Off-Centered Pb Interstitials in PbTe.

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Tellurium (Te) vacancies are known major defects in lead telluride (PbTe).
• Experimental data show Pb-rich PbTe volume increases with higher Pb content, suggesting other Pb defects are significant.
• Previous theoretical studies focused on higher-symmetry, on-centered interstitial defects.

Purpose of the Study:

• Investigate the defect properties of low-symmetry lead (Pb) interstitials in lead telluride (PbTe).
• Determine the ground state configuration and formation energies of Pb interstitials.
• Explain the observed experimental properties of Pb-rich PbTe, including lattice expansion and anharmonicity.

Main Methods:

• Utilized density functional theory (DFT) calculations.
• Explored off-centered interstitial configurations by breaking higher symmetry constraints.
• Calculated defect formation energies and lattice relaxation patterns.

Main Results:

• The lowest energy ground state for Pb interstitial defects is off-centered along the [1¯1¯1¯] direction.
• Identified four multi-stable structures with low defect-formation energies for Pb interstitials.
• Predicted a defect density for Pb interstitials approximately six times higher than previously estimated for PbTe synthesized at 900 K.
• Observed long-range lattice relaxation (>1 nm) in the [111] direction for off-centered Pb interstitials, suggesting weak local dipole formation.

Conclusions:

• Off-centered Pb interstitials are crucial defects in PbTe, significantly increasing defect density.
• The lattice relaxation induced by off-centered Pb interstitials provides a new explanation for the strong anharmonicity observed in PbTe at high temperatures.
• This study refines the understanding of defect physics in PbTe, impacting materials design for thermoelectric applications.