Nmr Study On The Transition Metal Chalcogenide Materials

Due to the serious environmental problem and the exhausting of non-renewable energy sources,more and more attention to renewable and environment friendly power sources have been paid in recent years.Thermoelectric materials could make direct conversion between electric and thermal power,and thus this type of material can be used to recycle waste heat which consume two thirds of the total energy.Besides,because of no emissions and moving parts,these materials have the advantages of no noise,low pollution and high reliability.However,their low power conversion efficiency limited the application of such materials.To improve their performance,a genius idea of phonon glass-electron crystal?PGEC?was proposed by Slack in 1990 s.It stresses lowing lattice thermal conductivity while keeping high electric performance.It has been found that the thermal conductivity of Clathrates and Skutterudites can be effectively reduced by introducing guest atoms.The underlying physics for such phenomenon was considered to originate from strong scattering between lattice phonons and the rattling of guest atoms.Transition metal chalcogenides have been considered as a new type of PGEC thermoelectric materials with extremely low thermal conductivity approaching amorphous limit.The motion of ions in these materials has been suggested to be relevant for understanding their low thermal conductivity.At higher temperature beyond superionic transformation,liquid-like diffusion of ions results in a removal of transverse phonon mode and further lowered thermal conductivity as a consequence,which promotes the concept of “Phonon liquid”.That means there could exists strong correlation between lattice thermal conductivity and atomic/ionic motion in this type of materials.Thus an element sensitive local probe with appropriate time scale related to atomic/ionic migration is desired.Nuclear magnetic resonance?NMR?is one of ideal techniques,which is widely accepted and applied in investigating structure and dynamics in various materials.In this dissertation,NMR will be employed as the key method to systematically study atomic motion in the noble metal chalcogenides.In the beginning chapters,the research background,NMR technique and experimental method will be introduced in detail.In the next chapters,transition metal chalcogenide material CuAgSe will be firstly introduced as a model system for NMR investigation.By taking the advantages of NMR,a strong experimental evidence for the existence of correlation between atomic/ionic diffusion and lattice thermal conductivity has been found for the first time.A schematic description of the abnormal cation mobility versus temperature will be discussed in the following chapter.Further,a new approach on the vacancy mediated atomic mobility will be proposed and then verified by NMR experiments.At last but not the least,NMR is performed to study the so-called solid-like and liquid-like behaviors in such materials.The transition and hybrid of solid/liquid-like behavior of ionic diffusion in Cu₂ S will be presented in detail in this section.