Light-induced phase transition in AlD 3 at high pressure

Trivalent aluminum hydride in the rhombohedral $\ensuremath{\alpha}$ phase ($R$$\overline{3}$$c$ space group) was studied at high pressures in a diamond-anvil cell by means of Raman scattering, x-ray diffraction, observation of optical transmission, and the density functional simulations. At $P\ensuremath{\approx}53$ GPa the heavier isotope AlD${}_{3}$ undergoes a first-order structural phase transition which was found to be stimulated by the laser irradiation used for the Raman-scattering measurements. In the new high-pressure phase Al atoms form a lattice with a monoclinic unit cell ($P{2}_{1}/c$ space group) over which a superstructure is developed when pressure is varied. The superstructure is formed by regular displacements of the Al atoms with the period over three unit cells; the propagation vector is ${\mathbf{k}}_{2}=(\frac{1}{3}\frac{1}{3}\frac{1}{3})$. The undistorted $P{2}_{1}/c$ lattice itself appears as superstructure over the rhombohedral $R$$\overline{3}$$c$ one resulting from the displacive structure transformation with the propagation vector ${\mathbf{k}}_{1}=(\frac{1}{2}0\frac{1}{2})$. The band gap as given by the density functional calculations and evidenced from the sample transparency behavior at high pressures remains greater than the laser photon energy used (${E}_{ph}=2.41$ eV). That indicates that bond weakening/breaking due to electron excitation across the band gap is not the cause of the phase transition. A likely mechanism of the light action is that structure transformation is driven by phonons, which are excited due to strong electron-phonon coupling in the $\ensuremath{\alpha}$ phase.