Broadly Tunable Emission from CaMoO4: Bi Phosphor Based on Locally Modifying the Microenvironment Around Bi3+ Ions

The trivalent green element of bismuth, when doped into different compounds, can produce multiple emissions in ultraviolet, blue, green or even yellow spectral regimes. The emission adjustability comes from the susceptibility of bismuth naked 6s electrons to the crystal field particularly surrounding Bi3+. However, this has never been observed in a single compound. In this work, we report that broadly tunable bismuth emission indeed occurs even within the same host (CaMoO4) when the local microenvironment around Bi3+ is modified by intentional introduction of alkali ions. As the radius of the selected alkali ion decreases, the emission regularly blue-shifts from 586 to 554 nm, and its intensity is enhanced greatly. For instance, the integrated intensity for the Li+ and Bi3+ codoped sample is 4.46 times stronger than the Bi3+ single-doped sample upon excitation at 300 nm. Low-temperature photoluminescence spectra surprisingly reveal that energy transfer can partially take place from the MoO42- group to Bi3+ at lower temperature when excited into the charge transfer state of the MoO42- group, however, this does not occur at higher temperature. The mechanism needs further study. Dynamic luminescence analysis between 10 and 300 K implies a population dependence of the excited states P-3(0) and P-3(1) on temperature that is responsible for the evolution of the Bi3+ emission lifetime with temperature.