Negative linear compressibility and phonon softening in Mg(IO3)2 under pressure

We investigate pressure-induced changes in the structure, lattice dynamics, mechanical properties, and bonding of the strongly anisotropic, monoclinic P21 phase of Mg(IO3)2 via the density functional theory with a global hybrid density functional approximation. Infrared and Raman spectra are simulated, which highlight a peculiar pressure-induced phonon softening of certain bands assigned to I-O stretching modes. As pressure increases, a redistribution of the electron density around the iodine atoms is observed leading to the elongation and weakening of the three I-O interactions within [IO3] structural units, with a corresponding pressure-induced expansion of the a and c lattice vectors, which translates into a negative linear compressibility of the material in the ac plane. This is accompanied by the shortening and strengthening of the interaction of iodine atoms with their three second-neighboring oxygen atoms, with a corresponding compression of the b lattice vector along which they are mainly oriented. Quasiharmonic lattice dynamical calculations reveal a negative thermal expansion in the ac plane. The strength of various interatomic interactions, as well as their evolution with pressure, is assessed in terms of local mode adiabatic force constants obtained through a new periodic implementation of the local vibrational mode theory. We complement and corroborate the analysis with a topological description of the electron density through the quantum theory of atoms in molecules.