The phase diagram of single crystal [(CH3)4N]2CuBrxCl4−x solid solutions

Differential scanning calorimetry (DSC) and X-ray measurements give evidence on a complete miscibility within the solid solution system [(CH₃)₄N]₂CuBr_xCl_4−x. In addition to the known phase sequences of pure components, some new phases appear for intermidiate concentrations. Out of five triple points observed in the phase diagram, one seems to be a Lifshitz point between a paraelectric, an incommensurate and a ferroelectric phase.     

Determination of λ and κ in a mosaic of single crystal YBa2Cu3O6.95

We present transverse fieldSR measurements of the magnetic field distribution in a mosaic of single crystal YBa₂Cu₃O_6.95 in high fields (1.9–6.5 T) applied parallel to the crystallographicc-axis. TheSR lineshapes are shown in detail and the results of fits to the lineshapes are summarized.     

Structure of the magnetic-field-induced modulated phase of an α-Fe2O3: Ga single crystal

The field and orientational dependences of magnetic linear birefringence in α-Fe₂O₃: Ga single crystals are investigated. The experimental results make it possible to refine and specify the structure and reveal the features of technical magnetization of the modulated magnetic phase of this weak ferromagnet. It is shown that the model of the modulated magnetic structure used in this study adequately describes the experimental situation.     

Magneto-resistance and vortex phase diagram of BaNi_（0.1）Fe_（1.9）As_2 single crystal

The transition from vortex glass to a liquid phase is studied in Ba Ni0.1Fe1.9As2 single crystal with Tc = 19.4 K by magneto-resistance measurements. The resistivity curves are measured in magnetic fields in a range of 0 T–13 T for H‖c and H⊥c. Good scalings for all values of resistivity ρ（H, T） and the effective pinning potential U0（H, T） are obtained with the modified vortex glass theory by using the critical exponents s and H0. Phase diagrams for H‖c and H⊥c are determined based on the obtained vortex glass temperature Tg, the vortex dimensionality crossover temperature T＊, and the upper critical magnetic field Hc2. Our results suggest that both below and above 5 T, single vortex pinning co-exists with collective creep, and collective creep is dominant. There is a narrower vortex liquid region for H⊥c than for H‖c in the vortex phase diagram, which may originate from a stronger pinning force.     

The vibration spectrum and high temperature phase transition of NaBF_4 single crystal

The polarized Raman spectrum of NaBF_4 single crystal measured by use of different geometric arrangements are reported. The obvious anisotropy of the crystal results in the degradation of degenerated modes of BF_4. Experiments show that the vibration frequencies decrease with the increasing of temperature. But when it above the phase transition point the v_1 symmetric stretching vibration is restored. From this, the phase transition temperature can be estimated to be 250℃ (523K).     

Generic phase diagram of Nd2−xCexCuO4

We investigated the generic phase diagram of the electron doped superconductor, Nd_2?xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. Based on the discussion on the tremendous influence of O_ap “impurities” on superconductivity in T′ cuprates, we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2?xCe_xCuO₄.     

Low-temperature growth of γ phase in thermally deposited In2Se3 thin films

In the present report, we have studied the structural and optical change in the In₂Se₃ thin films prepared by the thermal evaporation method, deposited on a glass substrate and annealed at 250°C. Both the structural and optical studies revealed the formation of γ-In₂Se₃ phase on annealing at such low temperature as 250°C which is not observed before. Raman analysis indicates that the as-prepared film consists of both α and γ-In₂Se₃ phases and the annealed film contains only γ-In₂Se₃ phase. The absorption mechanism in the studied film follows the direct allowed transition. The optical band gap is found to be decreased with annealing due to the increase in the width of localized states near to the band edges. Transmittance is found to be decreased and the absorption is increased with annealing due to the change in the film density which enhances its suitability for solar cell applications.     

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.     

