Crystal structure solution of a high-pressure polymorph of scintillating MgMoO4 and its electronic structure

The structure of the potentially scintillating high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ ($\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$) has been solved by means of high-pressure single-crystal x-ray diffraction. The phase transition occurs above 1.5 GPa and involves an increase of the Mo coordination from fourfold to sixfold accommodated by a rotation of the polyhedra and a concommitant bond stretching resulting in an enlargement of the $c$ axis. A previous high-pressure Raman study had proposed such changes with a symmetry change to space group $P2/c$. Here it has been found that the phase transition is isosymmetrical ($C2/m\phantom{\rule{4pt}{0ex}}\ensuremath{\longrightarrow}\phantom{\rule{4pt}{0ex}}C2/m$). The bulk moduli and the compressibilities of the crystal axes of both the low- and the high-pressure phase, have been obtained from equation of state fits to the pressure evolution of the unit-cell parameters which were obtained from powder x-ray diffraction up to 12 GPa. The compaction of the crystal structure at the phase transition involves a doubling of the bulk modulus ${B}_{0}$ changing from 60.3(1) to 123.7(8) GPa and a change of the most compressible crystal axis from the (0, $b$, 0) direction in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ to the ($0.9a$, 0, $0.5a$) direction in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$. The lattice dynamical calculations performed here on $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ served to explain the Raman spectra observed for the high-pressure phase of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ in a previous work demonstrating that the use of internal modes arguments in which the ${\mathrm{MoO}}_{n}$ polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ was also calculated and compared with the electronic structures of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ and ${\mathrm{MgWO}}_{4}$ shedding some light on why ${\mathrm{MgWO}}_{4}$ is a much better scintillator than any of the phases of ${\mathrm{MgMoO}}_{4}$. These calculations yielded for $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ a ${Y}_{2}\mathrm{\ensuremath{\Gamma}}\ensuremath{\longrightarrow}\mathrm{\ensuremath{\Gamma}}$ indirect band gap of 3.01 eV in contrast to the direct bandgaps of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgMoO}}_{4}$ (3.58 eV at $\mathrm{\ensuremath{\Gamma}}$) and ${\mathrm{MgWO}}_{4}$ (3.32 eV at $Z$).