Preparation And Physical Properties Of (BEDT-TTF)(FeBr₄ ) Crystal

Because of the rich crystal structure and electronic structure, BEDT-TTF based charge-transfer salt have the changing properties, showing the electrical properties such as insulators, semiconductors, metals, superconductors and the magnetic properties such as Pauli paramagnetism, low-dimensional antiferromagnetism, three-dimensional ordered antiferromagnetism and weak ferromagnetism. In this thesis, we prepare crystal named (BEDT-TTF)[FeBr4] and use a variety of experimental methods to study its physical properties.In ChapterⅠ, we first introduce the invention, development and principles of the electrochemical crystallization technology. Detailed steps and precautions in crystal preparation have also been discussed. And then we introduce fundamental principles and related technology of X-ray diffraction , scanning tunneling microscopy, superconductivity quantum interference device,physical property measurement system and electron paramagnetic resonance.In ChapterⅡ, we briefly introduce the phenomena and the nature of the organic molecular crystals, especially BEDT-TTF based charge-transfer salts, including crystal structure, electrical conductivity, magnetism and so on. We also introduce organic molecular crystals which have the composite properties by mixing the magnetic elements, such as magnetic molecular metals, magnetic molecular superconductors and so on.In ChapterⅢ, the (BEDT-TTF)[FeBr4] single crystal has been prepared by electrochemical crystallization. We study the surface structure of the (BEDT-TTF)[FeBr4] films obtained by molecular beam evaporation and find that the structure of the insulating surface is equivalent to the bulk bc face structure. The magnetism of the crystal is determined mainly by magnetic ferric ions, and the ferric ions in crystal are not ferromagnetic coupling or antiferromagnetic coupling. From 300K to 185K, the crystal resistivity is related to the efficiency of electronic thermal excitation and the strength of the electron scattering by ionized impurities, while from 185K to 10K, the conductivity mainly depends on the ionization of the impurities and defects in crystal. The effect of magnetic field on the conductivity has two parties which are electron scattering increased by the Lorentz force and electron scattering decreased by polarized spin. The resonance signals of theπelectrons and the d electrons are found in the EPR spectra, and theπ-d coupling is enhanced as the temperature is decreased. When the crystal rotates along different axis, the resonance signal changed with the angle of rotation is enormously different.