Thermoelectric properties of PbTe-based materials driven near the ferroelectric phase transition from first principles

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Murphy, Aoife Rose
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University College Cork
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The thermoelectric figure of merit, ZT, is a dimensionless parameter used to characterise the efficiency of a thermoelectric material. One of the most common techniques to improve ZT is to suppress lattice thermal conductivity via nanostructuring. However, this approach often comes with the cost of reduced electrical conductivity and Seebeck coefficient, resulting in only modest increases to ZT. Using the first principles Boltzmann transport framework, we show that driving PbTe-based materials to the verge of the soft mode (ferroelectric) phase transition could be a powerful strategy to reduce their lattice thermal conductivity while potentially preserving the electrical properties beneficial for a large ZT. The proposed concept is based on the induction of considerably softened zone-centre transverse optical modes, increasing their anharmonic coupling with low-frequency heatcarrying acoustic phonons and reducing phonon lifetimes at all frequencies. We illustrate this concept by applying biaxial tensile (001) strain to PbTe and its alloys with PbSe; and by alloying PbTe with a rhombohedral material, GeTe, reducing the lattice thermal conductivity by a factor of 2−3 compared to PbTe. Furthermore, by tuning the chemical composition of Pb1−xGexTe alloys, we also investigate the roles of proximity to the phase transition, mass disorder, and phase of the alloy in reducing the lattice thermal conductivity. Finally, we take the first steps towards a fully ab initio calculation of the figure of merit of PbTe-based materials near the phase transition by considering electron-phonon interactions, and find fair agreement between the calculated electronic mobility of PbTe and experiment. Due to certain symmetry forbidden electron-phonon coupling, we anticipate that our outlined approach may not degrade electronic properties beneficial for a large ZT. This proposed concept is general and offers a promising new strategy to potentially increase thermoelectric efficiency.
Thermoelectric materials , Phase transition , Ferroelectric , Soft mode , Thermal conductivity , Seebeck coefficient , Electrical conductivity , Thermoelectric figure of merit , Phonon lifetimes , Phonon scattering , Electron phonon coupling , Electronic mobility , DFT , DFPT , Relaxation time approximation , BTE RTA , PbTe , GeTe , Anharmonicity , Lattice thermal conductivity , Electron phonon , Soft optical modes , Soft zone centre optical modes , Anharmonic phonon coupling , Electronic lifetimes , Lifetimes , Broadband phonon scattering , Tensile strain , Alloy , Resonant bonding , Density functional theory , Density functional perturbation theory , Boltzmann transport equation
Murphy, A. R. 2017. Thermoelectric properties of PbTe-based materials driven near the ferroelectric phase transition from first principles. PhD Thesis, University College Cork.