Solving Static and Dynamic Disorder in Cu₄TiTe₄: Crystal Structure and Thermodynamic Properties.

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Cu4TiTe4 exhibits positional disorder due to mobile copper atoms, hindering accurate crystalline model development.
• Understanding this disorder is key to capturing the material's notable thermodynamic and optical properties.

Purpose of the Study:

• To develop reliable crystalline models for Cu4TiTe4 that account for atomic disorder.
• To investigate the influence of copper atom positional disorder on the material's properties.

Main Methods:

• Utilized Density Functional Theory (DFT) coupled with quasi-harmonic approximations.
• Employed supercells to represent various copper atomic environments and identified nonequivalent structural configurations.
• Incorporated Boltzmann weights based on total energies to derive average properties.

Main Results:

• Calculations using 2x2x1 supercells revealed 16 nonequivalent configurations, highlighting the inadequacy of single-configuration models.
• Identified low energy barriers (<0.5 eV) for copper atom diffusion, indicating significant dynamic disorder.
• Observed negative thermal expansion in Cu4TiTe4 at low temperatures when dynamic disorder is considered.

Conclusions:

• Accurate modeling of Cu4TiTe4 requires incorporating dynamic atomic disorder, not just static configurations.
• The dynamic nature of copper atoms significantly impacts the material's low-temperature thermal expansion behavior.