Reverse Monte Carlo modelling of amorphous Si₃B₃N₇using scattering and¹⁵N NMR data

Abstract

We present an enhanced reverse Monte Carlo approach that includes fitting to NMR data in the form of chemical shifts in addition to the usually used scattering data. Furthermore, the internal energy is accounted for in the cost function to prevent unphysical structures. This approach was applied to generate structural models of amorphous Si 3 B 3 N 7 , which is a prototype of a new class of high performance ceramics exhibiting interesting features like high thermal and mechanical stability.We fitted our structural models in direct space to radial distribution functions from x-ray, neutron and electron scattering experiments, and to the 15 N NMR spectrum of the ceramic. This spectrum could not be interpreted before since it exhibits a broad structureless signal which is a superposition of peaks related to different chemical environments NB x Si 3−x (x = 0-3) whose chemical shifts were only partly known experimentally. Therefore we based the calculation of NMR data in the reverse Monte Carlo optimizations on previous theoretical work that was done in our group. All generated models reproduce the experimental radial distribution functions very well. This good agreement does not deteriorate when the NMR data are taken into account. Fitting the models to 15 N NMR chemical shifts in addition to scattering data results in structural changes that not only improve the agreement with the experimental magic-angle spinning (MAS) NMR spectrum but yield also significantly better second-nearest neighbour coordination statistics. M Supplementary data are available from stacks.

Computational study of impact of composition, density, and temperature on thermal conductivity of amorphous silicon boron nitride

“…A typical experimental density of polymer‐derived a‐Si 3 B 3 N 7 is 1.9 g cm −3 . In previous simulations of a‐SiBN a density of 2.5 g cm −3 was adopted . While this is lower than a weighted average of β‐Si 3 N 4 (ρ = 3.20 g cm −3 ) and h‐BN (ρ = 2.34 g cm −3 ), it is still larger than the experimental density .…”

Synthesis and processing of silicon nitride and related materials using preceramic polymer and non-oxide sol-gel approaches

“…TEM shows no variations in elemental composition the amorphous phase, suggesting it to be homogeneous at the nm level, and PDF studies observe only Si-N, B-N and Si-Si distances [149]. Neutron PDF and MAS-NMR studies confirmed the structure to be random even at the shortest length scales [150,[242][243][244], and this homogeneity even at the ~1 nm length scale is likely responsible for the high temperature behaviour of the ceramic. The distribution has been explained in terms of the probability of the various reactions within the synthesis process [245].…”

“…Modelling has shown that ordered regions of ~15 nm are necessary for the crystalline phases to become more stable than the amorphous [246]. Nonetheless some heterogeneity is suggested to occur at very short length scales (<1 nm) in the form of nanovoids and also of B-rich and Si-rich regions [244,247].…”