Crystal Structures and Phase Stability of the Li₂S-P₂S₅ System from First Principles

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Materials Science

Background:

• The Li₂S-P₂S₅ system yields superionic conductors with high room-temperature Li-ion conductivity.
• The metastability of these phases necessitates understanding their thermodynamic accessibility.

Purpose of the Study:

• To computationally determine the thermodynamic stability of crystalline phases in the Li₂S-P₂S₅ system.
• To investigate the role of electronic, configurational, and vibrational free energy in phase stability.
• To propose new ground-state orderings for key superionic conductor compositions.

Main Methods:

• First-principles calculations of electronic, configurational, and vibrational free energies.
• Construction of a phase diagram for the crystalline Li₂S-P₂S₅ space.
• Analysis of entropy contributions (vibrational and configurational) to phase stability.

Main Results:

• A phase diagram for the Li₂S-P₂S₅ system was constructed, predicting new ground-state orderings for Li₃PS₄ and Li₇PS₆ polymorphs, and Li₇P₃S₁₁.
• Experimental phase stability trends, including polymorphic transitions and high-temperature instability, were reproduced.
• All studied superionic conductors were predicted to be metastable yet thermodynamically accessible at ambient temperatures.

Conclusions:

• Vibrational and configurational entropy are essential for accurately describing the stability of superionic conductors.
• Configurational disorder in Li sublattices is critical for predicting configurational entropy.
• A strong correlation exists between fast Li-ion diffusion and thermodynamic stability, driven by configurational entropy.