Stefan Riegg
Relationship between composition, crystal structure, and physical properties in perovskite-related ruthenates
Supervisor: Prof. Dr. Alois Loidl [Experimental physics V]
Date of oral examination: 01/30/2014
289 pages, english
For this work a considerable number of pure and substituted (rare-earth metals, Ti, and Mn) La2RuO5 samples with an unconventional layered perovskite-related structure were synthesized by a new soft-chemistry approach. Besides the crystal structure investigated by x-ray and neutron diffraction as well as x-ray absorption spectroscopy, the magneto-structural phase transition was studied in detail by measurements of the magnetic susceptibility and specific heat. Below the transition temperature T_d = 161 K a rare spin-singlet ground-state was identified consisting of Ru4+ (S = 1) spin dimers, which can be imagined as the rungs of a ladder-like arrangement parallel to the crystallographic c-axis. In this low-temperature state a spin gap of approximately 48 meV opens as derived from the magnetic susceptibility. This was corroborated by modelling the entropy changes of the transition observed as peak in the specific heat using the intradimer exchange J_0 = 23.5 meV. Furthermore, it was found that rare-earth substitution merely influences the structure rather than the magnetic properties, while titanium suppresses both the structural and magnetic phase transition when almost half of the ruthenium becomes replaced. Manganese substitution leads to a second magnetic phase transition at lower temperatures than the dimerization. It was characterized as spin-glass state but the type of exchange between Mn4+ (S = 3/2) and Ru4+ (S = 1) driving this spin-glass state within the dimerized Ru matrix has still to be clarified. Polycrystalline samples of a second group of ruthenates with a 2x2x2 - perovskite superstructure (ACu3Ru{x}Ti{4-x}O12}) were also synthesized as solid-solution series with A = La, Pr, Nd. The complex phase diagram depending on the Ru content was studied in detail. Pure titanates (known for their colossal dielectric permittivities) show an antiferromagnetically ordered ground-state below 25~K even remaining for small Ru doping. For 0.25 < x < 2.25$ a spin-glass phase was found, while above x = 2.25 a heavy-fermion phase with a behavior characterized by a disturbed Kondo-lattice was identified. Concomitant with this a metal to insulator was found at roughly x = 2.25 with decreasing Ru content. The combination of detailed XANES and thermogravimetric investigations revealed a constant Ru4+ valence while the Cu valence decreases linearly from +2 for the titanates to +1.6 for the ruthenates. In addition, ESR measurements and DFT calculations were performed allowing to conclude that the Cu 3d electrons become itinerant and interact Kondo-like with the localized Ru 4d electrons, resulting in the heavy-fermion state.