Ammonothermal synthesis of crystalline oxonitride perovskites LnTaON₂ (Ln = La, Ce, Pr, Nd, Sm, Gd)

The perovskite type oxonitridotantalates LnTaON₂ 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 NaN₃ 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 LaTaON₂ (a=5.7156(1), b=8.0675(1), c=5.7465(1) Aa, R_wp=0.0471), CeTaON₂ (a=5.6761(11), b=8.0386(16), c=5.7891(12) Aa, R_wp=0.183), PrTaON₂ (a=5.6920(1), b=8.0197(1), c=5.6804(1) Aa, R_wp=0.0349), NdTaON₂ (a=5.6884(1), b=8.0037(2), c=5.6554(1) Aa, R_wp=0.026), SmTaON₂ (a=5.6827(1), b=7.9656(2), c=5.6103(1) Aa, R_wp=0.042), GdTaON₂ (a=5.6160(10), b=7.9359(12), c=5.5962(10) Aa, R_wp=0.118). LaTaON₂ crystallizes in space group Imma (no. 74) and the other compounds LnTaON₂ 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 (LaTaON₂ 1.8 eV, CeTaON₂ 1.7 eV; PrTaON₂ 1.9 eV, NdTaON₂ 2.0 eV, SmTaON₂ 2.0 eV, GdTaON₂ 1.8 eV) were estimated by using UV/Vis measurements and the Kubelka-Munk function. (copyright 2017 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim)