Silicon Boron Nitrides: Hypothetical Polymorphs of Si ₃ B ₃ N ₇

Abstract

A simple construction principle is provided for the generation of Si-B-N structures. With this protocol, for example, starting from α-Si N (1) a nitrogen atom is removed (→2), three silicon atoms are replaced by boron (→3), and the structure is relaxed to give 4 (Si: large empty circles, N: small empty circles, B: small filled circles). For Si B N over 100 structures were derived in this way, whose geometries and energies were determined by density functional calculations. Two likely candidates for crystalline Si B N were found.

Reverse Monte Carlo modelling of amorphous Si3B3N7using scattering and15N NMR data

“…On such small length scales the density is ill defined, resulting in a strong dependence of the calculated local densities on the radius of the volume element used for density measurement. Probing the local densities using spherical volume elements with radii between 3 and 4 Å, we obtained mean densities of the occupied regions of about 2.9-3.1 g cm −3 (calculated from the mean values of the local density in regions with increased local density) which is comparable to the densities of the hypothetical crystalline structures of Kroll and Hoffmann [33] of 2.8-2.9 g cm −3 . This result supports the hypothesis of Hannemann et al that the surprisingly low experimental density of a-Si 3 B 3 N 7 can be explained by the presence of small voids generated during the synthesis which are surrounded by regions with higher density [18,7].…”

“…Chemical shifts in β-Si 3 N 4 were calculated as −298 and −315 ppm. Because the chemical shifts in mixed environments NB 2 Si and NBSi 2 in the solid state are not known experimentally, we had performed calculations using clusters of hypothetical crystalline Si 3 B 3 N 7 polymorphs suggested by Kroll and Hoffmann [33] and had found values between −272 and −290 ppm for NB 2 Si and −293 and −302 ppm for NBSi 2 environments (referring to the central atoms of the clusters) [27]. Contrary to previous assumptions, the chemical shift of the threefold coordinated N thus does not steadily increase with the number of Si neighbours.…”