X-ray and neutron scattering studies of unconventional spin-Peierls systems

TiOCl and TiOBr belong to a select group of low-dimensional magnetic systems which exhibit a spin-Peierls phase transition. The spin-Peierls transition arises in materials with quasi-one-dimensional (or chain-like) physical properties, which possess an unusual combination of spin 1/2 magnetic moments, short-range antiferromagnetic interactions, and strong spin-lattice coupling. This transition is magnetoelastic in nature, and is characterized by the dimerization of quasi-one-dimensional spin chains and the formation of a non-magnetic singlet (S = 0) ground state. Very few materials have been found to undergo a spin-Peierls phase transition, and TiOCl and TiOBr are among only three known inorganic spin-Peierls compounds. These compounds have recently attracted particular attention, as they have been shown to differ from the standard spin-Peierls scenario in several important respects. Unlike conventional spin-Peierls systems, which undergo a single continuous phase transition, TiOCl and TiOBr exhibit two successive phase transitions upon cooling - a continuous transition from a uniform paramagnetic phase to an incommensurate spin-Peierls phase at TC 2, followed by a discontinuous transition into a commensurate spin-Peierls phase at TC1. In addition, TiOCl and TiOBr differ from conventional spin-Peierls systems due to their unusually high transition temperatures and the anomalously large energy gap which separates the singlet ground state from the first triplet (S = 1) excited state. This thesis consists of a series of x-ray and neutron scattering experiments on these two unconventional spin-Peierls compounds.;X-ray measurements were performed on the doped compound Ti1- xScxOCl to study the effect of quenched non-magnetic impurities on the spin-Peierls ground state. These measurements show that the spin-Peierls behaviour of TiOCl is extremely sensitive to doping, and prove that even Sc concentrations of ∼ 1 % are sufficient to eliminate commensurate fluctuations and suppress commensurate spin-Peierls order down to the lowest measured temperatures. There is evidence that Sc-doping may also result in changes to the critical properties of the phase transition at TC2.;X-ray scattering measurements were performed on TiOBr in order to carry out a detailed study of the critical behaviour associated with the paramagnetic-to-incommensurate spin-Peierls phase transition at TC2. These measurements reveal that the critical properties of TiOBr are surprisingly conventional in nature, and can be well-described by the standard three-dimensional universality classes. In contrast to TiOCl, the low temperature phase diagram of TiOBr appears to contain no evidence of commensurate fluctuations.;Time-of-flight neutron scattering measurements were used to perform the first study of the magnetic excitation spectrum of TiOBr, providing the first direct measure of the singlet-triplet energy gap in this system. These measurements reveal n = 1 and n = 2 triplet excitations with relatively little dispersion, suggesting the presence of excited triplets which are well-localized and weakly-interacting in nature.;Lab-based x-ray scattering measurements were performed on TiOCl in order to study the properties of the low temperature phase diagram and examine the superlattice Bragg peaks which characterize the commensurate and incommensurate spin-Peierls phases. These measurements reveal the presence of commensurate fluctuations in the incommensurate spin-Peierls and ''pseudogap'' phases of TiOCl, and provide evidence of unusual competition and coexistence of commensurate and incommensurate states between TC1 and TC2.;Finally, synchrotron x-ray measurements were performed on TiOBr in pulsed magnetic fields of up to 30 Tesla. These measurements show that the structural and superlattice Bragg peaks exhibit no apparent field-dependence up to 30 Tesla, placing important constraints on the field-temperature phase diagram of this compound.