Enhanced superconductivity of the Ti-Zr alloys in the high-pressure BCC phase

A high-pressure study of the crystal structure and superconductivity of Ti-Zr alloys demonstrates an increase in the ω-β-transition pressure from about 30 to 43–57 GPa when the titanium content in the alloys increases from 0 to 50 at. %. The isobaric values of the BCC β-phase superconducting temperature (at 46 GPa) increase from 5.7 to more than 15 K between 0 and 50 at. % Ti, the latter value being the absolute maximum for BCC d-metal alloys. These data correlate with the earlier assumption of an s-d electron transfer in Zr under pressure.     

Energy analysis of diffuse ω-reflections in NbZr by Mössbauer X-ray scattering

When Nb is added to Zr the hexagonal phase of the latter is suppressed. The bcc alloy shows a strong instability towards the hexagonal ω-phase. The dynamic nature of this instability has recently been a subject of considerable experimental and theoretical effort. We have energy analyzed diffusely scattered Mössbauer gamma rays to estimate the dynamic nature of the instability. The central peak of the diffuse scattering at several hexagonal points in k-space shows no inelastic features to a resolution ΔE = ±3×10^?9 eV. Under the assumption that source and scatterer line shapes are Lorentzian, the diffuse scattering in both the 20% and 30% Nb samples is elastic to 3×10^?9 eV. It appears more appropriate at this point to describe the ω- phase scattering in terms of static incoherent precipitates.     

Strengthening in high-pressure quenched Zr

The present study examined the effects of pressure (range: 1–6?GPa) on microstructure and mechanical properties of pure Zr. Pressure significantly affected refining of Zr microstructure. When 5?GPa pressure was applied, ω-phase was observed in processed specimen, and volume fraction sharply increased to 57.4% for specimen pressurized-quenched at 6?GPa. Benefitting from refinement of acicular-shaped α (α′) plates and the formation of equiaxial ω-phase, the yield strength of the sample quenched from 6?GPa reached ～616?MPa, which is almost twice as that of coarse-grained Zr.     

Neutron diffraction determination of the number and displacement of the atoms in the diffuse ω-phase of Zr0·8Nb0·2

Alloys of Zr and Nb with Nb concentrations between 5 and 17 per cent undergo an athermal displacive transformation from b.c.c. to the hexagonal ω-phase. The ω start temperature depends on Nb concentration. The ω structure is derived from b.c.c. by the displacement of atom rows parallel to [111]. For Nb concentrations greater than ? 17 per cent the Bragg reflections characteristic of the ω-phase never develop under athermal conditions but a diffuse scattering in their vicinity is present. The alloy in this state is referred to as diffuse omega phase and omega domains cannot be imaged in the electron microscope. Coherence of the displaced atoms with the b.c.c. lattice allows the determination of the number of displaced atoms and their mean displacement from intensity measurements of the b.c.c. Bragg reflections. At 300°K some 32 per cent of the atoms in the Zr_0·8Nb_0·2 alloy are displaced 56·2 per cent of the way to the ideal ω positions as represented by the AlB₂ structure. Both the number of atoms and their mean displacement increase as the temperature is lowered. We believe these results are the first to quantify the diffuse omega phase.     

A synchrotron study of high-pressure transformations in TiH0·74

Abstract

The crystal structure of the TiH_0·74 alloy was studied by the energy dispersive X-ray diffraction technique in the pressure range to 30·5 GPa at temperatures to 630 K. A phase transformation to the (η + ω) two-phase state was found to occur above 7 GPa at room temperature, then (η+ω)-TiH_0·74 remained stable up to P=30·5 GPa. Another phase transformation resulting in a single-phase state, ζ-TiH_0·74, was found to occur upon heating (η+ω)-TiH_0·74 above T ? 560 K. Both high-pressure phases, η and ζ, were indexed on the basis of the tetragonal sublattices of the Ti atoms with nearly the same specific volumes. It is assumed from the relation of the specific volumes that the hydrogen atoms occupy the tetrahedral interstices in the ζ-phase and the octahedral interstices in the η-phase.     

