New layered perovskite materials obtained by low temperature topochemical methods

Due to interesting properties that layered perovskite materials can manifest, there is a continuous search for new compounds in this specific area. Soft chemistry methods such as ion exchange, intercalation and deintercalation have proven excellent synthetic techniques in targeting specific frameworks. Thus, the concept of rational design of new materials has been demonstrated by topotactic reactions.; New transition metal oxyhalides, (MCI)LaNb2O7 (M = Mn, Fe) have been prepared by ion exchange reactions. Layered perovskite hosts (ALaNb2O7, A = Li, Na, K or Rb) were reacted with the corresponding anhydrous metal halides under mild reaction conditions (<400°C). The crystal structures of the obtained compounds were analyzed by Rietveld method. The new structures consist of edge sharing octahedra of MCl4 O2 (M = Fe, Mn) separating perovskite layers. Thermal analysis in oxygen, argon and hydrogen shows the materials to be metastable where both compounds decompose exothermically above 690°C. At high temperatures the compounds are paramagnetic following the Curie-Weiss law, while at temperatures lower than 100K they become antiferromagnetic. The exchange path is achieved by the chlorine ions bridging between the transition-metal ions. In the iron system, another maximum arises at 8K and the ferromagnetic nature of this transition was assigned by ac susceptibility measurements and subsequently by neutron diffraction analysis.; Compounds of formula M0.5UNb2O7 (M = Fe, Ni) were obtained by an ion exchange reactions between RbLaNb2O 7 and anhydrous MBr2 at low temperature (350°C). Cu 0.5UNb2O7 was prepared by topochemical deintercalation of CUCl2 from (CuCl)LaNb2O7 under a dynamic vacuum at 550°C. The series of compounds are isostructural with the layer spacing following the trend expected from the transition-metal cationic radii. The compounds are paramagnetic and evidence for weak ferromagnetic coupling is observed in the copper compound at low temperature (<10K).; The versatility of the new obtained structures to topotactic manipulations is further demonstrated when one of the oxychloride is investigated. It was found that (CuCl)LaNb2O7 intercalates lithium with copper getting reduced. Novel-layered type structures of formula (A2Cl)LaNb 2O7 (A = Li, Na) were obtained. After copper removal, another intercalation step gave (Li3Cl)LaNb2O7 in which niobium is in the formal oxidation state of +4.5 and neutron diffraction analysis by Rietveld method of both lithium and lithium intercalated chloride compounds showed that the perovskite layers are now separated by double alkali metal chloride (LiCl2O2) arrays. The susceptibility data on (Li2Cl)LaNb2O7 revealed a diamagnetic behavior while weak ferromagnetic interactions even at room temperature were observed on (Li3Cl)LaNb2O7.