Origin of luminescence in ZnMoO4 crystals: Insights from spectroscopic studies and electronic structure calculations

A set of ZnMoO₄ polycrystalline samples was synthesized by solid state reaction method and characterized by XRD structural analysis, ICP elemental analysis, SEM-EDX analysis, UV–Vis diffuse reflectance spectroscopy. The photoluminescence (PL) properties of ZnMoO₄ samples were studied in 8–300 K temperature range with use of synchrotron radiation as an excitation source. The geometry-optimized electronic structure calculations of perfect and defective ZnMoO₄ crystals were carried out by the DFT-based band-structure methods, plane-wave pseudo-potential (CASTEP program package) and FP-LAPW (Wien2k package). A number of point defects which can influence the luminescence properties of ZnMoO₄ was considered in calculations, namely the oxygen vacancy V_o, compensated vacancies V_o + V_Zn, tungsten impurity W_Mo and the MoO₃-deficient phase Zn₃Mo₂O₉. Comparative analysis of experimental data and calculation results allowed explanation of the origin of the red and blue-green luminescence emission bands of ZnMoO₄ as well as formation of their excitation spectra. It is assumed that the green component of ZnMoO₄ luminescence emission, like in other molybdates of M^IIMoO₄ family (M^II = Ca, Sr, Cd, Pb) originates from radiative annihilation of excitons self-trapped on regular MoO₄ groups. The Red component of ZnMoO₄ emission has a defect-related origin and it originates from radiative transitions in MoO₄ groups located near oxygen vacancies.