Ammonothermal Synthesis of Crystalline Oxonitride Perovskites LnTaON₂ (Ln=La, Ce, Pr, Nd, Sm, Gd)

Abstract

Abstract The perovskite type oxonitridotantalates LnTaON 2 with Ln=La, Ce, Pr, Nd, Sm, and Gd were synthesized by the ammonothermal method employing custom‐built autoclaves made of nickel‐based superalloy. Metal powders were reacted with NaOH and NaN 3 as mineralizers under supercritical conditions with purified ammonia at temperature range of 870–1070 K and pressure values of 150–300 MPa. Crystal structures and the space groups were determined by using powder X‐ray diffraction and refined by the Rietveld method. The refined lattice parameters are for LaTaON 2 ( a= 5.7156(1), b= 8.0675(1), c= 5.7465(1) Å, R wp =0.0471), CeTaON 2 ( a= 5.6761(11), b= 8.0386(16), c= 5.7891(12) Å, R wp =0.183), PrTaON 2 ( a= 5.6920(1), b= 8.0197(1), c= 5.6804(1) Å, R wp =0.0349), NdTaON 2 ( a= 5.6884(1), b= 8.0037(2), c= 5.6554(1) Å, R wp =0.026), SmTaON 2 ( a= 5.6827(1), b= 7.9656(2), c= 5.6103(1) Å, R wp =0.042), GdTaON 2 ( a= 5.6160(10), b= 7.9359(12), c= 5.5962(10) Å, R wp =0.118). LaTaON 2 crystallizes in space group Imma (no. 74) and the other compounds LnTaON 2 with Ln=Pr, Nd, Sm, Gd in Pnma (no. 62). SEM measurements were performed to investigate the elemental composition and morphology of the oxonitride perovskites. The bandgap values of the oxonitrides (LaTaON 2 1.8 eV, CeTaON 2 1.7 eV; PrTaON 2 1.9 eV, NdTaON 2 2.0 eV, SmTaON 2 2.0 eV, GdTaON 2 1.8 eV) were estimated by using UV/Vis measurements and the Kubelka–Munk function.