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Lead chalcogenide quaternary systems have been shown to provide high thermoelectric (TE) efficiency superior to those of binary andternary lead chalcogenides, arising from both altered electronic band structures and a reduction in lattice thermal conductivity. Here, we have synthesized single-phase samples of the quaternary compound (PbTe)0.55(PbS)0.1(PbSe)0.35 dopedwith Na and characterized their TE properties. We show that the dopant solubility is limited to 1 at. %. A very low lattice thermal conductivity of∼0.6 Wm−1K−1at 850 K is achieved at all dopant concentrations because of phonon scattering from point defects associated with solute atoms with high contrastatomic mass. As a result, a high TE figure of merit of approximately 1.5 is achieved at 823 K in heavily doped samples. Moreover, the figure of merit is greater than 1 over a wide temperature range above 675 K.

Magnetic moment measurements performed on a single crystal sample of (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy reveal an increase in magneticmoment at intermediate fields. Magnetic hysteresis (m-H) loops were taken at different constant temperatures using either a MPMS-5T SQUID magnetometer or a PPMS-9T magnetometer and within the temperature range 0.1 < T/Tc < 0.85. Vortex dynamics at fieldsbelow the peak in hysteresis loops for the sample is shown to be well explained by a quasi-lattice Bragg glass state and elastic collectiv evortex creep.

Magnetic moment measurements performed on a single crystal sample of (Tl,Pb)(Sr,Ba)[subscript 2]Ca[subscript 2]Cu[subscript 3]Oy reveal an increase in magnetic moment at intermediate and high fields over a wide temperature range below H[subscript c2] Magnetic hysteresis (m-H) loops were taken at different constant temperatures using either a MPMS-5T SQUID magnetometer or a PPMS-9T magnetometer and within the temperature range 0.1 < T/Tc < 0.85. The main objective of this thesis was to study the mechanisms involved in the emergence of the peak in the magnetic hysteresis loops in the two samples (the so-called peak effect). It is argued that point disorder in the form of oxygen vacancies or substitution is responsible for pinning at intermediate and high field. Vortex dynamics at fields below and above the peak in hysteresis loops for the sample are shown to be well explained by a quasi-lattice Bragg Glass state and elastic Collective and Plastic Vortex Creep, respectively. For this purpose, relaxation in magnetic moment was measured at different fields and temperatures. Following that, the field and temperature dependence of the normalized relaxation rate, S, the effective activation energy, U, and the critical exponent were calculated. Analysis of all these quantities in the material led to the conclusion that the transition from a highly ordered vortex lattice (Abrikosov lattice) to an amorphous vortex lattice is responsible for the rise in magnetic moment at low fields, and subsequently the crossover from this amorphous state to a plastically deformed vortex lattice, which is assisted by dislocations, is responsible for the suppression of the peak effect in m-H loops. The investigation of flux pinning by twin planes has attracted considerable attention, since naturally-formed twin planes occur regularly in Y123 crystals. One of the characteristic properties of vortex pinning by twin planes is a cusp-like dependence of the irreversibility field (H[subscript irr]) o