Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems

Abstract

In recent years, LAST-m (AgPb_mSbTe_m+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb²⁺ ions by (Ag¹⁺, Sb³⁺) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T^2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κ_el,E) and for constant current (κ_el,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κ_el,J = L_oσT, where L_o =2x10⁻⁸WΩ/K is the Lorentz number, overstimates κ_el,J by nearly 60% at 800K for carrier concentration n=5x10¹⁹/cm³. As a result, one tends to underestimate the lattice contribution κ_latt = κ_exp - κ_el,J. We give theoretical values of effective Lorentz number L = κ_el.J/σT for different n and T.