Stacking Characteristics of Close Packed Materials.

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Materials Science

Background:

• Close-packed structures exhibit diverse stacking sequences.
• Predicting phase transitions in materials remains challenging.
• Existing models struggle to capture the full spectrum of structural variations.

Purpose of the Study:

• To develop a unified theoretical framework for describing and predicting close-packed structures.
• To represent infinite stacking sequences in a finite, continuous parameter space.
• To understand and predict phase transitions in various materials.

Main Methods:

• Characterizing enthalpy as a linear expansion in continuous stacking variables (αn).
• Defining continuous parameters (Hn) representing stable structures.
• Utilizing density functional theory (DFT) and interatomic potentials for calculations.
• Analyzing phase diagrams as a function of thermodynamic variables (pressure, temperature, composition).

Main Results:

• Developed a continuous parameter space (Hn) for infinite discrete stacking sequences.
• Demonstrated that only adjacent structures in Hn space are thermodynamically accessible.
• Identified 11 stable structures and over 50 phase transitions for the Lennard-Jones potential.
• Unified the understanding of disparate phase diagrams for elements like Sc, Tl, Y, and lanthanides.
• Showed the 9R-fcc transition is not thermodynamically allowed in equilibrium.

Conclusions:

• The Hn parameter space provides a powerful tool for describing and predicting material structures and phase transitions.
• The model offers a unified approach to understanding diverse phase behaviors across different elements.
• Experimental observations of non-equilibrium transitions, like 9R-fcc in Li and Na, are consistent with theoretical predictions of their thermodynamic impossibility.