Light-induced phase transition in AlD₃ at high pressure

Trivalent aluminum hydride in the rhombohedral α phase (R3̄c 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 P53 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 (P2₁/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 k₂=(131313). The undistorted P2₁/c lattice itself appears as superstructure over the rhombohedral R3̄c one resulting from the displacive structure transformation with the propagation vector k₁=(12012). 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 α phase.