Two-phase equation of state for lithium fluoride.

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Accurate equations of state (EOS) are crucial for understanding material behavior under extreme conditions, particularly for ionic solids like lithium fluoride (LiF).
• Previous models for LiF have shown discrepancies with experimental data, especially concerning phase transitions under high pressure and temperature.
• The existence and pressure of a B1-B2 phase transition in LiF have been debated for decades, with some EOS models predicting it along the Hugoniot.

Purpose of the Study:

• To develop a comprehensive two-phase equation of state for lithium fluoride (LiF) covering solid and liquid phases.
• To provide a thermodynamically consistent model that integrates pressure, heat capacity, and free-energy functions.
• To critically evaluate existing LiF EOS models and address the long-standing debate on the B1-B2 phase transition.

Main Methods:

• Fitted pressure and heat-capacity functions for solid (B1 phase) and liquid LiF to experimental data and quantum molecular dynamics (QMD) simulations.
• Thermodynamically integrated these functions to derive free-energy functions for a consistent EOS.
• Compared the developed EOS with existing models (SESAME 7271v3, LEOS 2240, Smirnov's model) using experimental data and recent velocimetry results.

Main Results:

• The developed two-phase EOS for LiF demonstrates superior agreement with experimental data compared to SESAME 7271v3, LEOS 2240, and Smirnov's model.
• The model refutes the predicted B1-B2 phase transition at ~140 GPa along the Hugoniot, as proposed by Smirnov's EOS.
• Recent velocimetry data supports the inconsistency of the B1-B2 transition at 140 GPa in LiF.

Conclusions:

• The presented thermodynamically consistent equation of state provides a more accurate representation of lithium fluoride's behavior across a wide range of P-T conditions.
• The findings challenge the existence of the B1-B2 phase transition in LiF at the pressure predicted by Smirnov's model, aligning with recent experimental observations.
• This improved EOS is valuable for shock and ramp-compression studies, offering enhanced predictive capabilities for LiF under extreme environments.