Structural and physical–chemical properties of the CuGa5Se8, CuGa3Se5 and CuIn3Se5 compounds

Large-block (8×4×3 mm³) CuGa₅Se₈ crystals were obtained by the horizontal Bridgman method. Homogeneous CuGa₃Se₅ and CuIn₃Se₅ single crystals 12 mm in diameter and up to 40 mm in length were grown by directed crystallization of the melt. All three compounds were found to have the chalcopyrite-related structure. The melting points of these compounds were defined by means of the differential thermal analysis. The lattice parameters a and c as well as the axial thermal expansion coefficients α_a and α_c were determined as a function of temperature in the range from 90 to 650 K by the X-ray diffraction method. It is revealed that for all three compounds the coefficients of expansion along the a-axis are larger than those along the c-axis over the entire temperature range studied.     

On the phase diagram of a two-dimensional electron–hole system

The phase diagram for a system of spatially separated electrons and holes in coupled quantum wells or graphene double layers is studied in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. We find a second order transition between an electron–hole plasma and a BCS phase, as well as a first-order transition between the BCS phase and a bosonic Mott phase of tightly bound electron–hole pairs without phase coherence. The electron–hole plasma exists at low and at high densities for weak interaction, the BCS phase at moderate density and the Mott phase at high density and strong interaction.     

Electrical conductivity and phase diagram of the Na2SO4CaSO4 system

Very high vacancy concentrations may be obtained in solid solutions of the high temperature phase of Na₂SO₄. In this paper the Na₂SO₄CaSO₄ system has been studied using differential scanning calorimetry (DSC), impedance spectroscopy and X-ray powder diffraction. The phase diagram, especially the stability range of the solid solution of the high temperature Na₂SO₄ phase, has been redetermined. The electrical conductivity of this solid solution increases rapidly with increasing CaSO₄ content and reaches a maximum for about 5 mol% CaSO₄; the conductivity at e.g. 300°C, 3.5×10⁻³ (μ cm)⁻¹, is almost three orders of magnitude higher than that of pure Na₂SO₄.     

Single crystal growth of YBa2Cu3O7−δ through a Pt compound intermediate phase

We report the synthesis of YBa₂Cu₃O_7−δ (Y-123 phase)-single crystals, through a different path to the traditional one – which involves Y₂BaCuO₅ (Y-211 phase) as a precursor –. In our case, at 970°C a Pt compound appears: Y₂Ba₃Cu₂PtO₁₀ which dissolves in the melt when heated at 1100°C allowing the growth of Y-123 phase. Large single crystals of Y-123 phase obtained through the route developed in this study showed good superconducting properties with a transition temperature onset at about 85 K; this transition temperature is shifted towards higher temperatures through oxygenation processes that are favoured by their dimensions.     

Infrared active phonon modes and ionicity of single crystal MgAl2O4

Near-normal incident infrared reflectivity spectra of （100） MgAl2O4 spinel single crystal have been measured at different temperatures in the frequency region between 50 and 6000 cm^-1. Eight infrared-active phonon modes are identified, which are fitted with the factorized form of the dielectric function. The dielectric property and optical conductivity of the MgAl2O4 crystal are analysed. From TO/LO splitting, the effective Szigeti charges and Born effective charges at different temperatures are calculated for studying the ionicity and the effect of polarization. Based on the relationship between the （LO-TO）1 splitting, which represents the transverse and longitudinal frequencies splitting of the highest energy phonon band in the reflectivity spectrum, and the ionic-covalent parameter, the four main phonon modes are assigned. MgA1204 can be considered as a pure ionic crystal and its optical characters do not change with decreasing temperature, so it may be used as a suitable substrate for high-Tc superconducting thin films.     

Calculation of the structure properties and P–T phase diagram of hafnium

Abstract

The full-potential linear muffin-tin orbitals (FP-LMTO) method is used to calculate the total energy and equilibrium lattice properties for the observed phases of Hf. The temperature and pressure dependences of the Gibbs energy are found for these structures within the Debye model. A quantitative agreement with the experimental points of the P-T phase diagram is obtained.     

