Stacking Characteristics of Close Packed Materials

TL;DR

This paper presents a unified theoretical framework for analyzing the stacking characteristics of close packed materials, enabling prediction of stable structures and phase transitions through a continuous parameter space derived from density functional theory.

Contribution

It introduces a linear expansion model for stacking configurations, allowing the representation of infinite stacking sequences within a finite, continuous space of parameters, and applies it to various materials.

Findings

Identified 11 stable structures in Lennard-Jones models.

Discovered over 50 phase transitions as a function of cutoff range.

Explained differences in phase diagrams of various elements.

Abstract

It is shown that the enthalpy of any close packed structure for a given element can be characterised as a linear expansion in a set of continuous variables $\alpha_n$ which describe the stacking configuration. This enables us to represent the infinite, discrete set of stacking sequences within a finite, continuous space of the expansion parameters $H_n$. These $H_n$ determine the stable structure and vary continuously in the thermodynamic space of pressure, temperature or composition. The continuity of both spaces means that only transformations between stable structures adjacent in the $H_n$ space are possible, giving the model predictive and well as descriptive ability. We calculate the $H_n$ using density functional theory and interatomic potentials for a range of materials. Some striking results are found: e.g. the Lennard-Jones potential model has 11 possible stable structures and over 50 phase transitions as a function of cutoff range. The very different phase diagrams of Sc, Tl, Y and the lanthanides are understood within a single theory. We find that the widely-reported 9R-fcc transition is not allowed in equilibrium thermodynamics, and in cases where it has been reported in experiments (Li, Na), we show that DFT theory is also unable to predict it.