1 research outputs found
Stacking Characteristics of Close Packed Materials
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 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 . These 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 space are
possible, giving the model predictive and well as descriptive ability. We
calculate the 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