Lattice thermal conductivity reduction in Ca3AlSb3 and Ca5Al2Sb6 by manipulating the covalent tetrahedral chain

Abstract

Understanding the structural and physical origins of low thermal conductivity is critical to improving and designing efficient thermoelectric materials. For two distinct Zintl Ca–Al–Sb compounds with different stoichiometric ratios (Ca₃AlSb₃ and Ca₅Al₂Sb₆), experimental measurements suggest the low lattice thermal conductivities (∼1.43 W mK⁻¹ for Ca₃AlSb₃ and 1.52 W mK⁻¹ for Ca₅Al₂Sb₆ at 300 K). In order to understand the physical origin of the low thermal conductivity, we present the first-principles studies on the lattice dynamics and phonon-transport properties. The theoretically calculated lattice thermal conductivity of Ca₃AlSb₃ and Ca₅Al₂Sb₆ is ∼1.61 W mK⁻¹ for Ca₃AlSb₃ and 1.85 W mK⁻¹ for Ca₅Al₂Sb₆ at 300 K, which is in reasonable agreement with the experimental measurements. The low lattice thermal conductivity is attributed to the low acoustic Debye temperature and strong optical-acoustic phonon couplings in the two Ca–Al–Sb compounds. It is worth noting that the thermal conductivity of Ca₃AlSb₃ and Ca₅Al₂Sb₆ along the x direction (along the Al–Sb chain) is obviously higher than that along the y/z direction (perpendicular to the chain). The high lattice thermal conductivity along the Al–Sb chain is due to the strong Al–Sb covalent bond. From the phonon density of states (PDOS), the obviously frequency regions dominated by different atoms suggest that forming defects with one atom would only shift its related PDOS and might not affect the PDOS of others. Based on the understandings of the crystal structure, PDOS and atomic displacement parameter, we represent a methodology to further lower their lattice thermal conductivity: substituting heavier atoms along the Al–Sb chain to strongly scatter phonons. When using Tl to substitute Al, the vibration frequency of the Tl dopant is only 1/3 of that of the substituted Al atom. The significantly decreased vibration frequency will introduce a low phonon band within the PDOS, which will suppress the lattice thermal conductivity. Our work not only elucidates the physical mechanism of low lattice thermal conductivity in Ca₃AlSb₃ and Ca₅Al₂Sb₆ Zintl compounds, but also offers an efficient approach (breaking the covalent tetrahedral chains) to further block the heat transport.