The thesis focuses on frustrated systems of mixed-valence transitional metal oxides. That is to say, on systems, the symmetry of which leads to the development of competing interactions that can hardly minimize their energy simultaneously. The main mixed-valence transition metal is Mn. We exploit the capability of multi-valence state and synthesise compounds with multi coordination numbers. In its oxides the structure is composed of Mn³⁺O₆ octahedral units with common edges or vertices, forming prototype porous complexes. Their microporous and layered structure further intensifies the geometric frustration, while their spin-lattice interactions become more intriguing, when alkali metal cations get intercalated in it. The derived compounds belong to the generic group of delafossites, A⁺M³⁺O₂.

     The parent delafossite is α-NaMnO₂, the lattice of which consists of MnO₂ layers with intercalated Na⁺ cations. It is further studied, regarding the crystal structure stability and the magnetic properties of its non-stoichiometric compounds, the phase transition, its structural changes employing synchrotron radiation and microscopic magnetic excitations by inelastic neutrons scattering. The sub-stoichiometric compounds are two-phase, while α-NaMnO₂ exhibits a spin-gap at Τ≤Τ_Ν=45 Κ, the magnetic interactions are mainly developed along b-axis and the spin-fluctuations are one-dimensional, along the same axis. Additionally, the isostructural CuMnO₂ is studied, in the structure of which the intercalant cations have been replaced by Cu⁺. It has been synthesized under a new synthesis protocol and characterised, with respect to its thermal stability and magnetic properties. Moreover, its crystal and magnetic structure have been analysed by synchrotron and neutron powder diffraction, respectively. It demonstrates a phase transition from monoclinic to triclinic at T_N=68 Κ with large structural changes, while it also has a spin-gap at Τ≤Τ_Ν. Eventually, the two delafossites are compared. Furthermore, there have been attempts to synthesise and study the complementary compounds: the β-NaMnO₂ polymorph, the NaVO₂ homologues system and the K_0.125MnO₂ analogue. The goal is the comparison of their structures and the comprehension of the correlation between the magnet ostructural mechanisms and their physicochemical properties.

 

KEYWORDS

Metal oxides, crystal / magnetic structure, Χ-ray diffraction / neutron scattering, transition metals, frustration, microporous / layered structure, magnetoelectronic properties.