Synthesis, structure and properties of carboxylate coordination polymers

Abstract

The work in this thesis focuses on several coordination polymers, their crystal structures and their potential applications as functional materials, in particular, their ferroelectric and magnetic properties.

The mechanism of the known ferroelectric phase transition of (C₃NO₂H₇)₃CaCl₂ (TSCC) was studied in detail using both X-ray and neutron diffraction data. The key structural changes occurring in the vicinity of TC were identified in the Ca-O-C bond angles, which change by up to 12˚. The structure of the compound was determined between 300 and 20 K to investigate potential phase transitions as suggested in the literature. The diffraction data did not suggest an additional phase transition within this temperature range. However, the detailed crystallographic studies did uncover new evidence of ferrielectricity, which is consistent with electrical hysteresis data.

The origins of the ferroelectricity of TSCC were also probed through the synthesis and characterisation of a range TSCC-related compounds which incorporated a range of alternative halides and metal 2+ cations. Each compound formed coordination polymers which were linked by sarcosine molecules, although the connectivity and structures formed were dependent upon the composition. Whilst none of the TSCC derivatives had ferroelectric properties, diffraction experiments were limited to a low temperature of 90 K and there could be additional phase transitions below this point.

A family of oxalate-based coordination polymers ALi₃M(C₂O₄)₃ with a perovskite-like structures were also synthesised. The structural flexibility of these systems was investigated through the incorporation of different A site cations and their magnetic properties were investigated. Polyhedral distortions increase significantly upon the inclusion of rubidium or caesium. Both KLi₃Co(C₂O₄)₃ and KLi₃Ni(C₂O₄)₃ have weak antiferromagnetic exchange and μ[sub](eff) of 4.82 μ[sub]B and 3.62 μ[sub]B, respectively. The syntheses and structures of four new oxalate containing compounds, namely Rb₂Co(C₂O₄)₂.4H₂O, Rb₂CoCl₂(C₂O₄), K₂Li₂Cu(C₂O₄)₃ and Li₄Co(C₂O₄)₃, which could have interesting magnetic and electrochemical properties, are also reported.