Cathodoluminescence of Mo-bearing scheelite

Scheelite generally shows a remarkable blue luminescence in ultraviolet light, whereas powellite shows intense luminescence in a yellow region. The scheelite luminescence rapidly changes from blue to yellow light emission when it is replaced by CaMoO₄ of several mol% (e.g., Tyson et al, 1988). Still now, the luminescence mechanism in this process has been under discussion. In this study, we have investigated the cathodeluminescence (CL) of the scheelite with various powellite composition to clarify luminescence centers and CL emission mechanism.

Color CL images at room temperature shows a blue emission in most samples and a yellow emission in the samples including Mo. CL spectral analysis of the CL emissions was performed after a total sensitivity correction of digital data obtained using an SEM-CL by a Gaussian peak-fitting method. Blue CL is attributed to the emission caused by the charge transfer transition between the 2p orbit of four oxygen ions O^2- arranged in the tetrahedron and the central metal W, which is the WO₄^2- ion. In a few samples, a broad band emission in a yellow region was detected at around 500 nm with not so intense intensity. The scheelite with yellow emission has a certain amount of Mo estimated by EPMA. According to fluorescence emission analyses (e.g., Tyson et al, 1988), when WO₄^2- is replaced with MoO₄^2- up to around 10 mol%, the blue emission of the WO₄^2- changes to yellow emission assigned to MoO₄^2- center unaccompanied by WO₄^2- center. The reason for this is believed that thermal quenching of WO₄^2- excitation would be more efficient than that of MoO₄^2- excitation, but not proved by the measurements. In this study, same behavior has been recognized in the CL of the scheelite with 10 mol% powellite. The CL spectra of the scheelite were obtained at various temperatures here. The results of the spectral analysis based on the Mott-Seitz mechanism reveals that the end-membered scheelite shows almost no thermal quenching, and the scheelite with 10 mol% of powellite shows a significant temperature quenching above -70 °C, suggesting no evidence the energy transfer between WO₄^2- center and MoO₄^2- center. The facts are inconsistent with the conventional explanations. It should be reconsidered to clarify the emission mechanism in the series between scheelite and powellite famous for a classic phosphor.