Structure and properties of some triangular lattice materials

Solid state chemistry;Ferromagnetic materials;Ferroelectric crystals

Downie, Lewis James ; Lightfoot, Philip ; Lightfoot, Philip

University:University of St Andrews

Department:Chemistry (School of)

关键词: Solid state chemistry;    Ferromagnetic materials;    Ferroelectric crystals;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/4423/LewisDowniePhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

This thesis is concerned with the study of two families of materials which contain magnetically frustrated triangular lattices.Each material is concerned with a different use; the first, analogues of YMnO₃, is from a family of materials called multiferroics, the second, A₂MCu₃F₁₂ (where A = Rb¹⁺, Cs¹⁺, M = Zr⁴⁺, Sn⁴⁺, Ti⁴⁺, Hf⁴⁺), are materials which are of interest due to their potentially unusual magnetic properties deriving from a highly frustrated Cu²⁺-based kagome lattice. YFeO₃, YbFeO₃ and InFeO₃ have been synthesised as their hexagonal polymorphs.YFeO₃ and YbFeO₃ have been studied in depth by neutron powder diffraction, A.C. impedance spectroscopy, Mössbauer spectroscopy and magnetometry.It was found that YFeO₃ and YbFeO₃ are structurally similar to hexagonal YMnO₃ but there is evidence for a subtle phase separation in each case.Low temperature magnetic properties are also reported, and subtle correlations between the structural, electrical and magnetic properties of these materials have been found.InFeO₃ was found to adopt a higher symmetry and is structurally similar to the high temperature phase of YMnO₃.TbInO₃ and DyInO₃ have also been synthesised and studied at various temperatures.The phase behaviour of TbInO₃ was analysed in detail using neutron powder diffraction and internal structural changes versus temperature were mapped out – there is also evidence for a subtle isosymmetric phase transition.Neither DyInO₃ nor TbInO₃ show long-range magnetic order between 2 and 300 K, or any signs of ferroelectricity at room temperature.The new compounds Cs₂TiCu₃F₁₂ and Rb₂TiCu₃F₁₂ have both been synthesised and shown to be novel kagome lattice based materials.The former shows a transition from rhombohedral to monoclinic symmetry in the powder form and from rhombohedral to a larger rhombohedral unit cell in the single crystal – a particle size based transition pathway is suggested.For Rb₂TiCu₃F₁₂ a complex triclinic unit cell is found, which distorts with lowering temperature.Both materials show magnetic transitions with lowering temperature.The solid solution Cs₂₋ₓRbₓSnCu₃F₁₂ (x = 0, 0.5, 1.0, 1.5, 2.0) was synthesised and investigated crystallographically, demonstrating a range of behaviours.Rb₂SnCu₃F₁₂ displays a rare re-entrant structural phase transition. In contrast, Cs₀.₅Rb₁.₅SnCu₃F₁₂ shows only the first transition found in the Rb⁺ end member. CsRbSnCu₃F₁₂ adopts a lower symmetry at both room temperature and below.Cs₁.₅Rb₀.₅SnCu₃F₁₂ and Cs₂SnCu₃F₁₂ show a rhombohedral - monoclinic transition, which is similar to that found in Cs₂TiCu₃F₁₂.