Shift of close-packed basal planes as an order parameter of transitions
between antiferromangetic phases in solid oxygen: II. Temperature/pressure
dependence of sound velocities and lattice parameters
In the present paper we generalised a phenomenological model developed in
\cite{gomo:2005} for the description of magnetostructural phase transitions and
related peculiarities of elastic properties in solid oxygen under high pressure
and/or low (below 40 K) temperature. We show that variation of all the lattice
parameters in the vicinity of αβ-phase transition is due to both the
shift of basal closed packed planes and appearance of the long-range magnetic
order. Competition between these two factors from one side and lattice
compression below Tαβ from another produces non monotonic
temperature dependence of lattice parameter b (along monoclinic axis). Steep
decrease of the sound velocities in the vicinity of Tαβ can be
explained by the softening of the lattice with respect to shift of the
close-packed planes (described by the constant K2) prior to phase transition
point. We anticipate an analogous softening of sound velocities in the vicinity
of αδ-phase transition and non monotonic pressure dependence of
sound velocities in α-phase.Comment: 11 pages, 8 figures, to appear in Low Temperature Physics, v.33, N6,
2007, materails presented at the Conference CC-2006, Kharkov, 200