First-principles study of rare earth adsorption atβ-Si3N4interfaces

Abstract

Structural characterization of rare earth adsorption at surfaces or interfaces of $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ grains within silicon nitride ceramics has recently been reported by three different groups using $Z$-contrast scanning transmission electron microscopy (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 set of possible bond sites is a characteristic of the $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ interface; however the strength of the rare earth--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 $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase transformation and on the $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}$ grain growth at high temperature as well as the subsequent microstructure of the ceramic.