that the KMgCl3 crystal undergoes two pure ferroelastic phase transitions from cubic prototypic phase to tetragonal, and to orthorhombic structure at about 312 °C and 224 °C, respectively, and can be classified into m3mFmmm(p) species. The permissible domain walls of this species were analyzed by both the strain tensor and group theoretical method. 相似文献