Ammonothermal Synthesis and Crystal Structure of the Ternary Amide Na₂Ba(NH₂)₄

Abstract

The ternary amide Na2Ba(NH2)4 was synthesized at ammonothermal conditions (870 K, 135 MPa) in custom‐built high‐pressure autoclaves. The compound was structurally characterized using X‐ray diffraction and crystallizes in space group Pccn (no. 56) with lattice parameters a = 10.6492(2), b = 7.8064(2) and c = 8.1046(2) Å. To the best of our knowledge, the structure type has not yet been observed in any ternary amide before and can be described as a defective variant of the NaCl structure type. The presence of amide ions in the compound is verified by Fourier‐transform infrared (FTIR) spectroscopy and the experimental spectrum is compared to the theoretical spectrum obtained through density functional theory (DFT) calculations. Na2Ba(NH2)4 complements the range of reported ternary alkali metal alkaline‐earth metal amides with the smallest Shannon radius ratio of rA/AE = 0.76. The influence of this ratio on the formation of the new structure type is discussed as well. The characterization of intermediate species such as this ternary amide extends the understanding of the ammonothermal synthesis and can be useful for synthetic control in the formation of nitrides at ammonothermal conditions.