| Summary: | <p>The work presented in this thesis focuses on the topochemical reduction of Ir-containing oxides with perovskite-type structures to form novel anion-deficient and mixed-anion phases.</p>
<br>
<p>Reactions between <em>n</em> = 1 Ruddlesden-Popper La<sub><em>x</em></sub>Sr<sub>4−<em>x</em></sub>CoIrO<sub>8</sub> (0 ≤ <em>x</em> ≤ 2) phases and LiH yields the formation of a series of oxyhydride phases, which represent the first oxyhydrides containing a 5d transition metal. For the Sr-rich members of the series (<em>x</em> = 0, 0.5), the extent of oxide-to-hydride exchange is limited by the stability of the overall Ruddlesden-Popper framework and, therefore, the temperature at which the reaction can be performed. Meanwhile, the extent of anion exchange in the remaining members of the series is limited by the formation of the Co<sup>I</sup>/Ir<sup>III</sup> oxidation state combination, as reduction of octahedrally-coordinated iridium below Ir<sup>III</sup> would require occupation of a strongly antibonding <em>e</em><sub>g</sub> orbital.</p>
<br>
<p>Similarly, reaction of LaSr<sub>3</sub>NiIrO<sub>8</sub> with LiH yields the formation of LaSr<sub>3</sub>NiIrO<sub>5.4</sub>H<sub>2.6</sub>, which contains nickel as Ni<sup>I/II</sup> and iridium as Ir<sup>III</sup>. Whilst iridium is present in its lowest energetically accessible oxidation state, it is not possible to fully reduce nickel to Ni<sup>I</sup> without the onset of non-topochemical decomposition. In an attempt to prepare an analogous anion-deficient phase, LaSr<sub>3</sub>NiIrO<sub>8</sub> was reacted with a Zr getter, which yielded LaSr<sub>3</sub>NiIrO<sub>6.4</sub>. This adopts an infinite-chain type structure, which contains Ir<sup>II</sup> that is likely to be disproportionated into Ir<sup>I</sup> and Ir<sup>III</sup>.</p>
<br>
<p>Reaction of Sr<sub>4</sub>MnIrO<sub>8</sub> with CaH<sub>2</sub> and LiH yields the oxyhydride phases, Sr<sub>4</sub>MnIrO<sub>6.58</sub>H<sub>1.42</sub> and Sr<sub>4</sub>MnIrO<sub>5.32</sub>H<sub>2.68</sub>, respectively. Whilst the hydride ions are exclusively located at the equatorial positions in Sr<sub>4</sub>MnIrO<sub>6.58</sub>H<sub>1.42</sub>, consistent with the other phases discussed in this thesis, hydride ions are also located at the axial positions in Sr<sub>4</sub>MnIrO<sub>5.32</sub>H<sub>2.68</sub>. This is attributed to a combination of factors, including the strong Jahn-Teller effect associated with Mn<sup>III</sup>, the tendency for hydride ions to adopt <em>cis</em>-arrangements with respect to each other and the preference for iridium to adopt highly symmetric coordination environments.</p>
<br>
<p>To explore the reactivity of other Ir-containing phases with LiH, reactions were performed using the 6H-hexagonal perovskite phase, Ba<sub>3</sub>CoIr<sub>2</sub>O<sub>9</sub>, and the double perovskite La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>6</sub> (<em>x</em> = 0, 1) phases. This yielded the oxyhydride phases, Ba<sub>3</sub>CoIr<sub>2</sub>O<sub>6.2</sub>H<sub>2.8</sub> and La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>4.4</sub>H<sub>1.6</sub>, in which the extent of anion exchange is limited by the stability of the perovskite frameworks, rather than the obtainment of the Co<sup>I</sup>/Ir<sup>III</sup> oxidation state combination. Whilst the La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>4.4</sub>H<sub>1.6</sub> phases adopt anion-disordered arrangements, Ba<sub>3</sub>CoIr<sub>2</sub>O<sub>6.2</sub>H<sub>2.8</sub> adopts a partially anion-ordered arrangement, in which the hydride ions are preferentially located at the corner-sharing sites and adopt a <em>trans</em>-arrangement with respect to each other to optimise the π-bonding interactions.</p>
<br>
<p>To investigate the reactivity of the La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>6</sub> phases further, they were heated in the presence of a Zr getter. This led to the formation of La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>4</sub> infinite-layer phases, which occurs via La<sub><em>x</em></sub>Sr<sub>2−<em>x</em></sub>CoIrO<sub>5</sub> intermediates. Below 135−140 K, Sr<sub>2</sub>CoIrO<sub>4</sub> adopts a ‘type I’ antiferromagnetic state at the cobalt sites, with very little ordering at the iridium sites. Meanwhile, LaSrCoIrO<sub>4</sub> adopts a magnetic structure based on C-type antiferromagnetic ordering with a possible incommensurate modulation that could arise from spin canting.</p>
<br>
<p>Finally, the effect of ball-milling on the reactivity of the perovskite phase, La<sub>2</sub>CoRhO<sub>6</sub>, was investigated. Whilst ball-milling showed no enhancement in reactivity for the reaction between La<sub>2</sub>CoRhO<sub>6</sub> and NaH, reaction of the ball-milled sample with LiH allowed a greater extent of oxide-to-hydride exchange to be achieved. This was attributed to the fact that the oxyhydride product arising from the reaction with LiH is intrinsically highly strained, so ball-milling destabilises the starting material to a greater extent than the oxyhydride product.</p>
|
|---|