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Ruddlesden-Popper (RP) Phase •	Crystal Structure •	Synthesis •	Applications •	References Ruddlesden-Popper phases are a form of layered perovskite structure consist of two-dimensional perovskite slabs interleaved with cations, The general formula of RP phase is An-1A’2BnX3n+1, where A, A’, and B are cations, X is an anion and n is the number of the layers of octahedra in the perovskite-like stack.[1] Generally, it has a phase structure that result a from the intergrowth of perovskite-type structure and NaCl-type stucture. The structure was named after S.N. Ruddlesden and P. Popper, who first synthesized and described the structure in 1957.[2][3]

Crystal Structure In the general formula of RP phases, An-1A’2BnX3n+1, A and A’ represent alkali, alkaline earth, or rare earth metal while B refers to transition metal. The A cations are located in the perovskite layer and have a 12-fold cuboctahedral coordination to the anions. The A’ cations have a coordination number, CN = 9 and is located at the perovskites boundary with an intermediate block layer. The B cations are located inside the anionic octahedral, pyramids and squares. [1] The unit cell of Ruddlesden-Popper phase (a)Sr2RuO4 (n = 1) and (b)Sr3Ru2O7 (n = 2). The polyhedra represent the perovskite-like structure. Synthesis The first series of Ruddlesden-Popper phase, Sr2TiO4, Ca2MnO4 and SrLaAlO4 were confirmed by powder X-ray diffraction (PXRD) in 1957.[2] These compounds were formed by heating up the appropriate oxide or carbonate, in the molecular proportion.

In recent years, interest on perovskite-like strucutre has been growing and the synthetic methods for this compound have been further developed. Alternative to the conventional solid state method, chimie douce or soft chemistry solid-state technique has been utilized to synthesize this class of material. These soft chemistry solid-state techniques involve ion-exchange reactions of layered perovskites, ion exchange reactions involving interlayer structural units, topochemical condensation reactions and other techniques such as intercalation-deintercalation reacions and multistep intercalation reactions of layer perovskite.[4]

Applications Similar to its parent perovskite structure, Ruddlesden-Popper phases possess interesting property such as colossal magnetoresistance, superconductivity, ferroelectricity, and catalytic activity.

Ruddlesden-Popper phase LaSr3FeO10 is an example of layer perovskite that finds its application in the rechargeable metal-air battery. Due to the layered structure of Ruddlesden-Popper phase, the oxygen that located in between in the layered perovskite is easily removable. The easily removable oxygen are essential to increase the efficiency of Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR). In the metal-air battery, OER is a process of charging reaction at the air electrode, whie ORR is a a process of discharging reaction. References [1]	Beznosikov, B. V., & Aleksandrov, K. S. (2000). Perovskite-like crystals of the Ruddlesden-Popper series. Crystallography Reports, 45(5), 792-798. doi: 10.1134/1.1312923

[2]	Acta Cryst. (1958). 11, 54-55 [doi:10.1107/S0365110X58000128]

[3]	Acta Cryst. (1957). 10, 538-539 [doi:10.1107/S0365110X57001929]

[4]	Schaak, R. E., & Mallouk, T. E. (2002). Perovskites by Design:  A Toolbox of Solid-State Reactions. Chemistry of Materials, 14(4), 1455-1471. doi: 10.1021/cm010689m

[5]	Takeguchi, T., Yamanaka, T., Takahashi, H., Watanabe, H., Kuroki, T., Nakanishi, H., Ueda, W. (2013). Layered Perovskite Oxide: A Reversible Air Electrode for Oxygen Evolution/Reduction in Rechargeable Metal-Air Batteries. Journal of the American Chemical Society, 135(30), 11125-11130. doi: 10.1021/ja403476v