Ammonothermal Synthesis of EAM O 2 N ( EA = Sr, Ba; M = Nb, Ta) Perovskites and 14 N Solid‐State NMR Spectroscopic Investigations of AM (O,N) 3 ( A = Ca, Sr, Ba, La)

Alkaline earth oxonitride perovskites EAM O 2 N ( EA = Sr, Ba; M = Nb, Ta) were synthesized by the ammonothermal method at temperatures of 900 K and maximum pressures of up to 300 MPa in custom‐built autoclaves starting from Nb or Ta and Sr or Ba metals. The reactions were performed under ammonobasic conditions using NaN 3 and NaOH as mineralizers. Powder X‐ray diffraction and Rietveld refinement were used to determine the crystal structures. The elemental composition and morphology of the obtained products were investigated by scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. Crystals of 1–10 µm size were obtained. Optical band gap values were determined from UV/Vis measurements applying the Kubelka–Munk function with the Tauc method ( E g = 1.9 ‐ 2.1 eV). In addition to that, A Nb/Ta(O,N) 3 with A = Ca, Sr, Ba, La were synthesized by ammonolysis reactions of the respective ternary oxide precursors. The products were investigated with 14 N solid‐state (MAS) NMR spectroscopy, all showing a single resonance at about 270 ppm.