Synthesis and characterisation of metal oxides isostructural with La₁₈Li₈Rh₅O₃₉

Description

The structural chemistry and magnetic properties of compounds isostructural with La₁₈Li₈Rh₅O₃₉ have been studied. In this cubic structure (space group Pm^{^_}<sub style='position: relative; left: -.5em;'>3</sub>n), [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 Nd₁₈Li₈Rh_5-xFe_xO₃₉ (1 &le; x &le; 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 La₁₈Li₈Rh_5-xFe_xO₃₉. 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 Nd₁₈Li₈Co₃FeO_39-y, Nd₁₈Li₈CoFe₃O_39-y and Nd₁₈Li₈Co₃TiO_39-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 Nd₁₈Li₈CoFe₃O_39-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. Nd₁₈Li₈Co₃TiO_39-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.

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546.6

Additional Fields

Identifer	oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:589762
Date	January 2012
Creators	Thammajak, Nirawat
Contributors	Battle, Peter D.
Publisher	University of Oxford
Source Sets	Ethos UK
Detected Language	English
Type	Electronic Thesis or Dissertation
Source	http://ora.ox.ac.uk/objects/uuid:5a97db26-edb3-45e0-972d-e516da0384d3