First Principles Study of Rare Earth Adsorption at Beta-Si3N4 Interfaces

Abstract

Structural characterization of rare earth adsorption at surfaces/interfaces of ��-Si3N4 grains within silicon nitride ceramics has recently been reported by three different groups using Z-contrast STEM imaging. Here we report the electronic structure basis for these observations and discuss the origin of similarities and differences among the lanthanides characterized in that work. Along with the features that are well-described by a first-principles cluster and surface slab models, we identify those differences in the experiment and theory that warrant further investigation. Stereochemical bonding factors are found to determine adsorption site preferences as opposed to ionic size effects. The 2 set of possible bond sites is a characteristic of the ��-Si3N4 interface, however the strength of the rare earth �V interface bonding is determined by the electronic structure of the nitride surface and the specific adsorbate. This is the principal factor controlling the effects of dopants on the �� �� �� phase transformation and on the ��-Si3N4 grain growth at high temperature as well as the subsequent microstructure of the ceramic.