Susumu Nanao

Persistence of Covalent Bonding in Liquid Silicon Probed by Inelastic X-Ray Scattering

Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.

Thermal phase transition of RbMnFe(CN)6 observed by X-ray emission and absorption spectroscopy

The thermal phase transition of RbMnFe(CN)6 has been observed by Mn and Fe 3p–1s X-ray emission spectroscopy (XES) and 1s X-ray absorption spectroscopy (XAS). The thermal variations of the spin states and the valences of Mn and Fe were determined to be Mn2+(S=5/2)–NC–Fe3+(S=1/2) for the high-temperature (HT) phase and Mn3+(S=2)–NC–Fe2+(S=0) for the low-temperature (LT) phase. These transitions are thus caused by charge transfer between Mn and Fe. The temperature dependences of Mn and Fe 3p–1s XES and 1s XAS were observed as the composition of the spectra of the HT and LT phases. The ratios of the HT component in each spectrum show good agreement with the thermal transition curves observed with magnetic susceptibility measurements.

Studies of defects at thermal equilibrium and melting in Cu and Ni by positron annihilation

Peak counting rates in the angular correlation were measured as a function of temperature above room temperature in the solid and liquid phases of Cu and in the solid phase of Ni. The peak counting rates in Cu increased about 17% upon melting. Temperature dependence of the peak counting rates in the liquid phase shows a hump at 1150 degrees C. Two possible explanations were proposed for this anomaly. The data in the solid phases were numerically analysed by the trapping model in which it was assumed that only single vacancies and divacancies are effective. The data at lower temperatures, where the effect of divacancies is sufficiently small, were analysed by the conventional trapping model of single vacancies. The results of both analyses are consistent. The characteristic temperature Tc in the trapping model was defined. It was demonstrated that Tc is proportional to E1VF. The experimental values of Tc were plotted against E1VF for various FCC metals and linear relationship between E1VF and Tc was also experimentally found.

Evaluation of the formation entropy of a single vacancy by means of positron annihilation

Abstract Analyzing positron annihilation data it was found that the formation energy of a single vacancy is proportional to the characteristic temperature Tc in f.c.c. metals. Assuming that the equilibrium fractional concentrations of single vacancies at Tc are constant, one finds that the formation entropy of a single vacancy is (1.6 ± 0.1)k for all f.c.c. metals. For zinc and cadmium, the formation entropies are 2.5k and 3.3k respectively.

Positron annihilation study of order-disorder transformation in Cu alloys

Abstract The anomalous change of N 0 ( T ) (the peak counting rates of angular distribution as a function of the temperature of a speciment) due to order-disorder transformation is observed in various Cu-based alloys. The detailed behavior of N 0 ( T ) in AuCu 3 is clearly different from those in CuZn or Cu-Pd alloys, which is attributed to the difference of the order of order-d disorder transformation.

Studies of vacancies and vacancy-impurity pairs in Cu-0.5 at. % Ge by means of positron annihilation

Positron annihilation in Cu-0.5 at. % Ge as a function of measuring temperature has been studied. By the use of the trapping model, the formation energy of a vacancy in copper and the binding energy between a germanium atom and a vacancy have been determined to be 1.28 and 0.27 eV, respectively.

The structure of icosahedral AlMgCu alloy

Abstract The pair distribution function π(r) was derived for an Al6Mg4Cu1 icosahedral alloy from the measurement of Mo Kα X-ray diffraction. The atomic structure proposed by Henley and Elser (Philos. Mag. B, 53 (1986) L59) was constructed using the projection method. The calculated π(r) from the Henley-Elser model was compared with the experimental π(r) of the icosahedral Al6Mg4Cu1 phase. While the agreement was not satisfactory, the Henley-Elser model still contains an attractive concept for the icosahedral AlMgCu alloy.

The measurement of the formation energy of a vacancy in Cu by positron annihilation

The temperature dependence of the coincidence counting rates of two annihilation photons was measured for Cu at theta =0. Analysing the results by the trapping model, the formation energy of a vacancy in Cu was determined to be 1.17+or-0.07 eV. This value is in good agreement with that by Simmons and Balluffi (1963).

Water-cooled quasi-doubly bent crystal monochromator for Compton scattering experiments

Commissioning and performance of a water‐cooled quasi doubly bent crystal monochromator for Compton scattering experiments are described. The monochromator consists of an array of twenty singly bent crystals. Each crystal is half immersed in liquid Ga‐ln alloy contained in a water‐cooled holder. This is necessary because the monochromator must be able to withstand heat load of 4 kW from synchrotron radiation from an ellipsoid multipole wiggler at the 6.5 GeV accumulation ring. The alignment procedure for twenty singly bent crystals is described in detail. Its performance is as follows; the focused beam size of 60 keV x rays is about 3 mm (vertical) and 8 mm (horizontal). The flux is 6×1012 photons/s at stored currents of 30 mA and the energy resolution is 88 eV for 60 keV x rays. Heating of various adjusting mechanisms by Compton scattered x rays from both the absorber used to suppress low energy x rays and the monochromator crystals is found to be a very serious problem for designing a high energy x‐ray ...

The Fermi surfaces of CuNi alloys by positron annihilation with crossed-slit geometry

Abstract The Fermi surfaces of Cu and CuNi alloys have been studied by positron annihilation with crossed-slit geometry. The results suggest that the Fermi surface detaches from the Brillouin zone boundaries for 78.7 Cu - 21.3 Ni and 58.1 Cu - 41.9 Ni alloys.