Superstructure and Order-Disorder Transformation of Interstitial Oxygen in Hafnium

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 \(a{=}√3a₀\), c =3 c 0 and \(a{=}√3a₀\), c =2 c 0 , where a 0 ∼3.2Å and c 0 ∼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 A\(\)B\(\)C\(\)... for HfO 1/6- to A\(\)B\(\)... for HfO 1/6+ . The concentration dependence of the ordered structures is interpreted in terms of strain ordering.