Domain Walls and Defects in Ferroelectric Inorganic Halide Perovskites CsGeX$_3$ (X = Cl, Br, I)

TL;DR

This work uses hybrid-functional DFT to show that all-inorganic CsGeX3 ferroelectrics possess highly mobile charge carriers, low-energy, weakly interacting domain walls, and defect chemistries favoring $p$-type doping. The key finding is that 71° and 109° DWs have low formation and migration energies with minimal defect pinning, while DWs do not significantly alter the electronic structure, limiting DW-based conduction but enabling low-energy, high-frequency switching. Collectively, these results position CsGeX3 as robust soft ferroelectrics with potential for high-frequency devices and transparent $p$-type conduction, contrasting with oxide ferroelectrics in terms of defect tolerance and DW behavior. The study highlights a distinct technological pathway for ferroelectric devices in inorganic halide perovskites, emphasizing defect insensitivity and polarisation dynamics over DW conduction.

Abstract

Among all-inorganic halide perovskites, the only known ferroelectrics are the family of CsGeX$_3$ (X = Cl, Br, I). Here, we study their ferroelectric domain walls (DWs) and common point defects by density functional theory (DFT) calculations and investigate the interplay between DWs and defects. The most stable defects are V$_{\text{X}}$ and V$_{\text{Cs}}$ and the former shows low migration barriers and high mobility. In contrast to oxide ferroelectrics, the affinity between point defects and DWs is negligible, reflecting the subtle structural distortions at CsGeX$_3$ DWs. Concomitantly, the formation energies and migration energy barriers of CsGeX$_3$ DWs are small compared to oxides, and neither V$_{\text{X}}$ nor V$_{\text{Cs}}$ pin migrating DWs. The band gap invariance across DWs and the lack of affinity towards intrinsic charged point defects imply that conducting DWs for nanoelectronics may be challenging to realise in CsGeX$_3$. However, shallow $p$-type defect levels and low hole effective masses suggest that high $p$-type conductivity may be achievable in nominally ferroelectric CsGeX$_3$. The low DW migration energy barriers and insignificant DW pinning by point defects make CsGeX$_3$ promising materials as robust soft ferroelectrics for high-frequency switching applications with low energy dissipation.

Figures (8)

• Figure 1: DFT optimised primitive structures of CsGeCl3, CsGeBr3 and CsGeI3 visualised together with their respective lone pairs. The lone pairs are extracted from the partial charge density of the top valence bands and shown with an isosurface level of 0.02.
• Figure 2: Electronic density of states for CsGeCl3, CsGeBr3 and CsGeI3 calculated using the HSE06 functional. Cs $p$, Cs $d$, Ge $s$, Ge $p$ and X $p$ (halogen-p) are shown and coloured in turquoise, purple, blue, orange and green, respectively. Other orbitals do not contribute significantly and are thus omitted.
• Figure 3: Electronic band structures for CsGeCl3, CsGeBr3 and CsGeI3 calculated using the HSE06 functional. The valence and conduction band are coloured in green and blue, respectively.
• Figure 4: Thermodynamic transition level diagrams for intrinsic defects in bulk CsGeCl3 (a) and CsGeI3 (b) calculated using the HSE06 functional. The three panels for each compound show the transition levels at three different chemical environments indicated in the stability window shown in SI Figure S6 and S7. The dashed lines at 0 eV, and $\sim3$ and $\sim1.5$ eV display the VBM and CBM, respectively.
• Figure 5: Migration barriers of V$_{\text{Cs}}$ and V$_{\text{Cl}}$ in bulk CsGeCl3 in charged and neutral cells calculated using the PBEsol functional.