Diffuse neutron scattering from an ω-phase ZrNb alloy

A back-scattering neutron spectrometer with an energy resolution of ～2μeV (FWHM) has been used to measure the diffuse quasielastic scattering from the ω-phase of Zr_1-xNb_x (0.18 < x < 0.19). It is found that the intrinsic energy-width of the scattering does not change between room temperature and 1000 K. At both temperatures the width is less than ～0.2μeV and consistent with zero.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

Optical excitations due to deuterium in niobium

The optical vibration of deuterium in the α′-phase of Nb D_0.6 has been measured by inelastic scattering of neutrons with a triple axis spectrometer at a hot source. The optical branches show no dependence on the phonon-wave vector and the polarization. The two observed branches fulfil the relation ω₂ = 1.41ω₁.     

Hydrogen absorption effects in the Zr(Fe0.5Cr0.5)2 compound

Zr(Fe_0.5Cr_0.5)₂ Laves phase samples have been made to absorb different hydrogen amounts up to 3.3 H/f.u. XRD showed all ZrFeCrH _y samples to have the C14 structure and, for 0 < y < 2.6, to be composed of a H-poor α-phase and a H-rich β-phase having different c/a ratios. Both phases could be distinguished in the Mössbauer spectra. The β-phase fraction was determined both from X-ray and Mössbauer analysis, with excellent agreement. β-phase lattice constants and hyperfine parameters vs. y exhibited a small jump at y ≈ 1.75, suggesting a phase transition. Mössbauer spectra of ZrFeCrH_3.3 in the 78–350 K range revealed that, for Fe at the 6h site, quadrupole splitting and linewidth increased for decreasing temperature. These effects were attributed to H atomic jump diffusion, and an activation energy of ≈70 meV was estimated.     

Optical phonon self-energies in titanium phases: effects of electron–phonon interaction

Temperature-dependent Raman investigations of titanium in the α and pressure-quenched ω-phase have been carried out. The results obtained suggest the possible coexistence of both phases at ambient pressure and low temperatures. Comparison of the low-temperature E_2g phonon self-energies in both phases with simulations based on the calculated electronic structures for α- and ω-Ti implies significant contributions from non-adiabatic electron–phonon interactions.     

Optical mixing in InSb

Abstract

In this paper, the authors have derived the expression for the combination frequency components ω₃ = 2ω₁-ω₂ in the current density by solving the appropriate Boltzmann transfer equation for electrons, when two laser beams of frequencies of ω₁ and ω₂ are incident on a degenerate nonparabolic semiconductor viz. InSb; the nonlinearity due to collision mechanism as well as the nonparabolicity of conduction band has been taken into account. The ionized impurity scattering has been considered to be the sole mechanism of electron scattering. The expression for the current density is further substituted in the wave equation to obtain the expression for the amplitude of combination frequency wave in the transmitted component. The calculated value of output power of frequency ω₂ is found to be 1/10th of the experimental value (Patel, Slusher and Fleury, 1966), ⁽⁷⁾ and thus is in better agreement as compared to earlier investigators (Wolff and Pearson, 1966; Kaw, 1968). ^{(5, 10)}     

The Electronic Absorption Spectrum of Hafnium Monochloride

Abstract

Intracavity laser spectroscopy has been applied for investigation of absorption spectrum of HfCl molecule. In the region 560–700 nm 59 bands have been obtained. Rotational structure analysis of 0–0 band indicated that Hund's case (c) of angular moment coupling applied to this molecule. The molecular constants (cm^?1) calculated for upper and ground electronic states are: ω′ = 353.05 cm^?1, ω″ = 379.65 cm^?1, B′=0.21486 cm^?1 B″ = 0.21801 cm^?1.     

Atomic Scale Chemistry of α2/γ Interfaces in a Multi-Component TiAl Alloy

We report on the distribution of micro-alloying elements in a multi-component TiAl-based alloy. The specimen contains 3 at.% Nb, 1.5 at.% Cr, 0.5 at.% Mn, 0.6 at.% (W + Hf + Zr), and 0.2 at.% each of B, C, and O. The distributions of all micro-alloying elements with respect to the heterophase interface between α₂ and γ lamellae are analyzed with a three-dimensional atom-probe (3DAP) microscope. All the elements partition except boron, which resides primarily in boride precipitates. Oxygen, C, Mn, and Cr partition to the α₂-phase, whereas Nb and Zr partition to the γ-phase. Both W and Hf exhibit excess concentration values within ca. 7 nm of the lamellar interface in the α ₂-phase, and their near interfacial excesses are 0.26 and 0.35 atoms nm^?2, respectively.     

