G. F. Syrykh

Experimental study of the vibrational spectrum and structure variations in NH4Cl under high pressure

Abstract In this paper application of the sapphire anvils technique to the study of vibrational spectra of hydrogen-containing systems by inelastic neutron Scattering under high pressure is described. The experiments were performed with the DN-12 time-of-flight spectrometer at the IBR-2 pulsed reactor in Dubna. The structural changes and pressure dependence of the libration and lattice modes in NH4Cl were studied up to 40 kbar. The results give some evidence of the NH4 + ion instability.

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH4Cl at pressures of up to 2.6 GPa and of NH4Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.

Size effects in the vibrational and electronic properties of Cu-Pb nanocomposites

Composites of Cu and Pb (immiscible in solid state) were prepared by melt spinning onto a copper disc. X-ray diffraction measurements showed the Cu-Pb composites thus obtained to consist of Pb nanoparticles of a certain size embedded in a copper matrix. The average size of the nanocrystalline Pb blocks was determined along the normals to the (111) and (200) reflecting planes, and their size distribution was measured. The vibrational, electronic, and superconducting properties of the Cu85Pb15 and Cu50Pb50 composites were derived from low-temperature heat-capacity, magnetic-susceptibility, and resistance measurements, and the contribution of Pb nanoparticles to the heat capacity was separated. The low-frequency excitation density in Pb nanocrystals was found to increase as compared to that in crystalline Pb. The observed decrease of Tc correlates with the variation of Pb nanoparticles in size, which is a consequence of the size effect in the properties of Pb nanocrystals.

Splitting of libration mode frequencies in the vicinity of orientation phase transition in NH4Br

Abstract Vibrational spectra of NH4Br at high pressures up to 4·5GPa have been studied by means of incoherent inelastic neutron scattering using sapphire anvil high pressure cell technique. Libration mode splitting was investigated in the vicinity of the orientation phase transition (Ptr = 2·7 GPa) and this effect disappears if pressure is less or higher than Ptr. This effect is explained in terms of two-well asymmetric potential.

Concentration dependence of partial vibrational spectra in Ni–Nb and Cu–Zr metallic glasses

The neutron inelastic scattering of the Ni44Nb56 and Cu33Zr67 metallic glasses with isotope substitution for Cu and Ni were measured. The results are compared to earlier data for Ni62Nb38 and Cu64Zr36 (Ni and Cu isotopic substitution). Availability of two samples with different isotope content for each system has enabled partial vibrational densities of states to be obtained for both materials. The ratios of average force constants, BNi/BNb and BCu/BZr, and mean-square atomic displacements, 〈u2〉, for Ni, Nb, Cu and Zr atoms are estimated. The interatomic interaction of the Ni and Cu atoms with respect to all neighbours is less than that of the Nb and Zr atoms. With decrease of the content of Ni and Cu atoms the differences of the average force constants in each system decline.

Experimental study of partial vibrational spectra in amorphous alloys

Abstract Inelastic neutron scattering study with isotope substitution for Ni has been performed on the Ni 64 Zr 36 amorphous alloy. The measurements were made for two samples: (1) natural mixture of Ni isotopes and (2) 99.5% enriched 60 Ni isotope (neutron cross sections — 18 barns and 1.09 barns, respectively). This has enabled the model-independent reconstruction of the Ni and Zr partial vibrational densities of states (VDOSs) and total VDOS in the system. Zr spectrum concentrates at lowering frequences and reveals a set of singularities (at ∼ 6, 11, 23 meV) as compared to Ni spectrum, which has a more flat character.

Dispersion relation of low-energy excitations in Zr50Be50 metallic glass

A high-resolution (1.5 meV) inelastic neutron scattering experiment is carried out to investigate in detail the low momentum (0.6–1.5 Å−1) dynamic response in Zr50Be50 metallic glass. The results are obtained on the hot neutron three-axis spectrometer IN1 in the Laue-Langevin Institute (Grenoble, France). A comparison with recent neutron scattering results for the momentum transfer region above 1.4 Å−1 shows a minimum in the dispersion relation of low energy excitations near the prepeak of the static structure factor. These excitations appear to be due to the short-range order in this amorphous system.

Vibrational and electronic properties of the amorphous systems Ni44Nb56, Ni62Nb38, and Cu33Zr67 as derived from specific heat measurements

The specific heats of the amorphous systems Ni44Nb56, Ni62Nb38, and Cu33Zr67 were studied in the temperature range 3–273 K. The data obtained allow one to isolate the contribution due to atomic vibrations from the experimentally measured specific heat, to determine the density of electronic states at the Fermi level and the temperature dependence of the characteristic Debye parameter Θ over a broad temperature range, and to calculate a few frequency moments that characterize the vibrational spectrum. The information derived on the average characteristics of vibrational spectra is in good agreement with earlier data on inelastic neutron scattering. In transferring from Ni44Nb56 to Ni62Nb38, the density of electronic states at the Fermi level decreases and the characteristic vibrational frequencies increase. The density of electronic states at the Fermi level for Cu33Zr67 is close to that for Ni62Nb38. The characteristic frequencies of the vibrational spectrum of the Cu33Zr67 system are substantially lower (by 30%) than those of the Ni44Nb56 and Ni62Nb38 systems.

Crystallization of Amorphous Zr-Be Alloys

The thermal stability and structure of binary amorphous Zr100 − xBex alloys have been studied using differential scanning calorimetry and neutron diffraction over a wide concentration range (30 ≤ x ≤ 65). The amorphous alloys have been prepared by rapid quenching from melt. The studied amorphous system involves the composition range around the eutectic composition with boundary phases α-Zr and ZrBe2. It has been found that the crystallization of alloys with low beryllium contents (“hypoeutectic” alloys with x ≤ 40) proceeds in two stages. Neutron diffraction has demonstrated that, at the first stage, α-Zr crystallizes and the remaining amorphous phase is enriched to the eutectic composition; at the second stage, the alloy crystallizes in the α-Zr and ZrBe2 phases. At higher beryllium contents (“hypereutectic” alloys), one phase transition of the amorphous phase to a mixture of the α-Zr and ZrBe2 phases has been observed. The concentration dependences of the crystallization temperature and activation energy have been revealed.

On the positions of impurity modes in phonon spectra of metallic alloys

Positions of local and quasi-local impurity frequencies in metallic alloys have been considered. It has been shown that the differences between the frequencies calculated in the isotopic approximation and the experimental values can be explained not only by a change in the force constants (force shifts), as it is usually done, but also by a change in the effective mass of the oscillations (mass shifts). Comparison of the calculations with the experimental data demonstrates that the main role in the difference between the frequencies calculated in the isotopic approximation and the experimental values is played by the mass shifts rather than the force shifts, at least for local modes.