A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8″, which meant it possessed high crystalline quality. The OH− absorption spectra shifted to 3534.7 cm−1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 × 10−5/cm, which exhibited high optical homogeneity.

We mainly investigated the effect of the valence state of photorefractive resistant elements on the photorefractive properties of codoped crystals, taking the Zn and Mo codoped LiNbO3 (LN:Mo,Zn) crystal as an example. Especially, the response time and photorefractive sensitivity of 7.2 mol% Zn and 0.5 mol% Mo codoped with LiNbO3 (LN:Mo,Zn7.2) crystal are 0.65 s and 4.35 cm/J at 442 nm, respectively. The photorefractive properties of the LN:Mo,Zn crystal are similar to the Mg and Mo codoped LiNbO3 crystal, which are better than the Zr and Mo codoped LiNbO3 crystal. The results show that the valence state of photorefractive resistant ions is an important factor for the photorefractive properties of codoped crystals and that the LN:Mo,Zn7.2 crystal is another potential material with fast response to holographic storage.

White light emitting diodes (WLEDs) are widely used due to their advantages of high efficiency, low electricity consumption, long service life, quick response time, environmental protection, and so on. The addition of red phosphor is beneficial to further improve the quality of WLEDs. The search for novel red phosphors has focused mainly on Eu2+ ion- and Mn4+ ion-doped compounds. Both of them have emissions in the red region, absorption in blue region, and similar quantum yields. Eu2+-doped phosphors possess a rather broad-band emission with a tail in the deep red spectral range, where the sensitivity of the human eye is significantly reduced, resulting in a decrease in luminous efficacy of WLEDs. Mn4+ ions provide a narrow emission band ~670 nm in oxide hosts, which is still almost unrecognizable to the human eye. Mn4+-doped fluoride phosphors have become one of the research hotspots in recent years due to their excellent fluorescent properties, thermal stability, and low cost. They possess broad absorption in the blue region, and a series of narrow red emission bands at around 630 nm, which are suitable to serve as red emitting components of WLEDs. However, the problem of easy hydrolysis in humid environments limits their application. Recent studies have shown that constructing a core–shell structure can effectively improve the water resistance of Mn4+-doped fluorides. This paper outlines the research progress of Mn4+-doped fluoride A2MF6 (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge or Sn), which has been based on the core–shell structure in recent years. From the viewpoint of the core–shell structure, this paper mainly emphasizes the shell layer classification, synthesis methods, luminescent mechanism, the effect on luminescent properties, and water resistance, and it also gives some applications in terms of WLEDs. Moreover, it proposes challenges and developments in the future.

A series of uranium-doped lithium niobate crystals (LN: U) were successfully grown by the modified vertical Bridgman method and their photorefractive properties in the visible band were systematically investigated for...

Φ1 inch Dy 3+ and Ge 4+ co-doped bismuth silicate (Bi 4 Si 3 O 12 , BSO) single crystals with the length of 80-100 mm were successfully grown by Bridgman method. They are transparent, free of cracks and inclusions. The white residual at the top parts of BSO crystals disappears with co-doping 1 mol% Dy 3+ and more than 3 mol% Ge 4+ . The FWHM values of X-ray rocking curves shows 1%Dy,3%Ge:BSO crystal possesses high crystallization quality. The intrinsic emission peak of BSO and the characteristic emission peaks of Dy 3+ ions are weakened with increasing the doping concentration of Ge 4+ . 1 mol% Dy 3+ and 3 mol% Ge 4+ are the optimal concentrations due to high crystallization quality and moderate emission intensity. The CIE coordinates and CCT values shift towards warmer white light region with increased Ge 4+ co-doping. The CCT values are close to the ideal value of 3000 K for warm white light when 1%Dy,3%Ge:BSO crystal is excited by various UV light. Increasing the temperature from 298 K to 573 K leads the luminescence lifetime to decrease from 659 µs to 645 µs. More than 95% and 80% photoluminescence intensity at room temperature is still retained at 423 K and 573 K respectively. Dy,Ge:BSO crystals are potential candidates for fabricating high power warm WLEDs.