Diffuse phase transition in a surface quartz layer with variations in temperature

The temperature dependence of the α-phase concentration in surface layers of a solution-grown quartz crystal has been studied in the range 290–820 K. This has been achieved by measuring the intensity of the 695.1, 785.0, and 1061.5 cm^?1 bands in the ?″(ν) IR damping spectra. It has been found that, in a surface layer ～0.15 μm thick, the concentration of the α-phase behaves with increasing temperature as expected for a first-order phase transition, namely, before 800 K, it remains constant, after which at T → 846 K, tends to zero. At a distance from ～1 to 20 μm from the surface, however, the concentration of the α-phase starts to decrease already at ～350 K, while at 812 K it decreases to one-fifth of the original value. This is paralleled by an increase in the intensity of the 804 cm^?1 band assigned to the β-phase. The diffusive pattern of the α-β phase transition is initiated by distortion of the quartz crystal lattice around growth dislocations. The internal stresses arising in these layers have been estimated from the shift of the band maxima. It has been established that at distances up to ～1 μm from the surface, tensile stresses reaching ～300–400 MPa appear at 400 K. These stresses drive in the surface layer of the macrocrystal microcracks culminating in total destruction of the sample. The appearance of tensile stresses is assigned to an increase in volume of the macrocrystal layer located at a distance from ～1 to 20 μm from the surface and the growth in it of the β-phase concentration. At the same time, compressive stresses develop in a layer ～1 to 20 μm thick at a temperature above 500 K, which reach a maximum at ～650 K, to fall off thereafter with increasing temperature. The compression is caused by vibrations of growth dislocation loops in the temperature range specified.     

Ultrasonic attenuation in superconducting Nb20Zr80

We have measured the low-temperature ultrasonic attenuation in quenched polycrystalline Nb₂₀Zr₈₀ at 30 and 90 MHz. The quenching process creates tunnelling states due to metastable regions of the athermal ω-phase. Our results can be explained on the basis of the interaction between electrons and tunnelling states.     

The crystal structures and phase transition of methyl bromide

The detailed crystal structures of deuterated methyl bromide in both its α-and β-phases have been determined at 175 K and 146 K respectively by neutron powder profile measurements. The α-phase has space group Cmc2₁ while the β-phase has space group Pnma and both have a quasi-two-dimensional character. The major characteristic of the first order transition is a large change in the relative orientation of the two molecules lying in the mirror planes. The transition is successfully interpreted in terms of changes in the topology of the lattice energy surface calculated as a function of temperature from pair-wise interatomic potentials.     

Ultrasonic attenuation due to resonant interaction with a distribution of level splittings of the ground state of shallow acceptors in Ge

The ultrasonic attenuation in Ge (Ga, In) has been measured in the frequency range from 500 MHz to 2.5 GHz, and from room temperature down to 1 K. Below 10 K the attenuation rises as ω²/T. For the first time saturation of the attenuation has been observed for ground state of a shallow acceptor. These results can be interpreted as due to resonance interaction with level splittings of a broad distribution with width of about 0.1 meV.     

Superconductivity in α- and ω-Zirconium under high pressure

The pressure dependence of the superconducting transition temperatureT _c (p) of α-Zr has been investigated in both solid and liquid pressure transmitting media. Up to about 45 kbardT _c /dp was measured to be + 3.5 × 10^?6 K/bar. Cold working at 4.2 K produced a strong irreversible effect onT _c . The superconductivity of the high pressure phase, ω-Zr, has been studied in its region of stability, i.e. above 60 kbar. For ω-Zr,dT _c /dp=+7.7 × 10^?6K/bar, andT _c (0)=0.72 K (by extrapolation).     

Perturbation theory for the angular correlation function

A perturbation expression for the angular pair correlation function g ⁽²⁾(r ₁₂, ω₁, ω₂) is derived for systems interacting via non central potentials based on the method developed by Gubbins and Gray [1]. The method uses the ‘correct’ (in the sense of Rushbrooke [3] and Cook and Rowlinson [4]) angle-averaged potential as the reference system about which the perturbation is made. A preliminary comparison between the original Gubbins-Gray expression for g ⁽²⁾(r ₁₂, ω₁, ω₂) and the present expression is made for a system of two-dimensional point dipoles.