Dion-Jacobson phase

The general formula for the Dion-Jacobson phase can be written as MIA(n-1)B(n)X(3n+1), where A and B are cations, M is a monovalent cation, and X is an anion. The structure consists of perovskite-like two-dimensional layers, constructed by stacking n corner-shared BX6 octahedra with the A cation in 12-fold cuboctahedral coordination with the anions. The perovskite-like layers are separated by the M cation, which is typically 8-coordinate. Depending on the identity of the monovalent cation, the perovskite-like layers can be displaced by 1/2 of the unit cell parameter, which changes the coordination environment of the M cation to 4-coordinate. The M and A cations can also be organic cations, such as methylammonium and ethylenediamine. This produces a hybrid organic-inorganic structure that can increase stability through interactions between two organic ligands. ==Synthesis==

Inorganic Dion-Jacobson perovskites, such as KLaNb2O7, have been synthesized via high temperature solid state reactions between the respective carbonate and oxide precursors. Two-dimensional Dion-Jacobson halide perovskites, such as (3AMPY)(MA)Pb2I7 (3AMPY = 3-(aminomethyl)pyridinium and MA= methylammonium) have been synthesized via precipitation reactions as well. ==Applications==

Dion-Jacobson oxides have applications as ferroelectrics, piezoelectrics, and photocatalysts. The layering between differently-charged M cations reduces the symmetry of the BO6 octahedra, which breaks inversion symmetry and allows for or enhances these properties. Both n = 2 and n = 3 Dion-Jacobson phases can be used for these applications. Hybrid organic-inorganic Dion-Jacobson phases have additional uses as photovoltaics and photodetectors, having better stability than three-dimensional hybrid perovskites. The stability of hybrid organic-inorganic Dion-Jacobson phases with photovoltaic properties facilitates their use in perovskite solar cells. Hybrid organic-inorganic Dion-Jacobson phases have a divalent organic cation which can bridge adjacent ABX3 layers, eliminating the van der Waals gap with a stable structure formed by hydrogen bonding. The improved structure allows for more efficient interlayer charge transport and environmental stability. Dion-Jacobson phases can be used as the absorbing layer in two-dimensional solar cells or as a layer for passivation on top of 3D perovskites to improve stability and power conversion efficiency. For example, an efficiency of 24.9% was achieved with a two-dimensional Dion-Jacobson solar cell, which retained 97% of its initial power conversion efficiency after 1000 hours in ambient air. When used as a top layer in a three-dimensional solar cell, a power conversion efficiency of 19.5% was achieved with 83% of the original efficiency retained after 1260 hours under continuous illumination. Through additive manufacturing, the performance of Dion-Jacobson phases can be improved even further. Incorporating chloride salts or thiocyanate salts into the precursor solution can coordinate ions from those compounds with undercoordinated ions often used in perovskite photovoltaics, such as Pb2+ and Sn2+, to reduce trap states and improve the power conversion efficiency of Dion-Jacobson solar cells. Other additives, such as dimethyl sulfoxide, can slow down crystallization to create larger grains and improve the structural properties of Dion-Jacobson phases. ==See also==