Crystal Chemistry Of Iron Based Superconductors And First-Principles Calculations In Search For New Superconductors

From the very beginning, structure prediction was considered impossible to be achieved. So far, there are many theories were built to optimize the process of obtaining the stable structure. As well as basing on total energy, we have proposed formation energy method to help us in new structure prediction. In this process, we have successfully set up the calculation platform, and com-pleted a series of new material calculation. These messages help us to understand the properties of new materials. In addition, we have designed a series of new structures in different systems and calculated the total energy of related materials. Then we researched their formation energy, In combination with a large number of literatures and experiments, we finally obtained the applica-bility of the formation energy method, that is,1) Formation energy of those materials which have been synthesized should be negative or very close to zero.2) It’s difficult to synthesize a structure with positive formation energy.3) But a new structure with negative formation energy may also can’t be synthesized successfully. Under the help of these conclusion, a lot of manpower and ma-terial resources will be saved, and improve the success rate, to a certain extent, of new structure prediction.We have summarized the features in structure and crystal chemistry of Iron based supercon-ductors, and also designed a serials of new structure, which are potential superconductors. After evaluating these new structure from formation energy, we have tried to synthesize part of them in experiment. Finally, we have successfully synthesized the new compound Sr2Cr3As2O2, and measured the physical properties, as well as carried out corresponding first principle calculations.We have calculated some new compounds which were synthesized by the members of our group. This part of job is listed in the following:1) We have calculated the band structure of Ba2Ti2Fe2As4O, and obtain the Fermi Surface. We also carefully compared the three compounds Ba2Ti2Fe2As4O, BaFe2As2, BaFe2As20. By calculate the electronic occupancy from the Fermi surface, we find that this compound indeed exits electronic transfer. And so on. More over, our research revealed that Ti2As2O sheet may exits week Neel AFM. The coexistence of AFM and superconductivity deserves deeper study.2) By GGSA+U, we have studied the electronic structure of EuBiS2F with U=3 eV. We find that the highest occupied band (HOB) is located around EF. Most of N(E) at EF is from Eu-4f electron, and form three sheets of FS with strong 2D property. From the DOS, N(E) at EF is about 20 times larger than other similar systems, which can well interpret the enhancements of heat capacity and Pauli magnetic susceptibility.3) The electronic structure of quasi-one-dimensional superconductor K2Cr3As3 has been s-tudied through systematic first-principles calculations. The ground state is paramagnetic. Around the Fermi level, the Cr-3dz2,3dxy,3dx2-y2 orbitals dominate the electronic states and form one 3D Fermi surface sheet and two quasi-1D sheets. The electron DOS at EF is less than 1/3 of the experimental value. The ASOC splitting is sizable (60 meV) on the 3D Fermi surface sheet as well as on one of the quasi-1D sheets. Finally, We conclude that abnormally large Hc2 may be caused by spin-triplet pariring mechanism.