Metastable (GaSb)(1−x)(Sn2)x alloys: Crystal growth and phase stability of single crystal and polycrystalline layers

Single-phase metastable (GaSb)_(1−x)(Sn₂)_x alloys with x ⩽ 0.23 far exceeding the equilibrium solid solubility limit of x ⪅ 0.02 for Sn in GaSb, have been grown by rf sputter deposition. The key feature allowing the growth of single phase alloys was the use of low energy ion-bombardment-induced collisional mixing during deposition. Films grown at elevated temperatures T_s on GaAs(100) substrates were found to be single crystals while polycrystalline films with very strong (220) perferred orientation and grain sizes of 20–40 nm were obtained on amorphous glass substrates. The maximum film growth temperatures T_m at which single phase films could be obtained was a function of the negative substrate potential V_s during deposition and the composition x. For V_s = 75 V, T_m ranged from 375 °C at x = 0.06 to 125 °C at x = 0.23. Crystal growth and post-annealing studies both indicated that the transformation from the single-phase metastable state to the equilibrium (GaSb + βSn) state occurs through a continuous set of metastable GaSb-rich phases and that Sn precipitates out first in the α-diamond structure and then transforms to the β-tetragonal structure. In single crystal films, the GaSb-rich phase precipitates out coherently with the alloy matrix. The activation energy for phase separation ranged from ∼ 1.4 eV for alloys with x = 0.06 to 0.9 eV at x = 0.23. A phase map for crystal growth, plotted as a function of T_s and x, and an annealing transformation diagram for (GaSb)_0.86(Sn₂)_0.14 was determined.     

Critical behavior of single crystal CuCr2Se4−x Br x (x=0.25)

The doping of bromine has prominent effect on the transport properties of CuCr₂Se₄, where the conductivity of CuCr₂Se_4?x Br _x changes from metallic behavior with x=0 to semiconductor with x=1. In this work, the critical behavior of single crystal CuCr₂Se_4?x Br _x with x=0.25, which lies at the boundary between the metallic and insulating state, is investigated through the magnetization under high magnetic field. The critical exponents β=0.39±0.03 and γ=1.25±0.05 are obtained by the Kouvel–Fisher method, and δ=4.79±0.01 is generated by the critical iso-thermal analysis. The obtained exponent β is close to the prediction of the three-dimensional (3D) Heisenberg model, while γ approaches that of the 3D-Ising model. Although the bromine doping has prominent effect on the transport behavior, the critical exponents are hardly changed. The obtained results indicate that the ferromagnetism is established through one-dimensional Cr–Se–Cr bonds. However, the nearest-neighbor magnetic interaction belonging to the 3D-Heisenberg model also contributes to the ferromagnetic exchange.     

Surface energy and crystal structure of nanowhiskers of III–V semiconductor compounds

A theoretical model has been proposed for calculating the surface energy of nanowhiskers in the nearest neighbor approximation. The surface energy has been calculated for different faces of III–V semiconductor crystals with cubic and hexagonal structures. The effect of the formation of the hexagonal wurtzite phase in nanowhiskers of III–V semiconductor compounds has been considered using the obtained data. Estimates for the critical radius of the phase transition in III–V semiconductor nanowhiskers are presented.     

Gas–crystal phase transition in a 2D dipolar exciton system

A system of dipolar excitons at temperatures exceeding the expected Bose–Einstein condensation temperature is considered. It is shown that a first-order phase transition with the formation of a phase close to the crystal of such excitons is possible at such temperatures. The phase diagram in the range of low concentrations and temperatures is constructed. The effect of this transition on the luminescence spectrum of the system is analyzed.     

Determination of the thickness and optical constants of amorphous Ge–Se–Bi thin films

The effect of varying bismuth concentration on the optical constants of amorphous Ge₂₀Se_{80? x} Bi _x (where x = 0, 3, 6, 9 and 12 at%) thin films prepared by thermal evaporation has been investigated. The transmission spectra T(λ) of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. An analysis proposed by Swanepoel [J. Phys. E: Sci. Instrum. 16 (1983) p.1214], based on the use of the maxima and minima of the interference fringes, was applied to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Increasing bismuth content was found to affect the refractive index and extinction coefficient of the Ge₂₀Se_{80? x} Bi _x films. Optical absorption measurements show that the fundamental absorption edge is a function of composition. With increasing bismuth content, the refractive index increases while the optical band gap decreases.