The structure of the potentially scintillating high-pressure phase of [Beta] - MgMoO 4 ( γ - 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 P 2 / c . Here it has been found that the phase transition is isosymmetrical ( C 2 / m -> C 2 / 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] - MgMoO 4 to the ( 0.9 a , 0, 0.5 a ) direction in γ - MgMoO 4 . The lattice dynamical calculations performed here on γ - MgMoO 4 served to explain the Raman spectra observed for the high-pressure phase of [Beta] - MgMoO 4 in a previous work demonstrating that the use of internal modes arguments in which the MoO n polyhedra are considered as separate vibrational units fails at least in this molybdate. The electronic structure of γ - MgMoO 4 was also calculated and compared with the electronic structures of [Beta] - MgMoO 4 and MgWO 4 shedding some light on why MgWO 4 is a much better scintillator than any of the phases of MgMoO 4 . These calculations yielded for γ - MgMoO 4 a Y 2 Γ -> Γ indirect band gap of 3.01 eV in contrast to the direct bandgaps of [Beta] - MgMoO 4 (3.58 eV at Γ ) and MgWO 4 (3.32 eV at Z ).

The authors thank I. Collings and M. Handfland from the ID15B beamline at the ESRF for their help during the experiments, and O. Gomis from the Universitat Politècnica de València for the discussions. Most of the work presented in this work benefited from the financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) under Projects No. PID2019- 106383GB-C41/43 (MCIN/AEI/10.13039/501100011033), MALTA Consolider-Team network RED2018-102612- T (MINECO/AEI/10.13039/501100003329), and from the Generalitat Valenciana under Project PROMETEO/2018/123. V.M. also thanks the MICINN for the Beatriz Galindo distinguished researcher program (BG20/00077).