The synthesis, structure and physical properties of layered hybrid copper (II) halide perovskites
Creators: name: Han, Ceng
Publisher: name: The University of St Andrews
Year: 2023
Subjects: subject: Layered perovskite
subject: Copper halide perovskite
subject: Crystal structure
subject: Ferromagnetism
subject: Optical property
subject: QE391.P47H2

description: Two-dimensional (2D) organic-inorganic hybrid copper (II) halide perovskites have attracted tremendous attention as promising multifunctional materials in recent years. The work presented in this thesis focuses on the synthesis, characterisation and physical properties of 2D hybrid layered copper (II) halide perovskites. The influence of halogen substitution (F, Cl or Br) in the benzylamine structure at ortho-, meta- and para- positions in the series hybrid layered copper (II) chloride perovskites A₂CuCl₄ are explored in chapter 3. F-substitution at the ortho and para-positions results in two centrosymmetric structures (2/4-FbaH)₂CuCl₄, whereas (3-FbaH)₂CuCl₄ adopts a polar space group. The incorporation of heavier halogen atoms, Cl or Br at the same position on benzylamine, leads to essentially isostructural final structures in each case, centrosymmetric structures (2-Cl/BrbaH)₂CuCl₄ and polar structures (3/4-Cl/BrbaH)₂CuCl₄. SHG measurements confirm the noncentrosymmetric nature of the polar structures, while dielectric measurements reveal the existence of phase transitions to higher symmetry. All compounds display ferromagnetic interactions within inorganic layers at low temperature. The trends in ferromagnetism and optical properties can be rationalised based on trends in the distortions of the [CuCl₄]∞ layers. The effect of positional isomerism of the halogen atom on the crystal structures A₂CuBr₄ will be discussed in chapter 4. F-substitution at the meta and para- positions produces two polar structures (3/4-FbaH)₂CuBr₄. Cl or Br-substitution at ortho and para- positions results in isostructural final structures. However, (3-ClbaH)₂CuBr₄ and (3-BrbaH)₂CuBr₄ adopt a centrosymmetric space group P2₁/c and a polar Cc, respectively, which are also distinct from (3-FbaH)₂CuBr₄ with Cmc2₁ space group. The dielectric measurements reveal the presence of phase transitions to higher symmetry and magnetic measurements confirm the appearance of ferromagnetic ordering at low temperature. The ferromagnetic ordering temperature and optical band gaps are strongly correlated with the distortions of the [CuBr₄]∞ layers. Five structures of hybrid layered copper (II) chloride perovskites ACuCl₄ or A₂CuCl₄ are discussed in chapter 5. One DJ phase structure ACuCl₄, (m-PdH₂)CuCl₄, incorporating a cyclic diamine is introduced, and synchrotron X-ray powder diffraction data reveal that it displays an unusual thermal contraction of the b-axis upon heating, which is due to the repositioning of the organic cation (m-PdH₂)²⁺. Two structures (3-AbaH)₂CuCl₄ and (4-AbaH)₂CuCl₄, including aromatic amino acids, adopt a space group of P1 ̅ and C2/c, respectively. The last two structures, (CpaH)2CuCl4 and (CbaH)₂CuCl₄, incorporating smaller cyclic amines, C3 and C4, adopt the same space group C2/c. All compounds possess ferromagnetic interactions within inorganic layers. The differences among these structures in terms of templating organic moieties and magnetic properties of these compounds are also discussed in chapter 5.