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

Hybrid layered double perovskites (HLDPs), representing the two-dimensional manifestation of halide double perovskites, have elicited considerable interest owing to their intricate chemical bonding hierarchy and structural diversity. This intensified interest stems from the diverse options available for selecting alternating octahedral coordinated trivalent [M(III)] and monovalent metal centers [M(I)], along with the distinctive nature of the cationic organic amine located between the layers. Here, we have synthesized three new compounds with general formula (R'/R'') 4/2 M(III)M(I)Cl 8 ; where R'=C 3 H 7 NH 3 (i.e. 3N) and R''=NH 3 C 4 H 8 NH 3 (i.e. 4N4); M(III)=In 3+ or Ru 3+ ; M(I)=Cu + by simple solution-based acid precipitation method. The structural analysis reveals that (4N4) 2 CuInCl 8 and (4N4) 2 CuRuCl 8 adopt the layered Dion Jacobson (DJ) structure, whereas (3N) 4 CuInCl 8 exhibits layered Ruddlesden Popper (RP) structure. The alternative octahedra within the inorganic layer display distortions and tilting. Three compounds show temperature-dependent structural phase transitions where changes in the staking of inorganic layer, extent of octahedral tilting and reorientation of organic spacers with temperature have been noticed. We have achieved ultralow lattice thermal conductivity (κ L ) in the HLDPs in the 2 to 300 K range, marking a distinctive feature within the realm of HLDP systems. The RP-HLDP compound, (3N) 4 CuInCl 8 , demonstrates anisotropy in κ L while measured parallel and perpendicular to layer stacking, showcasing ultralow κ L of 0.15 Wm -1 K -1 at room temperature, which is one of the lowest values obtained among Pb-free metal halide perovskite. The observed ultralow κ L in three new HLDPs is attributed to significant lattice anharmonicity arising from the chemical bonding heterogeneity and soft crystal structure, which resulted in low-energy localized optical phonon modes that suppress heat-carrying acoustic phonons.