Topochemical reaction strategies offer an important low-temperature (<500 °C) approach to the conscious manipulation of various inorganic host materials, allowing access to compounds that cannot be prepared by standard high-temperature methods. As the utility of these strategies continues to expand, researchers will be able to more effectively target materials with technologically significant properties. This Progress Report presents recent advances in topochemical reaction strategies as applied to perovskite and perovskite-related compounds. Emphasis is placed on structural modifications and corresponding variations in properties. Additionally, the future prospects of this approach to the rational design of intricate target compounds are discussed.

Topochemical manipulation of perovskites: low-temperature reaction strategies for directing structure and properties.

Research dealing with the ion exchange behavior of layered group 4 phosphates dates from World War II. Initially the exchangers were amorphous gels in which the exchange process was poorly understood. Crystallization of the gels and the subsequent determination of their crystal structures had led to an understanding of the ion exchange reactions. The most important group of exchangers have layered structures and can readily intercalate basic molecules in addition to exchanging ions. Their behavior can be used as the basis for understanding the properties of many proton containing layered compounds. The chemistry of these compounds has had a continuous growth in scope, particularly with the synthesis of organic-inorganic layered derivatives. Extension of this organic chemistry to divalent and trivalent metals further broadens the field. Potential applications as sequestrants, ion exchangers, catalysts, sensors and their possible use in the fields of electrochemistry and photochemistry are discussed.

Layered Phosphates, Phosphites and Phosphonates of Groups 4 and 14 Metals

Radiation stability of synthetic organic ion-exchange resins is not sufficient for the processing of very highly radioactive solutions because these solutions cause significant changes in the capacity and selectivity by hydrolysis of functional groups, chain scission and changes in degree of crosslinking1,2. Among the inorganic ion exchangers, zeolites and clay minerals have been used in decontaminating radioactive waste solutions3–6 because of their high ion-exchange capacity, selectivity and presumably their radiation stability7; however, their use is limited because of their instability in mildly acid solutions1. Another class of materials—insoluble acid salts formed by polybasic acids and certain hydrolysable polyvalent cations— also possess cation exchange properties. These salts include phosphates, arsenates, tungstates and molybdates of zirconium, thorium, titanium and other metals2. Among these insoluble acid salts, zirconium phosphate gels have been investigated8 very extensively. Crystalline phases of zirconium phosphate were first prepared in 1964 and 1968 from gels on refluxing in phosphoric acid by Clearfield and co-workers9,10. We report here the discovery of crystalline γ-zirconium phosphate as a highly selective caesium ion sieve which can be used to separate 137Cs from accident waste water or from circulating water in nuclear reactors.

John M. Kalnins

Papers

On the mechanism of ion exchange in zirconium phosphates—XIII: Exchange of some divalent transition metal ions on α-zirconium phosphate

On the mechanism of ion exchange in zirconium phosphates—XXIII exchange of first row divalent transition elements on γ-zirconium phosphate