Extended x-ray-absorption fine-structure and x-ray-absorption near-edge-structure studies of glassyNi66B33: A multishell modeling of the Ni-Ni distribution

Abstract

Amorphous ${\mathrm{Ni}}_{66}$ ${\mathrm{B}}_{33}$ is a novel metallic glass in that the metalloid content is much higher than those of conventional transition-metal—metalloid systems. More interestingly, this amorphous material crystallizes homogeneously into a single ${\mathrm{Ni}}_2$B phase of known crystal structure (that of ${\mathrm{Al}}_2$Cu) in which there is only one nonequivalent Ni site. The existence of the ${\mathrm{Ni}}_{66}$ ${\mathrm{B}}_{33}$ composition as a single phase in both the crystalline and glassy states makes it a useful model system for quantitative extended x-ray-absorption fine-structure (EXAFS) investigation of the local atomic environment in metal-metalloid glasses. The local structure of a Ni atom in crystalline ${\mathrm{Ni}}_2$B consists of four boron atoms at a distance of 2.14 Å, one Ni atom at 2.37 Å, two Ni atoms at 2.42 Å, and two sets of four Ni atoms at 2.63 and 2.70 Å. The ${\mathrm{Ni}}_2$B crystal was used to model the local structure of Ni in the metallic glass as follows: Crystallographic data (coordination numbers and bond distances) were used as fixed inputs to the Fourier-filtered EXAFS of the crystal to generate an envelope function for B and a set of self-consistent phase shifts for the Ni-B and Ni-Ni pairs. These scattering parameters were then transferred as fixed inputs to the Fourier-filtered EXAFS of the glass phase to extract structural parameters. The fitting of the glass EXAFS was performed systematically using sequentially 1 B + 1 Ni shells, 1 B + 2 Ni shells, 2 B + 1 Ni shells, 1 B + 3 Ni shells, 2 B + 2 Ni shells, and 1 B + 4 Ni shells. It was found that the 1 B + 3 Ni shell combination yields the best fit and physically most meaningful results for the glass. It was also found that the nearest boron shell about a Ni atom in the glass remains very similar to that of the crystal, but there is considerable rearrangement of the outer Ni shells. In the glass, the eleven Ni atoms are now distributed into three subshells with the shortest Ni-Ni distance measurably closer to the central Ni atom. This local rearrangement and closer packing of the Ni shells were reflected in the local electronic structure of the glassy phase and manifested in changes of the x-ray-absorption near-edge structure spectrum in going from the crystal to the glassy phase. Ge substitution was made to probe the metalloid environment in glassy ${\mathrm{Ni}}_{66}$ ${\mathrm{B}}_{15}$ ${\mathrm{Ge}}_{18}$. The present EXAFS findings are discussed in light of some recent diffraction and NMR results in Ni-B glasses.