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Cordes, Niklas; Schnick, Wolfgang (2017): Ammonothermal Synthesis of Crystalline Oxonitride Perovskites LnTaON(2) (Ln = La, Ce, Pr, Nd, Sm, Gd). In: Chemistry-A European Journal, Vol. 23, No. 47: pp. 11410-11415
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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 NaN3 as mineralizers under supercritical conditions with purified ammonia at temperature range of 870- 1070K 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) angstrom, R-wp = 0.0471), CeTaON2 (a= 5.6761(11), b= 8.0386(16), c= 5.7891(12) a, R-wp = 0.183), PrTaON2 (a= 5.6920(1), b= 8.0197(1), c= 5.6804(1) angstrom, Rwp= 0.0349), NdTaON2 (a= 5.6884(1), b= 8.0037(2), c= 5.6554(1) angstrom, R-wp= 0.026), SmTaON2 (a= 5.6827(1), b= 7.9656(2), c= 5.6103(1) angstrom, R-wp= 0.042), GdTaON2 (a= 5.6160(10), b= 7.9359(12), c= 5.5962(10) angstrom, R-wp= 0.118). LaTaON2 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 (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 KubelkaMunk function.