Ab Initio Study On The Structural And Thermodynamic Properties Of Fe-based Magnetic Compounds And Alloys

In this thesis,we studied the phase structures,mechanical properties,magnetic orders and thermodynamic properties of two different magnetic systems.The main context of this thesis can be divided into two parts.In the first part,the phase structure,mechanical properties and thermodynamic properties of Fe₂ P type FeMnP_1-xT_x?T=Si,Ga,Ge?compounds were studied theoretically.In the second part,the structure-magnetism correlation,mechanical properties and the effect of interstitial atoms?C and N?on stability on the tetragonal phases of dilute Fe1-xTmx?Tm=Cr,Mn,Co,Ni?binary alloys were studied.As a magnetic refrigerant material,good mechanical properties and mechanical stability at finite temperature are needed for FeMnP_1-xSi_x compounds,since it works under repeated thermal and magnetic cycles.It is found from experiments that FeMnP1-x Six compounds show poor mechanical properties,however,there are few studies on the mechanical properties of these compounds so far.Thus,we theoretically predict that Ga substituted Fe₂ P type FeMnP_1-xGa_x compounds could have a good mechanical properties and large magnetocaloric effect.The total energy,magnetic moment,formation enthalpy,mechanical properties and thermodynamic properties of FeMnP_1-xT_x?T=Si,Ga,Ge?compounds are studied by using the density functional theory in combination with elasticity theory and classical statistical theory.The total energy is calculated by VASP code,the phonon spectra are calculated by Phonopy code and the thermodynamical properties are calculated by Gibbs code.Firstly,we studied the phase stability of hexagonal and body centered orthorhombic phase those are possibly formed in Fe₂ P type compounds.The results indicate that the total energy,magnetic moment and the formation enthalpy of two phases are very close to each other,and theformation enthalpy is negative.The calculated elastic constants prove the mechanical stability of both phases.And further,the hexagonal phase is ductile and the orthorhombic phase is brittle.Phonon calculation indicates that the hexagonal phase is dynamically stable,while imaginary frequency was observed in phonon dispersion,demonstrating that the orthorhombic phase is dynamically unstable.And then,the mechanical properties of FeMnP_1-xGa_x compounds are compared with those of FeMnP_1-xT_x?T=Si,Ge?compounds.In order to estimate the mechanical properties of the paramagnetic phase of compounds,the antiferromagnetic ordered model is used to calculate the elastic constants.The results indicate that the hexagonal phase of FeMnP_1-xT_x?T=Si,Ga,Ge?compounds are mechanically stable,all the FM states of FeMnP_0.67T_0.33?T=Si,Ga,Ge?compounds are ductile,FeMnP_0.67Ga_0.33 compounds show best ductility,whereas ductility of FeMnP0.67Si0.33 compounds are the weakest.All the AFM states of FeMnP0.33T0.67?T=Si,Ga,Ge?compounds are ductile,FeMnP0.33Ge0.67 compounds show best ductility,whereas ductility of FeMnP_0.33Ga_0.67 compounds are the weakest.Other magnetic states of FeMnP_1-xT_x?T=Si,Ga,Ge?compounds are close to the ductile/brittle critical point.The disorder of atomic occupation may improve the ductility of the compounds.The variation of elastic constants with different doping atoms can be explained by the electron structure analysis and Force theorem.Finally,the Curie temperature of FeMnP_1-xGa_x compounds is determined by comparing the Gibbs free energy of the compounds in different magnetic orders.And then,the heat capacity,entropy change and the thermal expansion coefficient of the compounds are calculated and compared with those of FeMnP_1-xGe_x compounds.A discontinues transition of heat capacity as a function of temperature suggests that the FeMnP_1-xGa_x compounds undergoes a first-order phase transition.The Curie temperature of FeMnP_0.67Ga_0.33 compounds is Tc = 500 K,and the entropy change is?35?S = 69.34 J K^-1 kg^-1.The entropy change of FeMnP0.67Ge0.33 compounds at Curie temperature Tc = 590 K is?35?S = 66.69 J K^-1 kg^-1.Larger Curie temperature Tc = 770 K is observed for compounds withlarger Ga composition for FeMnP_0.33Ga_0.67,and the entropy change is?35?S =89.26 J K^-1 kg^-1.In summary,the predicted FeMnP_1-xGa_x compounds show similar structural and thermodynamical properties as FeMn P1-xGex compounds,therefore,it is believed that,FeMnP_1-xGa_x is a candidate refrigerant for room temperature magnetic refrigeration.A significant magnetism-structure correlation effect is present in Fe and Fe-based alloys,namely,the lattice structure determines the magnetic order.Therefore it is possible to modulate the magnetism through modulating the lattice structure.On the other hand,the magento-crystalline anisotropy of tetragonal dilute Fe-based alloys are related to the lattice tetragonality.Therefore,the study on phase stability and mechanical properties of Fe based alloys with different structures and magnetic orders is of great importance.Here we studied the relative stability and mechanical properties of Fe and Fe₁₄Tm₂?Tm=Cr,Mn,Co,Ni?binary alloys in different lattice structures?bcc,fcc and bct?and magnetic order?FM and AFM?.The results indicate that the FM states of bcc phase of Fe and Fe₁₄Tm₂?Tm=Cr,Mn,Co,Ni?alloys are energetically more stable than FM states and AFM states of fcc phase.The FM states of the fcc phase shows the high spin?HS?and low spin?LS?characteristics,and the AFM states of the fcc phase is more stable than FM states.For tetragonal phase,the AFM states of Fe14Cr2 and Fe₁₄Mn₂ are energetically more stable than their FM states,and the FM states of Fe14Co2 and Fe14Ni2 are energetically more stable than their AFM states.The FM-HS state of the fcc phase of the Fe₁₄Tm₂?Tm=Cr,Mn,Co,Ni?alloys and the Fe14Ni2 fcc phase of the FM-LS phase are mechanically unstable.The FM-HS tetragonal phase of Fe is mechanically unstable,and the other phases are mechanically stable.The results show that alloying makes the HS states mechanically stable.When the C or N atoms are filled in the octahedral interstitial of the tetragonal phase of Fe14Co2 alloys,the LS states become more stable than bcc phase,indicating that the filling of the interstitial atoms makes the system more stable.And Fe and Fe14Co2 alloys with interstitial C and N satisfy the mechanical stability criterion of tetragonal crystals.Theanalysis of the electronic structure shows that the alloys containing the octahedral interstitials may have larger magnetocrystalline anisotropy.