Chemical tuning of topological materials
Keywords: Layered Chalcogenides
Superconductivity
Intercalation
Magnetism
Solid-state Chemistry

This DPhil project is titled the ‘Chemical Tuning of Topological Materials’ in accordance with the Leverhulme grant project, which funded this work. However, this DPhil has moved to cover a broader topic covering materials that are not strictly topological. In this thesis, three distinct systems are investigated, with the common theme being that they are layered chalcogenide materials.
Intercalation of alkali and alkaline earth metals into ZrSe3 is shown here to have a significant effect on the selenide dimer linkages. PXRD studies on K, Rb and Cs intercalations will be compared to a high-temperature synthesis variant previously reported, where it will be shown that different polymorphs are obtainable at low temperatures. For example, for KxZrSe3, previous authors reported a structure with an Immm space group when synthesised at high temperatures. However, using low-temperature soft chemistry allows access to the Cmc21 structure, and the opposite is true for CsxZrSe3. The structures are related to each other by the relative shifting of the ZrSe3 layers. The intercalation of Ca using liquid ammonia is shown here to facilitate the co-intercalation of the solvent. This resulted in severe stacking faults, which are modelled in this thesis.
The Nb2PdxCh5 (Ch = S, Se) is a family of superconductors. Nb2Pd0.74S5 (Tc of 6.5 K) is intercalated with Li/NH3 or electrochemically to produce a phase with expanded lattice parameters. The structure maintains the C2/m symmetry and rigidity due to the PdS4 units linking the layers. Experimental and computational analysis showed that Li occupies triangular prismatic sites between the layers with an occupancy of 0.33(4). This level of intercalation suppresses the superconductivity. Deintercalation using I2 is shown to partially restore the superconductivity. Electrochemical intercalation reproduces the chemical intercalation product at low Li content (< 0.4), but at higher Li contents (≥ 0.4) accessed by this route, phase separation occurs with the indication that Li occupies another site. Similar results are found for the Nb2Pd0.82Se5 superconductor (Tc of 4.5 K). However, for the selenide, PND was not sufficient to locate Li and the combination of Li NMR and DFT suggest Li may occupy the octahedral site with ICP-MS suggesting an occupancy of 0.87.
The final system investigated is A2Ga2S5 (A = Mn, Fe), consisting of a buckled-honeycomb lattice which, in theory, could allow long-range magnetic ordering. However, the spin-glass behaviour of these systems using SQUID magnetometry will be shown, along with the absence of any sharp magnetic peaks using PND. A broad Warren-like peak is observed and can be modelled, indicating 2D magnetic behaviour with a very short correlation length. The diffuse peak will also be modelled using Monte Carlo simulations. The solid-solution Mn2-xFexGa2S5 is synthesised here for the first time, and the transition from the 1T (P-3m1) polytype to the 3R (R-3m) polytype is seen. The glassy behaviour remains, and an increase in the spin-freezing temperature with x is shown.