Exciton transition and electronic structure of PbMoO₄ crystals studied by polarized light

Polarized reflectivity spectra of PbMoO4 crystals have been measured using synchrotron radiation up to 20 eV. The optical constants for the crystallographic axes are derived by using a Kramers–Kronig analysis. It is found that the exciton band at 3.6 eV shows a doublet structure with distinct dichroism. X-ray photoemission spectroscopy (XPS) and the calculation of the electronic structure by a discrete variational Xα method are also carried out. The calculation shows that the valence band and the conduction band are mainly composed of the O 2p and Mo 4d states, respectively, and the Pb state contributes appreciably to the top of the valence band and the bottom of the conduction band. The valence-band XPS spectrum of PbMoO4 is compared with that of PbWO4, which reveals a remarkable difference between them. This difference reflects different magnitudes of hybridization of Mo 4d or W 5d state to the valence band. The exciton transition is explained in terms of the cationic Pb 6s → 6p excitation model taking into account the crystal-field splitting and the spin–orbit interaction of Pb 6p state. From a comparison of the doublet structure of the exciton band of PbMoO4 and PbWO4, it is suggested that the electron–hole exchange interaction plays an important role for the exciton transitions in both materials.