Universität zu Köln

Schilling, Henrik ORCID: 0009-0008-6536-5509 (2025). Crystal growth and characterization of 4d and 5d transition metal compounds. PhD thesis, Universität zu Köln.

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Abstract

In this work, both the flux method for growing single crystals from the melt and the chemical transport reaction method for growing crystals from the gas phase were employed. The flux method was applied to the crystal growth of quaternary perovskite compounds of the types Ba3MM’2O9 and Ba4MM’3O12. In these structure types, the metal atoms M and M′ occupy distinct crystallographic sites, both in octahedral coordination. The M′ atoms form face-sharing octahedral units: [M’2O9]-dimers in Ba3MM’2O9 and [M’3O12]-trimers in Ba4MM’3O12. These structural units are corner-connected via [MO6]-octahedra, resulting in isolated cluster motifs. Due to the short metal–metal distances within the [M’2O9]-dimers and [M’3O12]-trimers, the formation of molecular orbitals is possible. Therefore, these compounds are classified as "cluster materials". In this study, the clusters were occupied by either ruthenium or iridium, since these 4d and 5d transition metals exhibit strong spin–orbit coupling due to their high atomic numbers, leading to pronounced splitting of the valence electron levels. The interaction between molecular orbitals and spin–orbit coupling can give rise to novel electronic configurations, making these materials highly promising candidates in the field of quantum materials. The oxidation states of Ir and Ru are derived from the charge neutrality condition of the compound and are predominantly dictated by the valence of the M-site cation, with Ba2+ and O2- acting as constant components. This enables for targeted tuning of the electron count per dimer or trimer cluster, by selective substitution of differently valent elements on the M-site. Due to the limited understanding of the phase diagrams of such complex systems, experimental data were collected to identify suitable synthesis conditions. The effect of the BaCl2 flux on the crystal growth process was investigated, and parameters such as temperature, crucible material and chemical composition were optimized. As a result, several iridium- and ruthenium-based single crystals of Ba3MM’2O9 and Ba4MM’3O12 types were synthesized. Following optimization of the growth parameters, thermal, chemical and structural characterization of the synthesized crystals was carried out. It was found that the ruthenates exhibit greater thermal stability at elevated temperatures compared to their iridium analogues. Chemically, all compounds matched the target stoichiometry, with the exception of systems containing M = Nb and Ta, where partial substitution by Ir and Ru, respectively, was observed. Structural investigations using single-crystal X-ray diffraction were carried out on Ba3MRu2O9 (with M = Ce, Eu, Tb, Yb and Y), Ba4MRu3O12 (with M = Ce and Ta) and Ba4TaIr3O12. For Ba3MRu2O9 with M = Eu and Tb, potential structural distortions and symmetry reductions from P63/mmc to P63mc were observed. For all other compounds of this structure type, the hexagonal symmetry P63/mmc could be confirmed. Ba4TaRu3O12 crystallizes in space group R-3m, is isostructural with Ba4NbRu3O12, and exhibits Ru–Ta substitution. In contrast, the iridium analogue Ba4TaIr3O12 shows no such substitution. Ba4CeRu3O12 is monoclinically distorted and crystallizes in space group C2/m. Other synthesized trimer-type compounds with M = rare-earth elements showed severe structural disorder along the c-axis. These stacking faults hindered the indexing of the single-crystal data. In the final part of this work, the crystal growth of Ni2InSbO6 was investigated using the CTR method in combination with a transport balance to measure transport rates. The experimentally determined rates were compared with theoretically calculated values in order to enable controlled crystal growth. The results revealed that the chemical composition of the grown crystals depends on the transport rate and the partial vapor pressures of the starting materials.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Schilling, Henrik henrik.schilling@uni-koeln.de orcid.org/0009-0008-6536-5509 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-789800
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute of Geology and Mineralog
Subjects:	Physics Earth sciences
Uncontrolled Keywords:	Keywords Language cluster materials English crystal growth English single crystal English trimer compounds English dimer compounds English
Date of oral exam:	18 September 2025
Referee:	Name Academic Title Becker-Bohaty, Petra Prof. Dr. Lorenz, Thomas Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/78980