Structure of liquid transition metal hydrogen alloys under high pressure

Abstract

Hydrogen reacts with many metals and form solid solutions and/or metal hydrides [1]. In transition metal hydrides, hydrogen atoms usually occupy interstitial sites and the crystalline lattice expands. A solubility of hydrogen in iron (nickel) is small at ambient conditions but it increases with increasing pressure. To study effect of hydrogen to the structure of liquid metals, we have measured x-ray diffraction on liquid iron-hydrogen [2] and nickel-hydrogen alloys under high pressure and high temperature conditions. Measurements were carried out using a cubic-type multi-anvil press on BL14B1 at SPring-8. The metal sample was put in a NaCl sample capsule together with hydrogen source (LiAlH4). The hydrogen source decomposed and released hydrogen upon heating under high pressure. Structure factors, S(Q), of liquid FeHx and NiHx at about 4GPa and 1350°C and those of liquid pure Fe and pure Ni at about 3.5GPa and 1700°C were obtained. In both systems, S(Q) of pure metal and that of metal hydrogen alloy are very similar. The first peak in the pair correlation function slightly shifts to higher-r upon hydrogenation. Slight elongation of the metal-metal nearest neighbor distance supports a notion that hydrogen and iron form interstitial alloy in the liquid state.