High-temperature synthesis, single-crystal X-ray and neutron powder diffraction, and materials properties of Sr3Ln10Si18Al12O18N36 (Ln = Ce, Pr, Nd)

Abstract

The novel oxonitridoaluminosilicates (sialons) Sr3Ln10Si18Al12O18N36 (Ln = Ce, Pr, Nd) were obtained by the reaction of the respective lanthanide metals with Si(NH)2, SrCO3, and AlN using a radiofrequency furnace at temperatures between 1550–1650°C. The crystal structures of the isotypic sialons were determined by single-crystal X-ray investigations (Sr3Ce10Si18Al12O18N36: I3m, Z = 2, a = 1338.2(2) pm, R1 = 0.0333; Sr3Pr10Si18Al12O18N36: a = 1334.54(6) pm, R1 = 0.0296; Sr3Nd10Si18Al12O18N36: a = 1332.85(6) pm, R1 = 0.0271) and in the case of Sr3Pr10Si18Al12O18N36 with powder neutron diffraction as well. The three-dimensional sialon network is built up by SiON3, SiN4, and AlON3 tetrahedra. Besides the bridging O and N atoms of the sialon network there are isolated O2− which are tetrahedrally coordinated by Sr and Ln. The crystallographic differentiation of Si/Al and O/N seemed to be possible by a careful evaluation of the single-crystal X-ray diffraction data combined with lattice energy calculations using the MAPLE concept (Madelung Part of Lattice Energy). In the case of Sr3Pr10Si18Al12O18N36 the differentiation of O and N and the proposed ordering was completely confirmed by powder neutron diffraction.