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, R_wp =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.