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_ and HfO_ below 430℃. The lattice constants of the hexagonal supercells are respectively a=√, c=3c_0 and a=√, c=2c_0, where a_0～3.2A and c_0～5.1A are those of the metal lattice. The oxygen sublattices are described in terms of the stacking sequence of interstice layers parallel to the close-packed planes of the metal atoms, which varies continuously from A□B□C□…for HfO_ to A□B□…for HfO_. The concentration dependence of the ordered structures is interpreted in terms of strain ordering
