Distorted polyhedral architecture enabled high thermoelectric performance of columnar double halide perovskites Cs2AgPdCl5 and Cs2AgPtCl5

TL;DR

The paper addresses the challenge of achieving high thermoelectric efficiency with lead-free materials by investigating two distorted columnar double halide perovskites, Cs$_2$AgPdCl$_5$ and Cs$_2$AgPtCl$_5$, using DFT and semiclassical Boltzmann transport. It combines HSE06+SOC electronic structure calculations with AMSET and ShengBTE phonon analysis to reveal ultralow lattice thermal conductivity ($\kappa_l$ ≈ 0.20–0.27 W m$^{-1}$K$^{-1}$) and favorable band structures, including valley degeneracy that enhances transport. The materials exhibit strong thermoelectric performance, achieving $zT$ up to 1.30 for p-type Cs$_2$AgPdCl$_5$ at 800 K and 0.87–0.87 for Cs$_2$AgPtCl$_5$ under appropriate doping, driven by a combination of moderate band gaps, high Seebeck coefficients, and low electronic and lattice thermal conductivities. These findings establish distorted octahedral motifs as a viable route to ultralow $\kappa_l$ and competitive thermoelectric figures of merit, suggesting potential for high-performance, lead-free TE devices and guiding future design of columnar perovskites.

Abstract

We investigate the thermoelectric properties of two newly synthesized columnar double halide perovskites Cs$_2$AgPdCl$_5$ and Cs$_2$AgPtCl$_5$. These materials accommodate a distorted local polyhedral architecture with tetrahedral symmetry compared to traditional double halide perovskites. By employing density functional theory along with the semiclassical transport model, we have analyzed the electronic and transport properties of these materials. Our results show that at 800 K, the largest figure of merit ($zT$) is 1.30 (0.86) for p-type (n-type) Cs$_2$AgPdCl$_5$ and 0.87 for n-type Cs$_2$AgPtCl$_5$ at doping concentrations of $1.94 \times 10^{20}$ ($3.76 \times 10^{19}$) cm$^{-3}$ and $3.52 \times 10^{19}$ cm$^{-3}$, respectively. Remarkably, a very low doping concentration is required to achieve a high $zT$, setting these materials apart from others in this field. Our calculations demonstrate that Cs$_2$AgPdCl$_5$ benefits from the presence of conduction and valence band valleys near the band edges; however, the flat bands present in the valence band of Cs$_2$AgPtCl$_5$ do not improve its thermoelectric performance. Among these systems, hole doping in Cs$_2$AgPdCl$_5$ has shown remarkable thermoelectric performance. Interestingly, the local octahedral distortions present in these perovskites contribute to a marked reduction in the lattice thermal conductivity to 0.27 W/mK in Cs$_2$AgPtCl$_5$ and 0.20 W/mK in Cs$_2$AgPdCl$_5$ by causing enhanced phonon scattering, further improving the thermoelectric figure of merit. This drop in thermal conductivity, combined with the favorable electronic properties, underscores the potential use of these materials for applications in highly efficient thermoelectric devices.

Figures (26)

• Figure 1: Crystal structure of (a) Cs$_2$AgPdCl$_5$ and (b) Cs$_2$AgPtCl$_5$. Here, red, black, blue, green, and magenta color shows Pd, Pt, Ag, Cl, and Cs atoms, respectively.
• Figure 2: Projected -COHP(-pCOHP) curve for Ag-Cl1, Ag-Cl2, and Pd-Cl for (a) Cs$_2$AgPdCl$_5$ and (b) Cs$_2$AgPtCl$_5$.
• Figure 3: Electron localisation function map for (a) Cs$_2$AgPdCl$_5$ and (b) Cs$_2$AgPtCl$_5$.
• Figure 4: Electronic band structure along with the PDOS of (a) Cs$_2$AgPdCl$_5$ and (b) Cs$_2$AgPtCl$_5$ at the HSE06 level with spin orbit coupling. The band energies are scaled relative to the Fermi level.
• Figure 5: Phonon band structure along with partial phonon density of state (PDOS) of (a) Cs$_2$AgPdCl$_5$ and (b) Cs$_2$AgPtCl$_5$.