Ammonothermal Synthesis of Novel Nitrides: Case Study on CaGaSiN3.

The first gallium-containing nitridosilicate CaGaSiN3 was synthesized in newly developed high-pressure autoclaves using supercritical ammonia as solvent and nitriding agent. The reaction was conducted in an ammonobasic environment starting from intermetallic CaGaSi with NaN3 as a mineralizer. At 770 K, intermediate compounds were obtained, which were subsequently converted to the crystalline nitride at temperatures up to 1070 K (70-150 MPa). The impact of other mineralizers (e.g., LiN3 , KN3 , and CsN3 ) on the product formation was investigated as well. The crystal structure of CaGaSiN3 was analyzed by powder X-ray diffraction and refined by the Rietveld method. The structural results were further corroborated by transmission electron microscopy, 29 Si MAS-NMR, and first-principle DFT calculations. CaGaSiN3 crystallizes in the orthorhombic space group Cmc21 (no. 36) with lattice parameters a=9.8855(11), b=5.6595(1), c=5.0810(1) Å, (Z=4, Rwp =0.0326), and is isostructural with CaAlSiN3 (CASN). Eu2+ doped samples exhibit red luminescence with an emission maximum of 620 nm and FWHM of 90 nm. Thus, CaGaSiN3 :Eu2+ also represents an interesting candidate as a red-emitting material in phosphor-converted light-emitting diodes (pc-LEDs). In addition to the already known substitution of alkaline-earth metals in (Ca,Sr)AlSiN3 :Eu2+ , inclusion of Ga is a further and promising perspective for luminescence tuning of widely used red-emitting CASN type materials.