Structural and Magnetic Properties of Low-Dimensional Ternary Chlorides

Hänni, Nora (2016). Structural and Magnetic Properties of Low-Dimensional Ternary Chlorides. (Dissertation, Departement für Chemie und Biochemie, Philosophisch-naturwissenschaftliche Fakultät, Universität Bern)

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Magnetism as a quantum phenomenon has received considerable attention not only in the past but also in the present. Low dimensionality of the crystal lattice on the one hand and small spins on the other hand enhance the quantum character of a system. This PhD work is concerned with the identification, synthesis, and evaluation of model spin system candidate compounds according to a given set of criteria with the aim to prepare a solid basis for follow-up investigations on their physical properties. Based on various experimental methods, mostly X-ray and neutron diffraction in combination with thermal and magnetic bulk property measurements, the structural and magnetic properties of the target compounds were probed and the results were evaluated in the context of the scientific questions of the three main projects.
First, a series of quasi-one-dimensional hexagonal perovskites of the AMCl3 (A = Rb, Cs; M = Co, Ti, Sc) family was investigated. The focus was set on their magnetic properties regarding the characteristics of the respective M2+ ion as well as the dimensionality of the observed magnetic interactions. RbCoCl3 turns out to be a good realization of a Ising spin chain in the paramagnetic region with short-ranged antiferromagnetic correlations along the chains. Below TN1 = 28 K it undergoes two subsequent magnetic ordering transitions. The complex magnetic structures were successfully solved in a series of neutron diffraction experiments. The Ti-based isostructural compounds, in contrast, do not show magnetic long-range order down to 2 K and short-ranged antiferromagnetic correlations persist. Both RbTiCl3 and CsTiCl3 are highly anisotropic regarding the dominant magnetic exchange interaction. No structural phase transitions are observed.
According to X-ray powder diffraction and magnetic bulk property measurements CsScCl3 is severely scandium deficient. Interestingly, the proposed homogeneity range between CsScCl3 and Cs3Sc2Cl9 was not confirmed despite intense synthetic efforts.
Second, a group of quasi-two-dimensional ternary chlorides of the A2MCl4 (A = Rb, Cs; M = Cr, Mn) family was synthesized in order to study the critical perturbation of a weak 4-fold crystal field in the basal plane as a function of the external magnetic field and temperature. Neutron diffraction investigations on the Heisenberg ferromagnet Rb2CrCl4 are intrinsically difficult due to forces experienced by the ferromagnetic sample in a magnetic field. Up to 0.4 T no trend is established for the field-dependent evolution of the critical exponent and further experimental work is required. However, based on our diffraction data we can confirm space group I4/mmm as we do not observe any distortions in contrast to some literature propositions. For the first time, a series of magnetic susceptibility measurements investigates the magnetic hysteresis below 300 and 2 K. The antiferromagnetic Cs2MnCl4 undergoes a structural phase transition around 300�C upon cooling and only powder samples can be obtained of the I4/mmm-phase. However, for the first time the high-temperature crystal structure was solved on a metastable single crystal. It has the space group Pnma. A preparative procedure to produce large high-quality single crystals vii for planned neutron scattering experiments was successfully established for the isostructural but incongruently melting antiferromagnet Rb2MnCl4.
Third, on the quest for kagome lattice type compounds, we identified the Na2M3Cl8 (M = Ti, Mn, Mg) family of incongruently melting ternary chlorides as promising candidates. The kagome type phase was investigated in a series of X-ray and neutron diffraction experiments. No magnetic long-range order is established in the Ti-based compound down liquid helium temperature.
Single crystal magnetic susceptibility measurements display short-ranged antiferromagnetic correlations and a pronounced anisotropy. It structurally distorts upon cooling and the symmetry of the kagome type phase is lost in two successive phase transitions. For the first time, our comprehensive temperature-dependent single crystal X-ray diffraction data allows to acknowledge the intermediate phase as a crystallographically distinct phase. A qualitative structural model is proposed. For Na2Mn3Cl8, the symmetry of the lattice is preserved at least down to 100 K. Also at lower temperatures no features of magnetic long-range order or structural distortions are observed in the magnetic bulk data. Thus, the Mn-based compound is a promising candidate for follow-up investigations on the magnetic excitations in a kagome system. In the non-magnetic Mg-based compound, finally, a non-kagome type high-temperature phase was identified and the crystal structure was successfully solved on a metastable single crystal sample. Such crystal structure is observed for the first time in a halide compound.
The experimental work was carried out between 2012 and 2016 at the Department of Chemistry and Biochemistry at the University of Bern, at the Paul Scherrer Institute, at the Institute Laue-Langevin, and at the University College London.

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