Synthesis of a multinary nitride, Eu-doped CaAlSiN3, from alloy at low temperatures

A multinary nitride, Eu-doped CaAlSiN3, which had previously been synthesized at 1600-1800 degrees C as a red-emiting phosphor material, was obtained at 500-800 degrees C via the reaction of a CaAlSi alloy with a low concentration of Eu (composition Ca0.992Eu0.008AlSi) in ammonia in the present study. In supercritical ammonia (100 Wa), CaAlSiN3 was formed at temperatures >= 500 degrees C both with and without the addition of sodium amide. The addition of sodium amide significantly facilitated the synthesis and prevented the presence of unreacted silicon, which could be due to the formation of sodium ammonometallates as intermediates in the presence of sodium amide. CaAlSiN3 was synthesized even in an atmospheric ammonia flow, but the crystallinity of the product was rather low. Evidence suggested that CaAlSiN3 was insoluble in the pressurized sodium amide-ammonia medium under the present conditions, and the prolonged reaction of the alloy in ammonia at the CaAlSiN3-forming temperatures (500-800 degrees C did not result in an effective improvement of the crystallization. In contrast, well-crystallized samples with plate- and bar-like nanocrystals were synthesized by first converting the alloy at 300-400 degrees C into sodium ammonometallates and subsequently decomposing the ammonometa, Ilates up to 800 degrees C into CaAlSiN3. The products showed a red emission centered at 630-644 nm at a blue excitation of 460 nm. Instead of using sodium amide, the use of sodium azide, which was converted into sodium amide during heating, led to a product of plate-like crystals with significantly reduced oxide (oxide-free in the XRD spectrum) but did not result in an improved light emission. The size and lattice strain were calculated by refining against the XRD patterns, and the elemental composition was obtained via energy dispersive X-ray analysis on single nanocrystals with TEM observations. The synthetic conditions-structure and composition-light emission relationships were discussed. The deficiency in calcium was the determining factor for the decreased light emission.