Charge Transfer Energies Of Lanthanide Ions

Lanthanide (Ln)-doped functional materials play a significant role in the fields of luminescence, catalysis, magneto-optical, thermoelectricity, etc, which are closely related to the performances of electrons of Ln elements in these materials. The location of the4f energy levels of Ln impurities in the band gap of host crystals is very important, which controls the performances of materials and can be determined by charge transfer (CT) energy of Ln ions in hosts. Therefore, it is very important to establish a simple and effective method to determine CT energy. CT energy is the energy required to transfer an electron from the valence band of a host crystal to an empty or unfilled4f shell of a trivalent (or tetravalent) Ln impurity to create the divalent (or trivalent) Ln in the host. The process of CT transition reflects the behavior of electrons, which can be accurately described by the concept of electronegativity (EN). EN is an important parameter to describe the power of an atoms in a molecule, ion, or chemical group to attractive electrons to themselves, which is a useful tool and approach to link the microstructure with the macroscopic properties of materials, and it has been successfully applied in many researches of novel materials design. Thus, from the viewpoint of EN, we can deeply understand, interpret, and predict CT energy of Ln ions in inorganic crystals.In this thesis, based on the concept of electronegativity, the crucial factor of CT transition for Ln ions in inorganic crystals is investigated. Firstly, the systematic variation of CT energy is studied for the whole Ln series ions, which is independent on the type of host crystals and is the inherent property of Ln series ions. The shape of the zig-zag curve appears to be almost the same for Ln series ions in all types of compounds, even though its location relative to valence band and conduction band changes. EN and ionic radius of Ln ions are used to determine the systematic variation in CT energy for different Ln ions, which shows a satisfactory result. It is known that the larger the bond strength of host crystal, the more energy is needed to excite an electron from the host to impurity Ln ion. Bond strength is defined in terms of the basic atomic parameters such as EN and ionic radius. Thus, CT energies of Eu3+in host crystals, which act as the reference for other Ln CT energies, are calculated by the strength of constituent chemical bonds of hosts. By using this method, the CT energies of Ln ions in wide band gap inorganic crystals are calculated, including the CT energies of Eu3+in14binary compounds,12pseudobinary compounds, and40complex compounds (containing chemical group). Furthermore, this method is extended to calculate the CT energies of Eu3+in19â…¡-â…¥ and â…¢-â…¤ binary, and25AxB1-xC and ABxC1-x ternary alloyed semiconductors. This thesis provides us a method to compositionally predict the CT energies of Ln ions in inorganic crystals, which is useful to investigate and design novel Ln-doped functional materials through elemental screening.