Professor Hiromi Nakano from Toyohashi University of Technology used a material with a unique periodic structure (smart material: Li-M-Ti-O [M = Nb or Ta]) as a host material to synthesize novel Mn4 + activated phosphors which exhibit red light emissions at 685 nm when excited at 493 nm. Since the valence of Mn ions in the material changes from Mn4 + to Mn3 + depending on the sintering temperature, composition and crystal structure, there is a difference in the photoluminescence intensity of the phosphors. XRD, TEM, and XANES were used to clarify the relationship between the intensity of photoluminescence and sintering temperature, composition, crystal structure, and MgO co-doping.
The white color of white LEDs is usually achieved by exciting a yellow phosphor with blue light. However, the color rendering index with this method is rated as low because red light is insufficient compared to sunlight. Therefore, phosphors which emit red light have an important role as high color rendering index materials.
Previously, Professor Nakano’s team used a smart material (Li-M-Ti-O [M = Nb or Ta]) as a host material to synthesize a red phosphorus activated by Eu3 +. This time, they synthesized new red phosphors activated by Mn4 + without using rare earths.
Li-Nb-Ti-O (LNT) system and Li-Ta-Ti-O (LTT) system are both smart materials (see figure for example) that self-organize into a periodic structure with a intergrowth layer period which changes depending on the amount of TiO2 doping. The periodic structure area of the LTT system is narrower than that of the LNT system, and there is a difference in the sintering conditions for its creation. Therefore, while comparing the LNT and LTT systems, the team closely investigated how the intensity of photoluminescence and valence of Mn ions change with sintering temperature, composition, crystal structure, and MgO codoping.
As a result of this research, it was understood that LTT had a significantly higher photoluminescence intensity than LNT due to changes in crystal structure due to temperature and sinter composition. Generally, if the sintering temperature is high, Mn4 + will probably reduce to Mn3 +, explaining the decrease in photoluminescence intensity. Regarding the changes in crystal structure, when the amount of TiO2 doping is increased, the number of [Ti2O3]2+ periodic intergrowth layers also increase. Since the intergrowth layer is formed from Ti3 + ions, it has been understood that the surrounding oxygen deficiencies contribute to the reductions from Mn4 + to Mn3 +. In addition, when the MgO doping was performed to increase the intensity of photoluminescence, the LTT phosphor which had no periodic structure exhibited a Mn4 + ratio of 100% and the highest photoluminescence intensity.
The student who was originally involved in the experiment stated that “phosphorus Mn4 + did not show photoluminescence with the host material”, and the research was put on hold for about six months. The next year another student synthesized phosphorus and said, “It has poor photoluminescence, but I think there are some things we could try to improve it. Through repeated trial and error, the team discovered an important factor: in addition to the sintering temperature, there were significant differences in the changes in crystal structure when the Mn4 + ratio was controlled. Thanks to numerous trips to the Aichi Synchrotron Center, the team was able to measure the Mn4 + ratio and consolidate the results of its research.
Future prospects :
Phosphorus activated by Mn4 + had to be synthesized at a relatively low temperature of 850 ° C in order to increase the Mn4 + ratio. However, in this condition there is a problem with moderately low crystallinity. In the future, they will try various co-dopants to further explore the synthesis process to achieve brighter red phosphorus. In recent years, there has been more interest in dark red Mn phosphors activated without the use of rare earth materials, such as for use in LED grow lights, and one can expect this. as applications develop in the future.
Hiromi Nakano, Shota Ando (2021) “Relationship between the intensity of photoluminescence, oxidation of Mn ions and the crystal structure of new phosphors Li-M-Ti-O: Mn4 + (M = Nb or Ta)” Materials Research Bulletin flight. 143, pages 111445 (1-7). https: /
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