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 beta-MgMoO4 (gamma-MgMoO4) 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 -> 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 beta-MgMoO4 to the (0.9a, 0, 0.5a) direction in gamma-MgMoO4. The lattice dynamical calculations performed here on gamma-MgMoO4 served to explain the Raman spectra observed for the high-pressure phase of beta-MgMoO4 in a previous work demonstrating that the use of internal modes arguments in which the MoOn polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of gamma-MgMoO4 was also calculated and compared with the electronic structures of beta-MgMoO4 and MgWO4 shedding some light on why MgWO4 is a much better scintillator than any of the phases of MgMoO4. These calculations yielded for gamma-MgMoO4 a Y2 gamma -> gamma indirect band gap of 3.01 eV in contrast to the direct bandgaps of beta-MgMoO4 (3.58 eV at I') and MgWO4 (3.32 eV at Z).