An Unconventional Phase Transition in BaAl2O4 Driven by Two Competing Soft Modes

TL;DR

This study reveals an unconventional phase transition in BaAl2O4 driven by two competing soft modes, with experimental and first-principles evidence showing simultaneous condensation of these modes at the transition temperature.

Contribution

It uncovers a novel phase transition mechanism involving two soft modes competing, supported by synchrotron X-ray experiments and first-principles calculations.

Findings

Unconventional phase transition at 451.4 K in BaAl2O4.

Simultaneous condensation of two soft modes at Tc.

Structural instabilities at M- and K-points with similar imaginary frequencies.

Abstract

We investigated the temperature dependence of the superlattice intensity and thermal diffuse scattering intensity of BaAl2O4, which has a network structure with corner-sharing AlO4 tetrahedra, via synchrotron X-ray diffraction experiments. The temperature variation of the superlattice intensity revealed that the structural phase transition occurs at TC = 451.4 K from the P6322 parent crystal structure to the low-temperature superstructure with a cell volume of 2a x 2b x c. BaAl2O4 exhibits an unconventional structural phase transition driven by two competing soft modes, q1/2 ~ (1/2, 1/2, 0) and q1/3 ~ (1/3, 1/3, 0). When approaching the TC from above, the soft mode with q1/3 appeared first and was followed by the q1/2 soft mode. The thermal diffuse scattering intensities from both soft modes increased sharply at TC; therefore, both modes condensed simultaneously. The first principles calculation revealed that structural instabilities exist at the M- and K-points, at which the calculated imaginary frequencies are similar. The small energy difference of these structural instabilities generates the two competing soft modes and determines the eventual low-temperature crystal structure.