Ideal two-dimensional electronic structure in bulk lead titanate for superior thermoelectrics

Abstract

An ideal two-dimensional (2D) electronic structure is revealed in n-type tetragonal perovskite PbTiO3 through first-principle calculations. This opens exciting prospects for its applications in thermal-to-electrical energy conversion. The electronic structure is regarded as ideally 2D when it is highly dispersive only along two axes, while along the remaining axis, it is nearly non-dispersive with a bandwidth smaller than 1 meV – insignificant when compared to thermal energy above room temperature (i.e., kBT ≥ 25.9 meV). As a basis for comparison, the electronic structure of the cubic perovskite SrTiO3 is calculated. It is nonideal 2D with the bandwidth along the less dispersive axis larger than the thermal energy. The ideal 2D electronic structure in PbTiO3 leads to a 2D-like density of states, resembling a step function around the conduction minimum while for SrTiO3, the density of states remains 3D-like with a square-root energy dependence. Consequently, n-type PbTiO3 exhibits superior effective band degeneracy within the Fermi window compared to SrTiO3, even though PbTiO3 is nondegenerate at the conduction band minimum, while cubic SrTiO3 is three-fold degenerate. This demonstrates the important role of the ideality in the 2D electronic structure. The ab-initio results demonstrate the potential of PbTiO3 as a promising thermoelectric material.