First-Principles Calculations Of Pressure Effects On Perovskite-type Ferroelectric And Magnetic Materials

In the field of modern material science,applying field conditions（such as pressure, electric and magnetic fields,etc.） has become one of the effective approaches to developing new materials and improving material properties via driving structural phase transiton as well as corresponding changes in physical properties. Under the condition of high pressure, perovskite-type ferroelectric and magnetic materials have abundant structural phase transition.Based on the analysis of the research ground of this field, this paper carried out first-principles calculations on the high-pressure properties of G-type antiferromagnet SmFeO₃ and lead-free ferroelectric/piezoelectric material BiAlO₃.This paper focuses on hydrostatic pressure effects on magnetism、ferroelectricity and other properties of perovskite-type compounds.According to the theoretical results,we obtain the following conclusions:Ferrite SmFeO₃ is characterized by orthorhombic Pbnm symmetry with G-type antiferromagnetic order at atmospheric pressure.When the applied hydrostatic pressure is lower than the critical value（⁶8GPa）,the crystal structure parameters（lattice constants,volume,the length of Fe-O bonds and antiphase displacement） of SmFeO₃ decrease linearly with the increasing pressure;Fe³⁺ keeps high spin（HS） state with its G-type AFM moment retained ⁴ μB,while the weak magnetization of SmFeO₃ with Γ2 and Γ4 increase linearly;Meanwhile HS SmFeO₃ is an insulator,and the band gap decreases linearly with the increasing pressure.The spin crossover transition with crystal structural parameters,magnetic and electrical properties collapses can be induced when the pressure increases above the critical value（⁶8GPa）,and it is also isostructural phase transition（i.e.the crystal space group remains unchanged） via phonon spectra analysis.The Fe³⁺ tranforms from high spin to low spin state with the magnetic moment of ⁴μB,Meanwhile,the band gap reduces from >2 e V to 1.6 e V,i.e.insulator-semiconductor transition occurs.Since then,the above three properties os the low spin state of SmFeO₃ have the same hydrostatic pressure effect with the high spin state.As a strong correlation system,Hubbard U value in first principles calculations may influence the high pressure effect,so during the calculation differet Hubbard U values（U=2.0,3.0,4.0 e V） are considered,and the results show that the structure parameters and the magnetic moment of Fe³⁺ are almost independent on the Hubbard,while the spin crossover transition pressure and the band gap increase linearly with the increasing U.Theoretical invedtigation was carried out on the new ferroelectric material BiAlO₃（space group R3c） under positive and negative pressure loading modes.The comparative analysis of structure and physical properties under pressure is as following:（1） The crystal structure parameters the crystal axises and volume reduce under positive pressure,while the negative pressure has the opposite effect.And the c axis is more easier to deform under pressure.（2） The electrical property the band gap decreases linearly with the increasing positive pressure,while under negative pressure it changes non- monotonically.（3） The elastic property C11 and C33 are higher than other elastic constants significantly,which indicates the rhombohedral BiAlO₃ has obvious anisotropy.By analyzing the positive and negative pressure effects on the poisson’s ratio,it is found that the anisotropy is more sensitive to negative pressure.In the pressure range-8⁹GPa,BiAlO₃ has been showing brittleness,and the positive pressure is beneficial to improve the ductility while the deformability is poor under negative pressure.（4） The ferroelectric property The ferroelectric distortion is caused by lone pair electrons of Bi³⁺ due to the born effective charge analysis.The spontaneous polarization decreases with the increasing positive pressure,which indicates the ferroelectric properties of BiAlO₃ are suppressed by positive pressure,while piezoelectric coefficients d11、d33 decrease; With negative pressure,the spontaneous polarization and piezoelectric coefficients have the opposite tendency with the increasing pressure,what’s more at ⁸GPa,d11 and d33 become equal.