Synthesis and Photoluminescence Properties of Novel BaSi4O6N2:Eu2+ Phosphor for White- LEDs
Authors: Fukumura Kohei , Hattori Sayaka , Shibai Koki , Shih Shao-Ju , Murai Kei-ichiro , Moriga Toshihiro 
Published: February 2020

Presently commercial white-LEDs employ a blue LED chip and a yellow light-emitting phosphor, and are very poor in the color rendering index (CRI) [1]. Therefore, novel blue, green and red phosphors showing broad emission spectra have studied recently. BaSi4O6N2 has been recently reported to have a structure similar to Ba3Si6O12N2 green phosphor which shows not sharp but not broad emission [2-3]. To the best of our knowledge, there is no report that succeeded in synthesizing pure BaSi4O6N2 crystal phase, and no report that the materials related to BaSi4O6N2 shows fluorescence in visible region.
In this study, we investigated optimum sintering conditions and composition to obtain pure BaSi4O6N2 crystal phase by solid state reaction technique. We also examined the optical property of Eu2+ activated BaSi4O6N2 as well as the crystal structure through X-ray Rietveld refinement.
Although we optimized sintering temperature and duration (1100 °C and 2 h) to obtain exclusively BaSi4O6N2 crystal phase, impurity phases such as Ba3Si6O12N2 still remained. Therefore, we tried to suppress the formation of Ba3Si6O12N2 phase by adding Si3N4 raw material in excess of stoichiometric ratio of BaSi4O6N2 (Si/Ba = 4-6). As shown in Fig. 1, by increasing the Si/Ba ratio to be more than 4, the peaks which derived from Ba3Si6O12N2 and oxides (SiO2, Ba2Si4O10) decreased in intensity. No other impurity phases were observed in the XRD pattern of the sample with Si/Ba = 6. However, SEM-EDS analysis supported that the monophasic sample possessed the stoichiometric composition as BaSi4O6N2.
The lattice parameters of BaSi4O6N2 which crystallized in a hexagonal unit cell were a = 5.2573 (16) and c = 7.7295 (59)Å in the sample with Si/Ba = 6. Compared to the reported lattice parameters a = 5.351 and, c = 7.523Å of BaSi4O6N2 (JCPDS 82-4088), the a length was longer and the c length was shorter. As shown in Fig. 2, X-Ray Rietveld analysis of our BaSi4O6N2 demonstrated that the difference in the lattice parameters would come from the different occupation probability of two distinct Ba sites in the structure [4]. One is called the Ba1 site whose coordination number by anion is 12 and the other is 6-fold Ba2 site. The previous paper reported that Ba species were distributed to these sites with occupation probabilities of Ba1:Ba2 = 87:13 [3]. However, our oxynitride possessed neither Ba nor Eu at the Ba1 site.
As shown in Fig. 3, broad spectra with the emission peak of around 405 nm were observed under the excited light of 320 nm. This emission is ascribed to the 4f65d1→4f7 transition of Eu2+. In addition, the emission spectrum of BaSi4O6N2:Eu2+ possesses FWHM about 185 nm. It was obviously broader band compared with that of Ba3Si6O12N2:Eu2+ emission with FWHM about 70 nm. It indicates that this phosphor is fascinating for high CRI white-LEDs.
References
[1] Zhiguo Xia, Weiwei Wu, J. RSC, 2013, 42, 12989-12997
[2] Chen Guantong, Zhuang Weidong et al., J. Rare Earth, 2013, 31, 113-118
[3] Cordula Braun, Helmut Ehrenberg et al., Eur. J. Inorg. Chem., 2012, 3923-3928
[4] F. Izumi and K. Momma, Solid State Phenom., 2007, 130, 15-20

Figure 1