**Stacking Characteristics of Close Packed Materials**

CH Loach and GJ Ackland, PHYSICAL REVIEW LETTERS, 119, 205701 (2017).

DOI: 10.1103/PhysRevLett.119.205701

It is shown that the enthalpy of any close packed structure for a given element can be characterized 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 as well as descriptive ability. We calculate the H-n using density functional theory (DFT) 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.

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