New Lead-free Hybrid Layered Double Perovskite Halides: Synthesis, Structural Transition and Ultralow Thermal Conductivity.

Abstract

We have synthesized three new hybrid layered double perovskites (HLDPs) with general formula (R'/R'')4/2M(III)M(I)Cl8; where R' = C3H7NH3 (i.e. 3N) and R'' = NH3C4H8NH3 (i.e. 4N4); M(III) = In3+ or Ru3+; M(I) = Cu+ by simple solution-based acid precipitation method. The structural analysis reveals that (4N4)2CuInCl8 and (4N4)2CuRuCl8 adopt the layered Dion Jacobson (DJ) structure, whereas (3N)4CuInCl8 exhibits layered Ruddlesden Popper (RP) structure.Three compounds show temperature-dependent structural phase transition where change in the staking of inorganic layer, extent of octahedral tilting and reorientation of organic spacers with temperature have noticed. We have achieved ultralow lattice thermal conductivity (kL) in the HLDPs in the 2 to 300 K range, originating from the distorted crystal structures and soft lattice. The RP-HLDP compound, (3N)4CuInCl8, demonstrates anisotropy in kL while measured parallel and perpendicular to layer stacking, showcasing ultralow kL of 0.15 Wm-1K-1 at room temperature, which is one of the lowest values obtained among Pb-free metal halide perovskite. The observed ultralow kL in three new HLDPs is attributed to significant lattice anharmonicity arising from the chemical bonding heterogeneity within the complex crystal structure, coupled with the presence of low-energy localized optical phonon modes that suppress heat-carrying acoustic phonons.