Ruddlesden-Popper (RP) phases are a type of

perovskite structure A perovskite is any material with a crystal structure following the formula ABX3, which was first discovered as the mineral called perovskite, which consists of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural mou ...

that consists of two-dimensional perovskite-like slabs interleaved with

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

. The general formula of an RP phase is ''A_n+1B_nX_3n+1'', where ''A'' and ''B'' are cations, ''X'' is an anion (e.g., oxygen), and ''n'' is the number of octahedral layers in the perovskite-like stack. Generally, it has a phase structure that results from the intergrowth of perovskite-type and NaCl-type (i.e., rocksalt-type) structures. These phases are named after S.N. Ruddlesden and P. Popper, who first synthesized and described a Ruddlesden-Popper structure in 1957. Ruddlesden-popper phase 1

Crystal structure

The general RP formula ''A_n+1B_nX_3n+1'' can be written ''A_n-1A’₂B_nX_3n+1'', where ''A'' and ''A’'' are alkali, alkaline earth, or rare earth metals and ''B'' is a transition metal. The ''A'' cations are located in the perovskite layer and are 12-fold cuboctahedral coordinated by the anions (CN = 12). The ''A’'' cations have a coordination number of 9 (CN = 9) and are located at the boundary between the perovskite layer and an intermediate block layer. The ''B'' cations are located inside the anionic octahedra, pyramids and squares.

Synthesis

The first series of Ruddlesden-Popper phases, Sr₂TiO₄, Ca₂MnO₄ and SrLaAlO₄, were confirmed by

powder X-ray diffraction Powder diffraction is a scientific technique using X-ray, neutron, or electron diffraction on powder or microcrystalline samples for structural characterization of materials. An instrument dedicated to performing such powder measurements is cal ...

(PXRD) in 1957. These compounds were formed by heating up the appropriate oxides and carbonates in the correct proportions. In recent years, interest in perovskite-like structures has been growing and methods for synthesizing these compounds have been further developed. In contrast to the conventional solid-state method, '' chimie douce'' or soft chemistry techniques are often utilized to synthesize this class of materials. These soft chemistry techniques include ''ion-exchange reactions of layered perovskites'', ''ion-exchange reactions involving interlayer structural units'', ''topochemical condensation reactions'' and other techniques such as ''intercalation-deintercalation reactions'' and ''multistep intercalation reactions of layer perovskite''.

Applications

Similar to the parent perovskite phases, Ruddlesden-Popper phases can possess interesting properties such as colossal magnetoresistance, superconductivity,

ferroelectricity Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. All ferroelectrics are also piezoelectric and pyroelectric, with the add ...

, catalytic activity, white light emitting diodes, scintillators, fuel cell, and solar cells. Using Ruddlesden-Popper perovskite as light-emitting diodes has the advantages of low-cost solution-processing, tunable bandgap, and better stability compared to 3D perovskite. In 2018, Mohite et al. achieved a 14 hours stable operation of 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 Ruddlesden-Popper perovskite thin films as light-emitting diodes under operating conditions, while 3D perovskite as light-emitting diodes could degrade within minutes. The Ruddlesden-Popper perovskite can be used as solar cells, like common 3D perovskite. While it generally possesses lower power conversion efficiency (PCE) than 3D perovskites, it has been proven to have better stability. In 2021, using 4-chlore-phenylformamidinium (CPFA) as spacer cation in Ruddlesden-Popper perovskite thin films was studied by Yang et al. CPFA₂MA_n-1Pb_n(I_0.857Cl_0.143)_3n+1 perovskite thin film can still have ~80% of its original PCE, which is 14.78%, after 2000 hours storage under ambient conditions. The Ruddlesden-Popper phase LaSr₃Fe₃O₁₀ is an example of a layered perovskite being developed for use in rechargeable metal-air batteries. Due to the layered nature of Ruddlesden-Popper structures, the oxygen located between the perovskite layers is easily removed. The ease of removing the oxygen atoms is responsible for the efficiency of the

oxygen evolution Oxygen evolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. ...

reaction (OER) and oxygen reduction reaction (ORR) in the material. In a metal-air battery, OER is the process of charging that occurs at the air electrode, while ORR is the discharging reaction. The Ruddlesden-Popper phase perovskites are also prospective candidate materials in energy storage devices. The formula of (R-NH₃)₂A_n-1B_nX_3n+1 are being developed for

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

. Here, R-NH₃⁺ is long organic chain or cyclic ammonium cation, A is methylamine (MA) or formamidine (FA), B is Pb or Sn, and X is halogen ions. The Ruddlesden-Popper perovskites can also be used for cathode materials of solid oxide fuel cells (SOFC)

References

{{Titanium minerals Perovskites