The Interplay of Lattice, Spin, and Charge Degrees of Freedom in Layered Cobaltates

In this thesis the structural and magnetic properties of the single-layered perovskite La(2-x)Sr(x)CoO(4) were investigated by means of x-ray diffraction and neutron scattering over a wide doping (0.05 <= x <= 1.1) and temperature (1.5 <= T <= 1000 K) range. The interplay between structural distortions, antiferromagnetic spin correlations, and charge order was studied in detail and is discussed in comparison with the closely related compounds, La(2-x)Sr(x)CuO(4+delta) and La(2-x)Sr(x)NiO(4+delta). The so-called low temperature orthorhombic (LTO) distortion of the tetragonal (HTT) K(2)NiF(4)-type structure is lifted at x_LTO ~ 0.25 Sr doping at room temperature. It is shown that the variation of the lattice constants near the HTT/LTO phase boundary is due to steric effects, i.e. induced by a cooperative tilt of the corner-sharing CoO_6 octahedra. At higher doping the HTT structure is stabilized and the lattice volume decreases according to chemical pressure by the substitution of La(3+) ions with Sr(2+) ions and the removal of charge carriers from the CoO_2 layers. While the La coordination anisotropy is smoothly reduced with doping, the Co-O_apical bond-length exhibits a steep decrease near x ~ 0.45. This decrease cannot be attributed to the reduction of structural strains in the system, but to an electronic origin. The decrease suggests a Co(3+) spin-state transition from the high-spin to the low-spin state at room temperature with doping. A Co(3+) low-spin state in La(1.5)Sr(0.5)CoO(4) at room temperature is supported by the anomalous increase of the out-of-plane and the nearly constant in-plane lattice constant at higher temperatures. The quantitative analysis of the checkerboard-like charge order in La(1.5)Sr(0.5)CoO(4) on a large set of superstructure reflections reveals a oxygen-displacement modulation of the CoO_6 octahedra, which is not Jahn-Teller active. This is in contrast to the oxygen-displacement modulation and the Co(3+) intermediate-spin occupation, previously proposed in the literature. Commensurate, antiferromagnetic nearest-neighbor exchange between Co spins with a spin moment within the CoO_2 layer is found in the spin-glass phase up to x = 0.3. This doping level is below the site-percolation threshold (x_c ~ 0.41) on a square lattice. The nearest-neighbor exchange, similar to the parent compound La(2)CoO(4), is stabilized by a Co(3+) high-spin state in the insulating phase or the reduction of the hole mobility. Further Sr doping induces a short-ranged incommensurate magnetic order. The incommensurability increases nearly linear with increasing hole density and a maximum of the in-plane correlation length at half doping is observed. First indications are thereby found for a stripe-like spin arrangement in single-layered cobaltates with 0.4 <= x <= 0.6. An effective Co(2+) next-nearest-neighbor exchange and a weak interlayer coupling stabilize the spin order in La(1.5)Sr(0.5)CoO(4)