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

TL;DR

This paper extends a phenomenological model to describe how shifts in basal planes and magnetic ordering influence structural and elastic properties of solid oxygen near phase transitions, revealing non-monotonic behaviors in lattice parameters and sound velocities.

Contribution

It introduces a generalized model linking basal plane shifts and magnetic order to elastic properties during phase transitions in solid oxygen, including temperature and pressure effects.

Findings

Lattice parameters vary due to plane shifts and magnetic order near phase transitions.

Sound velocities soften before phase transitions due to lattice softening.

Non-monotonic pressure dependence of sound velocities in the alpha phase.

Abstract

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 $\alpha\beta$-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 $T_{\alpha\beta}$ from another produces non monotonic temperature dependence of lattice parameter $b$ (along monoclinic axis). Steep decrease of the sound velocities in the vicinity of $T_{\alpha\beta}$ can be explained by the softening of the lattice with respect to shift of the close-packed planes (described by the constant $K_2$) prior to phase transition point. We anticipate an analogous softening of sound velocities in the vicinity of $\alpha \delta$-phase transition and non monotonic pressure dependence of sound velocities in $\alpha$-phase.