Our ability to design new materials with desired electrochemical, thermoelectric or strongly-correlated electron properties is limited by thermodynamic control over reaction products in traditional high-temperature synthetic procedures. On the contrary, topotactic reactions, where extensive parts of the original framework are retained, allow for greater control of the structure of the final products; therefore, a combination of desired structural features, spin and oxidation states can be produced in a final material in a predictable fashion. A 'chimie douce' solvothermal reduction method was developed for topotactic oxygen deintercalation of complex metal oxides. The reduction of the Ruddlesden-Popper nickelate La4Ni3O10 was used as a test case to prove the validity of the method. The completely reduced phase La4Ni3O8 was produced via the solvothermal technique at 150°C - a lower temperature than by other more conventional solid state oxygen deintercalation methods. Unlike other techniques, a pure Ni1+ compound, LaNiO2, can be prepared by solvothermal reduction, demonstrating the advantages of the developed method for synthesis of highly metastable reduced compounds. Metastable oxyfluorides SrFeO2F and La4Ni3O8F2 were prepared through a low temperature, multistep synthesis via fluorination of the infinite layer intermediate phases. Influence of the synthetic pathway on O/F short range ordering and physical properties will be discussed. Mössbauer spectroscopy measurements revealed the predominance of cis fluorine configuration in FeO4F2 polyhedra, confirming a difference in local Fe coordination in comparison with O/F disordered SrFeO2F. When trying to achieve anion ordering during synthesis, it is important to perform anion intercalation at as low a temperature as possible to avoid thermal randomization of the anions. A low temperature solvothermal fluorination technique, which we developed, allows for fast and facile synthesis at lower temperatures than gas- solid fluorination by XeF2

Novel soft chemistry techniques for metastable materials synthesis

250th National Meeting (2015)

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