Synthesis and characterisation of metal oxides isostructural with La(18)Li(8)Rh(5)O(39)

The structural chemistry and magnetic properties of compounds isostructural with La18Li8Rh5O39 have been studied. In this cubic structure (space group Pm_3n), [111] chains of alternating RhO6 octahedra (2a, 8e sites) and LiO6 trigonal prisms (16i site) occupy channels within a La-O framework. In order to study the consequences of both reducing the dimensions of the channels and introducing paramagnetic cations into both the framework and the polyhedral chains, compositions in the series Nd18Li8Rh5-xFexO39 (1 ≤ x ≤ 4) have been synthesised and characterised by magnetometry, neutron diffraction and Mössbauer spectroscopy. Iron preferentially occupies the 2a site as low-spin Fe(IV) and the larger 8e site as high-spin Fe(III). Compositions having x > 1 show spin-glass-like behaviour below ~5 K. The behaviour of this series of compositions has been compared with that of the known series La18Li8Rh5-xFexO39. In order to facilitate this comparison, the structure of La18Li8Fe5O39 has been fully characterised by both neutron diffraction and EXAFS. The use of both techniques has revealed differences between the mean and local environments of iron.
In order to explore and extend the range and combinations of elements that can be accommodated in this structure, the Ti-containing compounds Ln18Li8M4TiO39 (Ln=La,Nd,Pr,Sm; M=Rh,Fe,Co) have been synthesised. Nd18Li8Fe4TiO39 was selected for a detailed study by magnetometry, neutron diffraction, Mössbauer spectroscopy and XANES. Cations were found to be stabilised in unusual oxidation states and disordered over three sites of the Nd18Li8Fe4TiO39 polyhedral chains. The 8e site is occupied by high-spin Fe3+, Ti3+ and Li+ in a ratio of 76:20:4; the 2a site by low-spin Fe4+ and Ti4+ in a ratio of 79:21 and the trigonal-prismatic 16i site by Li+ and Fe3+ in a ratio of 98:2. Nd18Li8Fe4TiO39 undergoes a transition to a spin-glass state at 4.25(5) K, whereas La18Li8Fe4TiO39 revealed a different type of magnetic transition at ~8 K. The nature of this transition is not yet clear. Monophasic samples could not be prepared in the Nd/Rh system and cation vacancies were found in Nd/Co. No pure samples of Sm-containing compositions could be prepared, while the only Pr composition which was obtained pure, according to X-ray diffraction, Pr18Li8Co4TiO 39 shows the similar magnetic properties to Pr18Li8Co3TiO39.
The compositions Ln18Li8M3 M'O39 (Ln=La,Nd,Pr,Sm; M,M'=Fe,Co,Ti) with 25% of the 8e sites vacant have been investigated. The Co-containing compositions Nd18Li8Co3FeO39-y, Nd18Li8CoFe3O39-y and Nd18Li8Co3TiO39-y were characterised by neutron diffraction. Cation vacancies on the 8e sites were found to coexist with anion vacancies around the 2a sites. The remaining octahedral sites are occupied by a disordered arrangement of transition-metal cations. The trigonal-prismatic sites are fully occupied by Li except in the case of Nd18Li8CoFe3O39-y where some Fe is present, as confirmed by Mössbauer spectral data. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.
The magnetic character of the Ln cations plays an important role in determining the properties of these compounds. Long-range magnetic order of the transition-metal cations was not observed in any compositions. Although this might be partly attributable to the cation disorder in the polyhedral chains, the apparent antiferromagnetic behaviour of Nd-containing compositions in which the transition metals have a low magnetic moment, e.g. Nd18Li8Co3TiO39-y, and the marked contrast between certain La and Nd compositions in which the transition metal content is the same, e.g. Nd18Li8Fe4TiO39 and La18Li8Fe4TiO39, suggests that the Ln sublattice is fully involved in determining the magnetic behaviour.