RuddlesdenPopper

Ruddlesden–Popper (RP) phases are a family of layered perovskite-related materials that extend the three-dimensional ABX3 perovskite structure into two dimensions. In halide RP phases the general formula is A_{n+1} B_n X_{3n+1}, where A is a large organic or inorganic cation, B is a divalent metal cation such as Pb2+ or Sn2+, and X is a halide (I−, Br−, or Cl−).

The structure consists of n consecutive MX6 octahedral layers forming a quasi-2D perovskite slab, which are separated by insulating AX rock-salt layers. The parameter n controls slab thickness: n = 1 yields a single MX layer, increasing n adds more octahedral layers until approaching the 3D limit.

Typical compositions use bulky organic cations such as butylammonium (BA) or phenethylammonium (PEA) as A-site spacers, together with lead or tin halides. Variants replace halides (I, Br, Cl) and can substitute A-site or B-site cations to tune properties.

RP phases exhibit strong quantum and dielectric confinement, leading to larger band gaps and high exciton binding energies compared with 3D perovskites. As n increases, the band gap gradually narrows toward the 3D perovskite value.

Synthesis and processing rely on solution-based methods or vapor deposition to form oriented, layered films. Parameters such as solvent, spacer cation, and antisolvent choice influence crystal orientation, phase purity, and stability. 2D RP films often demonstrate enhanced moisture tolerance relative to 3D counterparts.

Applications span light-emitting diodes, photodetectors, and solar cells, where the trade-off between stability, band gap, and charge transport is active. The family remains a focus of research for fundamental understanding and device optimization.

RP phases were described by Ruddlesden and Popper in 1957; their halide derivatives have attracted renewed interest since the rise of hybrid perovskites in the 2010s.