Abstract
Previous calculations have demonstrated that Te vacancies are energetically the major defects in PbTe. However, the Pb interstitials are also important because experiments have shown that the volume of Pb-rich PbTe increases at a higher Pb content. In this study, density functional theory calculations were used to investigate the defect properties of low-symmetry Pb interstitials in PbTe. By breaking the higher symmetry imposed on the on-centered interstitial defects, the lowest ground state of Pb interstitial defects is off-centered along the direction. Because of the four multi-stable structures with low defect-formation energies, the defect density of Pb interstitials is expected to be approximately six times higher than previous predictions for PbTe synthesized at 900 K. In contrast to the on-centered Pb interstitials, the off-centered Pb interstitials in PbTe can exhibit long-range lattice relaxation in the direction beyond a distance of 1 nm, indicating the potential formation of weak local dipoles. This result provides an alternative explanation for the emphanitic anharmonicity of PbTe in the high-temperature regime.
Highlights
Thermoelectric effects enable the direct conversion between thermal and electrical energies [1]
Because a large ZT value can lead to higher thermoelectric efficiency, reducing the lattice thermal conductivity and optimizing the thermoelectric power factor (α2 σ) have been key strategies for developing materials with high thermoelectric performance [1,3,4]
The formation energies of charged defects for the high-symmetry defect configFirst, the formation energies of charged defects for the high-symmetry defect configurations are revisited in PbTe
Summary
Thermoelectric effects enable the direct conversion between thermal and electrical energies [1]. The thermoelectric conversion efficiency can be estimated using the dimensionless figure of merit ZT = α2 σT/(κelec + κlatt ), where α is the Seebeck coefficient, σ is the electrical conductivity, κelec and κlatt are the electronic and lattice thermal conductivities, respectively, and T is the absolute temperature [1,2]. Thermoelectric performance can be improved by introducing imperfections into well-ordered lattices, introducing a second phase by extrinsic defects in disordered lattices, and nanoengineering [5,6]. This is because these strategies increase the electrical conductivity without changing the carrier mobility, reduce the thermal conductivity by phonon scattering, and enhance the Seebeck coefficient by controlling the density of states and charge carrier scattering.
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