K.D. Machado

X-ray and neutron diffraction studies and reverse Monte Carlo simulations of an amorphous Ni60Ti40 alloy produced by mechanical alloying

The local atomic order of an amorphous Ni60Ti40 alloy produced by mechanical alloying was studied by x-ray diffraction (XRD) and neutron diffraction techniques (ND). The total structure factors derived from the XRD and ND measurements were simulated by using the reverse Monte Carlo (RMC) method. The simulations were used to compute the partial pair distribution functions GijRMC(r) andthe partial structure factors SijRMC(K). From these functions, coordination numbers and interatomic distances for the first neighbours were calculated. There is a shortening in the heteropolar distance and a preference in forming unlike pairs, indicating a relatively strong chemical short-range order in the alloy.

Electronic and optical properties of amorphous GaSe thin films

The eletronic and optical properties of amorphous GaSe thin films produced by vacuum evaporation were investigated using X-ray photoemission spectroscopy (XPS) and transmittance spectroscopy techniques. XPS measurements allowed the determination of the valence band energy and showed the chemical bonding and the charge transfer between Se and Ga atoms. Transmittance measurements allowed the determination of the optical gap, refractive index and extinction coefficient in the low and high absorption regions. Using the Wemple and DiDomenico single oscillator model we also found the oscillator and the dispersive energies. From the valence band and optical gap energies, the conduction band was found and an energy level diagram for f-GaSe is proposed.

Structural and thermal investigations of an amorphous GaSe9 alloy using EXAFS, cumulant expansion, and reverse Monte Carlo simulations

In this article, we investigated structural and thermal properties of an amorphous alloy of the Ga-Se system. The amorphous GaSe9 alloy was produced by mechanical alloying and it was studied using EXAFS spectroscopy and cumulant expansion method. We also made reverse Monte Carlo simulations using the total structure factor S(K) obtained from x-ray diffraction and the EXAFS χ(k) oscillations on Se and Ga K edges as input data. Several parameters, such as average coordination numbers and interatomic distances, structural and thermal disorders, asymmetry of the partial distribution functions gij(r), and Einstein and Debye temperatures, were determined. The gij (E)(r) functions were reconstructed from the cumulants C1, C2, and C3 obtained from the Einstein model, and they were compared to the gij (RMC)(r) functions obtained from the simulations. The simulations also furnished the partial bond angle distribution functions Θijℓ(cosθ), which describe the angular distribution of bonds between first neighbors, and give information about the kind of structural units present in the alloy.