Anharmonic Interaction in Negative Thermal Expansion Material CaTiF₆.

Area of Science:

• Solid-state physics
• Materials science
• Computational materials science

Background:

• Quasi-harmonic approximation (QHA) is widely used but limited for highly anharmonic materials.
• Understanding anharmonic interactions is crucial for predicting material properties under varying temperatures.
• Negative thermal expansion (NTE) is an unusual phenomenon requiring advanced theoretical treatment.

Purpose of the Study:

• To quantitatively investigate the negative thermal expansion (NTE) of CaTiF6.
• To explore the role of anharmonic phonon interactions in NTE.
• To predict the lattice thermal conductivity (LTC) of CaTiF6.

Main Methods:

• Combining quasi-harmonic approximation (QHA) with improved self-consistent phonon approximation (ISCPA) for nonperturbative anharmonic effects.
• Calculating mode Grüneisen parameters to identify phonon contributions to NTE.
• Investigating phonon lifetimes from three-phonon interactions.
• Employing the Boltzmann transport equation with relaxation time approximation to predict LTC.

Main Results:

• ISCPA provides quantitative agreement with experimental NTE behavior, surpassing QHA's qualitative consistency.
• Low-frequency phonons, particularly acoustic phonons, are identified as major contributors to CaTiF6's NTE.
• Rigid unit modes (RUMs) associated with low-frequency optical phonons were identified.
• Calculated phonon lifetimes offer insights into the NTE mechanism upon heating.
• Anomalous, glasslike lattice thermal conductivity is predicted for crystalline CaTiF6.

Conclusions:

• The ISCPA method accurately captures the NTE of CaTiF6, highlighting the importance of anharmonic effects.
• Low-frequency phonons and RUMs are key to understanding the NTE mechanism in CaTiF6.
• The predicted glasslike LTC suggests potential for novel thermal management applications.