Topochemical reactions of ruthenium and rhodium containing oxide phases

This work is focussed on the topochemical reactions of Ru-containing and Rh-containing n = 1 Ruddlesden-Popper phases by using CaH2 as a reducing agent, to form oxide-hydride phases, or finely divided Zr powder as an oxygen getter, to form anion-deficient reduced oxide phases. The chemical doping of LaSr3NiRuO8 and LaSr3NiRuO4H4 is also been investigated.
Building on the work of the first Ru(II)-containing cation-ordered infinite layer phase LaSrNiRuO4 prepared via topochemical reduction of its parent double perovskite LaSrNiRuO6 by using CaH2 as a reducing agent, the n = 1 Ruddlesden-Popper equivalent LaSr3NiRuO8 was prepared via traditional ceramic synthesis and its topochemical reactions were well-investigated. The initial screening for the reactivity of LaSr3NiRuO8 was carried out by treating it with CaH2 in sealed silica ampoules and heated in the temperature range 350 ≤ T/ ºC ≤ 460. A number of distinct phases LaSr3NiRuO7, LaSr3NiRuO6 and LaSr3NiRuO4H4 were observed before non-topochemical decomposition of the starting material occurred. However, it was impossible to isolate the first two reduced oxide phases in this case as the presence of the by-product H2 gas strongly aided the formation of the oxide-hydride phase. LaSr3NiRuO4H4 preserves the highly symmetric structure of its parent LaSr3NiRuO8 oxide phase, a body-centred tetragonal unit cell, with all oxide ions which originally residing in ‘equatorial’ anion sites being completely substituted by hydride ions. The measured structural and magnetic data, along with the computational study, are consistent with the presence of transition metal ions in remarkably low oxidation states (Ru2+ and Ni+) in an extended solid and show no evidence of long-range magnetic ordering down to a temperature of 1.8 K. Since both LaSr3NiRuO8 and LaSr3NiRuO4H4 are antiferromagnetic insulators, doping chemistry of them was investigated by manipulating the ratio between La and Sr. Structural analysis shows that doped materials are structurally similar to undoped ones, however, magnetic behaviour of doped phases is significantly different from undoped ones. Magnetisation data collected from doped phases suggest that electrons of the B-site cations are no longer localised once the ratio between La and Sr deviates from 1:3.
The successful isolation of the previously mentioned reduced oxide phase, LaSr3NiRuO6, was achieved by treating LaSr3NiRuO8 with Zr powder in a getter, as there was no source of H- ions to form the competing oxide-hydride phase. LaSr3NiRuO6 adopts a body-centred orthorhombic structure, with ordered oxide vacancies along b-axis. The low temperature neutron powder diffraction data does not show any additional magnetic diffraction features, despite magnetisation data show strong evidence of a ferromagnetic ground state consisting of a d9, S = ½ Ni1+ centre and a d6, Ru2+ centre with two unpaired electrons (S = 1).
The topochemical reduction chemistry discussed above can be extended to the Co analogue LaSr3CoRuO8. Although both the reduced oxide phase LaSr3CoRuO6 and the oxide-hydride phase LaSr3CoRuO4H4 adopt the same crystal structures as their corresponding Ni analogues, statistical and visual fits of refinements are worse in Co cases due to the presence of stacking faults (n = 2, 3 and 4 Ruddlesden-Popper phases and separate perovskite blocks) across lattices.
Analogous chemistry was explored for another 4d transition-metal, rhodium. Data collected from these samples revealed La2Sr2CoRhO8 and LaSr3MnRhO8 had been converted into the oxide-hydride phases La2Sr2CoRhO6H2 and LaSr3MnRhO6H2 respectively, in which 50% of the oxide ions in the ‘equatorial’ anion sites have been exchanged for hydride anions, converting the –AO–BO2–AO– stacking sequence of the oxides into an –AO–B(O0.5H0.5)2–AO– sequence in the oxide-hydrides, with the oxide and hydride anions adopting a disordered configuration within the B(O0.5H0.5)2 sheets.

Advisor: Hayward, M