Microscopic optical and photoelectron measurements of MWO4 (M=Mn, Fe, and Ni)

Microscopic optical (absorption and reflection) and ultraviolet photoelectron spectroscopy (UPS) measurements were performed on single microcrystals of transition-metal tungstates, MWO4 (M=Mn, Fe, and Ni) at room temperature using Schwarzschild objectives and laboratory light sources. The diameters of the spots were 40 μm (optical) and 13 μm (UPS). From the reflectance spectra, the absorption coefficient spectra were obtained through Kramers-Kronig analyses. The weak structures of absorption spectra attributed to the d-d transitions in transition metals suggest that the M3d states contribute to the upper part of the valence band. The UPS spectra suggest that the O2p and M3d states hybridize and spread wide in the valence band. The bottom of the conduction band is attributed to the empty M3d state in NiWO4, but the empty M4s states in FeWO4 and MnWO4. The contribution of the W5d state in the conduction band is located in the higher energy side.