High Pressure Investigation of Structural and Electronic Behavior of β-ZnMoO4

In this study, the wolframite phase of ZnMoO₄ was investigated under compression using x-ray diffraction (XRD), Raman spectroscopy and ab-initio calculations. The enthalpy calculations show that β-ZnMoO₄ is a metastable phase at ambient pressure and becomes the most stable phase above 2 GPa. New diffraction peaks were observed in XRD patterns ~ 40 GPa, which indicate that it has undergone a phase transition to a lower symmetry phase. These diffraction peaks could be assigned to the (1 1 0), (2 0 -2), (2 2 0) peak of the base-centered monoclinic phase, Cm, determined by ab-initio crystal structure search. We observed a co-existence of the ambient and high pressure phases in the XRD data above 40 GPa, which clearly indicates the first order nature of the phase transition. The Raman spectroscopic studies showed an abrupt change in intensities and red shift of the highest intensity peak ~ 39 GPa, thus corroborating the x-ray diffraction and ab-initio results. This shift is correlated with a coordination increase from 6 to 9 in Mo⁺⁶ ions associated with this phase transition to Cm phase. This study also shows that β-ZnMoO₄ is an insulator with a direct band gap of 2.9 eV at ambient pressure as determined from UV visible absorption spectroscopy making it an ideal material for photocatalytic applications. Our electronic calculations corroborate this finding.