Exploration On The Structures And Dielectric Properties Of Nitrogen-containing Cyclic Compounds

Phase transition materials have gained intensive attention due to their applications in the fields of signal processing,infrared detection,data communication,switchable dielectric devices,etc.Much work has been done in order to design such materials.However,it is still a huge challenge for researchers to obtain compounds with structural phase transition by using molecular design and crystal assembly.Organic-inorganic hybrid compounds receive a considerable interest for possessing the desirable and useful properties of the two constituents within a single material and exhibiting excellent structures and properties.Among them,some simple small organic ammonium cations such as methylammonium([MeNH3]+),pyridinium and imidazolium,have greatly attracted our attention.This is because these small molecules can easily experience motions or rotations triggered by temperature and thus result in structural phase transitions.Inspired by this,we have designed and synthesized a series of small ammonium-based molecular compounds and discussed their crystal structures,phase transitions and dielectric properties.The main body of this thesis includes two parts.In the first part,we introduced a series of organic-inorganic hybrid compounds based on 2-pyridinemethylammonium,3-pyridinemethylammonium and 4-pyridinemethylammonium cations and discussed the structures and dielectric properties.No phase transitions were found in these compounds.In the second part,(cyclohexylmethylammonium)2PbBr4 and(cy clohexylmethylammonium)2SnI4 having two-dimensional:layered perovskite-like structures were designed and constructed.The two analogues exhibit reversible phase transition at the high temperature.From structural analyses,it is clear that the order-disorder transition of the cyclohexylmethylammonium cation is responsible for the phase transitions.