Abstract:

Multi-photoemissions from multi-luminescent centers that correspond to multi available sites in a given crystal are frequently noticed in previous works. Depending on the exact type of dopant built into, compounds having multi available sites should correspondingly offer multi luminescent centers, but some kinds of such compounds sometimes exhibit only one luminescent center, exhibiting the “controversial” centers. According to previous works, the akermanite(Ca₂MgSi₂O₇), which has been known as the classical afterglow host for a long term, belongs to this type of compounds having the “controversial” Ca sites. Here we report on utilizing the typical luminescent behavior of the non-rare-earth(non-RE) ion(Bi³⁺) to explore the issues of “controversial” Ca sites in Ca₂MgSi₂O₇ crystal. Our results show that the excitation bands centering at 250 nm and 276 nm can pump all samples of Ca₂MgSi₂O₇:Bi³⁺ to exhibit dual Bi³⁺-related photoemissions centering at 582 nm and ~350 nm, respectively. In combination with the crystal structure analysis, the spectral results indicate that there are dual Bi³⁺ luminescent centers[denoted as Bi³⁺(Ⅰ) and Bi³⁺(Ⅱ)] at two different Ca²⁺ sites coordinating with six- and eight- oxygen atoms in the Ca₂MgSi₂O₇ host. Surprisingly, the spectral results support the existence of the energy transfer from Bi³⁺(Ⅰ) to Bi³⁺(Ⅱ) and the process is found to be unidirectional. Further analysis indicated that the relative emission intensity of 582 nm and ~350 nm derived from Bi³⁺(Ⅰ) and Bi³⁺(Ⅱ) centers is tightly relied on the incident excitation wavelength and the Bi³⁺ doping contents. Our results clearly prove that there are two available Ca sites instead of only one Ca site in Ca₂MgSi₂O₇ crystal. Because of the reason, this work provides a new insight into verifying the available sites in some unknown or/and well-known crystals from another point of view.

Key words: bismuth(Ⅲ), akermanite, phosphor, energy transfer, controversial site