Topochemical manipulation of some complex transition metal oxides
Keywords: Anion exchanges
Cation exchages
Topochemical Manipulation
Oxyhydride
Novel Complex Transition Metal Oxides
Anion deintercalations
Soft Chemistry

This thesis is comprised of three parts. The first part concerns the investigation of the topochemical reduction of LaSrNiRuO6 in order to prepare LaSrNiRuO4 via anion deintercalation. The second part discusses the oxide-for-hydride anion exchanges performed in SrV1-xTixO3, and the resulting SrV1-xTixO2-yH1+y reduction products. Finally, the results from redox-neutral topochemical cation exchange reactions conducted in the three-dimensional perovskite structure of NaTaO3 are presented along with the characterisation of a novel product of composition Ni0.5TaO3.
The topochemical reduction of LaSrNiRuO6 using CaH2 was carried out to produce a novel extended oxide phase with composition LaSrNiRuO4. This phase is composed of sheets of apex-linked Ni1+O4 and Ru2+O4 squares in a checkerboard ordered arrangement. To the best of our knowledge, this material is the first example of a B-cation ordered infinite-layer oxide phase. The low oxidation states of the transition-metal cations are confirmed by DFT calculations from which a spin moment S = ½ is determined for the nickel while the ruthenium centres adopt an intermediate-spin S = 1 configuration. LaSrNiRuO4 behaves paramagnetically at room temperature. However, upon cooling (T &LT; 250 K) a phase transition is observed in which the nickel spins interact ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. A possible explanation is given for this observation based on an ordered arrangement of local Jahn-Teller distortions.
While investigating the preparation of LaSrNiRuO4, it was observed that different samples of the LaSrNiRuO6 starting materials exhibited markedly different reactivity. The observed differing reactivity is inconsistent with the crystal structure and composition of the LaSrNiRuO6 samples, from which all the materials are identical. Careful investigation of the X-ray diffraction data collected from the LaSrNiRuO6 materials revealed that the reactivity of the samples is a consequence of the microstructure. By quenching or slow-cooling the materials during their synthesis, the size of the crystalline domains formed is affected and this in turn is observed to define the extent to which the topochemical deintercalation of oxide anions takes place. A mechanism to explain this effect is presented in which the greater 'plasticity' of small crystalline domains helps to limit the influence of lattice strain during the reaction.
Similar with the observations for the LaSrNiRuO6 phases, it was found that the reactivity of SrV0.95Ti0.05O3 samples towards topochemical oxide-for-hydride exchange is also determined by the characteristics of the starting materials. The cooling rate can lead to phase segregation in SrV0.95Ti0.05O3 samples which in turn affects the reduction behaviour. A modification of the energy profile for the oxide-for-hydride exchange in SrV1-xTixO3 phases is proposed on the basis of the electronic configuration that the transition-metal cations adopt upon reduction (d2,V3+ and d1,Ti3+).
Finally, topochemical exchange reactions can also be carried out between cations in complex transition metal oxides when the mobility of the species to be exchanged is sufficiently greater with respect to the host lattice. The preparation of Ni0.5TaO3 from exchange of Na+ by Ni2+ in NaTaO3 represents a synthetic approach not yet widely explored in the long-standing challenge that the preparation of magnetoelectric multiferroic materials represents.
The topochemical reactions studied in this work highlight the possibility of directing and modifying the product phases, by tuning features of the reagents. This is in contrast with the limited control available in thermodynamic processes.