Superstructure and Order-Disorder Transformation of Interstitial Oxygen in Hafnium

Abstract

Crystal structure and phase transition in the hafnium-oxygen system containing 10–20 at% O have been studied by means of electron, neutron and X-ray diffraction and calorimetry as well. The oxygen atoms occupy the octahedral holes in h.c.p. metal lattice and two types of interstitial superstructures are found at the hypo- and hyper-stoichiometric composition HfO_1⁄6− and HfO_1⁄6+ below 430°C. The lattice constants of the hexagonal supercells are respectively a=\sqrt3a₀, c=3c₀ and a=\sqrt3a₀, c=2c₀, where a₀∼3.2Å and c₀∼5.1Å are those of the metal lattice. The oxygen sublattices are described in terms of the stacking sequence of interstick layer parallel to the close-pack planes of the metal atoms, which varies continuously from (Remark: Graphics omitted.) for HfO_1⁄6− to (Remark: Graphics omitted.) for HfO_1⁄6+. The concentration dependence of the ordered structures is interpreted in terms of strain ordering.