Nitridosilicates, oxonitridosilicates (sions), and oxonitridoaluminosilicates (sialons) - New materials with promising properties

The new oxonitridosilicate (sion) Ce-4[Si4O4N6]O and the sialons Sr(3)Ln(10)Si(18)Al(12)O(18)N(36) (Ln=Ce, Pr, Nd) and Sr10Sm6Si30Al6O7N54 were synthesised as coarsely crystalline products by high-temperature reaction using a radiofrequency furnace. The crystal structures were determined by single-crystal X-ray diffraction and powder neutron-diffraction (Ce-4[Si4O4N6]O and Sr3Pr10Si18Al12O18N36). Ce-4[Si4O4N6]O represents the first hyperbolically layered silicate. The Si/O/N network is built up of Q(3) type SiON3 tetrahedra enveloping the large [Ce4O](10+) cations. The sialons Sr(3)Ln(10)Si(18)Al(12)O(18)N(36) contain a complex network of corner sharing SiON3, SiN4, and AlON3 tetrahedra, while Sr10Sm6Si30Al6O7N54 is built up by a capped double-layer structure of SiON3, SiN4, AlON3, and AIN, tetrahedra. The crystallographic ordering of Al/Si and O/N, respectively, was investigated by refinement of the neutron powder diffraction data and lattice energy calculations (MAPLE) on the basis of the single-crystal X-ray diffraction data. Hardness investigations using single crystals of Sr3Pr10Si18Al12O18N36 yielded an averaged Vickers hardness of 22.0 GPa. (C) 2001 Elsevier Science Ltd. All rights reserved.