(a) Orthorhombic structure with stacking. ions occupy octahedral and interstices within the lattice of ions. (b) Monoclinic structure with stacking showing octahedra coordination. Molybdenum atoms of the b layer within a mirror plane compared with . (c) The new high-pressure phase with monoclinic structure. Purple circles represent the Mo atoms and red circles the O atoms.
Raman spectra recorded the nonreversible structural transition of at different pressures in the frequency regions. The assignment of the experimentally observed Raman peaks is based on the theoretical analysis. Symbols ▼ and show the appearance of new Bragg peaks corresponding to the and phases, respectively. The top panel is a Raman spectrum at 0.2 GPa after decompressing from 29.5 GPa.
Pressure dependence of phonon frequencies of during compression from 0 to 29.5 GPa. The three regions correspond to different phases. Different modes are labeled in the figure.
XRD patterns collected at various pressures for with incident wavelength . ◆: Au peak as an internal pressure marker in the diagram. Symbols ▼ and show the appearance of new Bragg peaks corresponding to and phases, respectively. The top panel is the XRD spectrum at 0.5 GPa after decompressing from 43 GPa.
Rietveld full-profile refinements of the diffraction patterns collected on compression at (a) 15.7 and (b) at 28.3 GPa. In the high-pressure phase, the fit is good for the diffraction pattern shown, with an . In the high-pressure phase, fit is good for the diffraction pattern shown, with an . Red, blue, and black solid lines represent experimental, calculated, and residual patterns, respectively.
Variation in lattice parameters of phases as a function of pressure. The solid blue symbols are obtained from , the solid green symbols are obtained from , and the solid red symbols are obtained from . The inset reports the behavior of the -angle of the and phases, respectively.
The molar volume as a function of pressure for phases at pressures up to 40 GPa. Solid blue symbols represent the phase data. The solid red symbols represent the phase data. The solid green symbols represent the phase data. Solid black lines are third-order Birch–Murnaghan equation of state fits to , , and phase data. Around 25 GPa is an estimated volume collapse of 10% from the to the phase.
Cell parameters and refined fractional coordinates for the phase of at 28.3 GPa.
Selected Mo–O distances and O–Mo–O and Mo–O–Mo angles in at ambient pressure and 15.7 and 28.3 GPa, according to three different phases.
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