First Principles Study On The Electronic Structure And Thermodynamic Properties Of FeMnP_0.83Ge_0.17 Compound

Manganese-based Fe₂P type giant magnetocaloric materials have many promising applications in room temperature magnetic refrigeration and magnetocaloric power generation.Being one representative material of this familyofcompounds,the?FeMn?₂P_1-xGe_xcompoundhasgood magnetocaloric properties and stability.At present,the macroscopic properties of this series of compounds are well characterized,but evolution of the lattice vibration mode and electronic structure of the compound across the phase transition,as well as the phase transition mechanism,have not been fully studied.In this paper,the first-principles calculation is employed to study the mechanical properties,electronic structure,X-ray absorption near-edge structure,phonon spectra as well as thermodynamic properties of FeMnP_0.83Ge_0.17.17 compound with different Ge site occupancy and different magnetic states,and then tries to demonstrate the evolution of the lattice vibration mode and the electronic structure of the compound across the magnetic phase transition.It is find that Ge atom has obvious 2c site preference in Ferromagnetic?FM?state,while Ge almost no site preference and the lattice parameters of the compound are not affected by Ge site occupation in Paramagnetic?PM?state.The phonon calculation shows that the phonon evolution of the compound across FM-PM phase transition is very sensitive to the Ge site occupation.Phonon strengthening as well as negative vibrational entropy change?S_vibib across phase transition were observed for the compound with Ge occupying 1b site.While the phonon spectrum of the compound with Ge at 2c site softens across the phase transition and shows large positive?S_vibib that are in same order of magnitude with magnetic entropy change.The electronic structure analysis demonstrates that the strengthening or weakening of adiabatic electron-phonon coupling related with the change of electronic density of state close to Fermi level is the physical origin of phonon evolution across magnetic phase transition.The thermodynamic calculation show that the Curie temperature T_c of FeMnP_0.83Ge_0.17.17 compound is very sensitive to the Ge site occupation,the T_c is 250K for Ge occupies 1b site,while T_c=490K for Ge at 2c site.Is is also observed that the K edge absorption spectra of Fe and Mn are not sensitive to the magnetic state and Ge site occupation,but interestingly,the K edge absorption spectra of metalloid atoms P and Ge are varies significantly across the magnetic phase transition and site occupation.The first absorption peak almost do not changed,while other peaks are significantly changed with the change of magnetic state,which demonstrates that the valence state of metalloid atoms are changed significantly across the transition.The results obtained in this thesis are helpful for the understanding of correlation between microscopic structure and macroscopic performance of FeMnPGe compounds.