Excitation energy transfer to luminescence centers in MIIMoO4 (MII=Ca, Sr, Zn, Pb) and Li2MoO4

Based on the results of spectroscopy studies and electronic band structure calculations, the analysis of excitation energy transformation into luminescence is performed for a set of molybdates MIIMoO4 (MII=Ca, Sr, Zn, Pb) and Li2MoO4. The bandgap energies were determined from comparison of experimental and calculated reflectivity spectra as 3.3 eV for PbMoO4, 4.3 eV for ZnMoO4, 4.4 eV for CaMoO4, 4.7 eV for SrMoO4, and 4.9 eV for Li2MoO4. It is shown that photoluminescence excitation spectra of these materials reveal the specific features of their conduction bands. The threshold of separated charge carriers' creation is shown to be by 1.3-1.9 eV higher than the bandgap energy in CaMoO4, SrMoO4 and ZnMoO4. The effect is explained by the peculiarities of conduction band structure, namely to the presence of gap between the subbands of the conduction band and to the low mobility of electrons in the lower sub-band of the conduction band.