First-principles Calculations On The Surfaces Of Perovskites KCaF₃,NaCaF₃ And KMgF₃

First-principles density functional theory（DFT）calculations have been performed to study the geometric structure,electronic structure and surface stability properties of perovskite fluorides KCaF₃,NaCaF₃ and KMgF₃ surfaces.The main contents of this thesis are as follows:（1）The cubic KCaF₃ and NaCaF₃（001）surfaces with MF（M = K or Na）and CaF₂ terminations were investigated.The MF termination has stronger surface.rumpling than the CaF₂ termination.All the computed band gaps for the KCaF₃ and NaCaF₃（001）surfaces are smaller than those of the bulks.Surface states are introduced at the top of the valance band of MF-terminated surfaces,while no such surface states were found for CaF₂ terminations.Calculation of the surface energies shows that the MF-terminated surface is energetically more favorable than the CaF₂-terminated surface.（2）The slab thickness effect on the properties of KMgF₃（001）surfaces were stuied.It is found the MgF₂ termination always displays stronger surface relaxation than the KF termination.Surface rumpling does not change much as the thickness of the surface slab increases,and slab with the thickness of 7 atomic layers could be thick enough to simulate the geometric peroperties such as surface rumpling etc.No distinct surface states were found in the band gap of KMgF₃（001）surfaces.The band gap for the KF termination is smaller than that of the MgF₂ termination.As the slab thickness increases to 7 atomic layers and above,the band gap of the surface alters only slightly.Surface energy keeps almost a constant with the change of the slab thickness,and the KF-and MgF₂-terminated KMgF₃（001）surfaces were found to have almost the same surface stability.（3）Calculations of five KMgF₃（110）polar surfaces have been carried out.These five surfaces are KMgF-and F2-terminated stoichiometric surfaces and MgF-,K-and F-terminated nonstoichiometric surfaces.The largest atomic displacement occurs on the surface layer Mg atom for the MgF termination,while for the other four surfaces,K atom on the surface or the second layer displays the largest relaxtion.Atomic layer rumpling decreases with the increase of the layer depth from the surface to the central layer.All the five KMgF₃（110）polar surfaces have smaller band gap with respect to that of the bulk,and the gap value for the five terminations decreases as follows:F2>F>KMgF>MgF>K.Analyzing of the surface energy shows that nonstoichiometric surfaces（i.e.MgF,K and F terminations）are energetically much more stable than stoichiometric surfaces（i.e.KMgF and F2 terminations）.