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

Abstract

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 ).