title: Structurally and magnetically complex materials: a magneto-caloric, a multiferroic skyrmion host and a ferrimagnetic semiconductor
Creators: name: Routledge, Kieran
Publisher: name: University of Liverpool Repository
Year: 2023

description: The study of structural subtleties and complications in novel materials is often neglected and instead their physical functionality is investigated directly. When this is done however, details in the crystal structure can be missed which have significant consequences for the physical properties of the material. This thesis aims to prove this with the use of investigations of three separate inorganic systems which have very different atomic and magnetic structure, and therefore functionality, from each other. These investigations include the synthesis of polycrystalline and single crystal samples, followed by characterisation with the use of diffraction, magnetisation and transport experiments, which are all introduced in Chapter 2. The most important concepts needed in the analysis of the collected data are explained in Chapter 3. Chapter 4 re-examines the structure of incommensurate composite ferrimagnetic semiconductor Ba1–pCr2Se4–p, finding that a single crystal sample has a modulation vector of q = (0,0,0.24144) partly characterised by crenel occupational modulations, creating Se2− dimers. It also has large disorder in its sixfold channel which is modulated in its basal plane with q ≈ (1/3,1/3,0), although chemical composition is found to vary greatly between samples. The magnetic structure has been determined to have P6/m symmetry with k = (0,0,0) with a total moment of 0.413 μB/Cr3+. There is uniaxial negative thermal expansion along the c axis and a previously undiscovered structural transition at 135 K is observed. It is also determined that Ba1–pCr2Se4–p is a p-type semiconductor with a high resistivity owed to hopping of the charge carriers but with very low thermal conductivity. Chapter 5 aims to ascertain the cause of the phase coexistence in the lacunar spinel GaMo4Se8 below its Jahn-Teller transition. It is found that the strain of the dominant R3m phase is large and so is relieved by a partial distortion to the metastable Imm2 phase, which is determined to likely be insulating with charge-ordering of ions, in contrast to the R3m phase which is likely itinerant and charge-disordered. Single domain crystals are grown, on which magnetisation measurements determine the critical behaviour at the Curie temperature, with critical exponents of β = 0.22(4), γ = 1.19(1) and δ = 6.42(1) not belonging to any one universality class. Magnetoentropic mapping confirms the existence of at least three different non-collinear magnetic phases below the Curie temperature, including a Néel-type skyrmion lattice. Chapter 6 scrutinises the PbFCl-like structure and disorder of magneto-caloric candidate material MnZnSb. In some powder diffraction patterns, there are unusual, asymmetric peak shapes that are fit with a reduction of symmetry to a triclinic cell. In another sample, it is apparent that Rietveld refinements are greatly improved with the inclusion of a partially occupied, Zn containing interstitial site. In variable temperature data, the cell volume changes significantly at the near room temperature magnetic transition, which is identified as a big contributor to the large change in entropy there. It is also theorised that due to differences in diffraction data and results from magneto-caloric calculations, that samples first exist in a metastable triclinic state which then becomes the tetragonal state over time.