Studies On Pressure Effect On The Optic And Electronic Properties Of Three Types Of Perovskite Oxide Functional Materials

The high-pressure effects on the physical properties, such as geometric structre, magnetic properties, the Curie temperature, band structure, spinning, absorption spectrum and refractive index, in double perovskite Sr2FeMoO6, La2FeMnO6and NiCrO3are investigated by using Density Functional Theory.The high-pressure effect on the physical properties in double perovskite Sr2FeMoO6is explored theoretically. The structure of Sr2FeMoO6exhibits14/mmm symmetry in the whole external hydrostatic pressure applied. A transition from ferrimagnetic half-metallic to nonmagnetic semiconductor state is found when the pressure is above36GPa, accompanied by a spin crossover of Fe ion from high-spin to low-spin state. When the pressure is above45GPa, the nonmagnetic compound behaves as full metal. By the mean-field theory, the estimated Curie temperature of the ferrimagnetic Sr2FeMoO6increases from472to639K when the pressure increases from0to30GPa. In mean time, the optic property of Sr2FeMoO6is studied as well. The result shows that the absorptional character is isotropic, and the energy of the peak of absorptional spectrum is increasing with the increase of the pressure. For the refractive index, it shows normal in the lower energy region. Furthermore, the effect of pressure on the refractive index is little. Simultaneously, the properties of Sr2FeMoO6under different biaxial mechanical strain have been investigated in detail. The results illustrate that Sr2FeMoO6possesses half-metallic characteristic in the whole range of strain applied in the present wor. There exhibits a spin crossover of Fe ion from the high-spin to intermediate-spin state happens when the compressive strain is above-7%. For Mo ion, a spin state transition from intermediate-spin to low-spin state is found when the strain is abover8%, accompanied by a magnetic phase transition from FIM to FM.The influence of biaxial strain on both the electronic and magnetic properties in double perovskite La2FeMnO6is investigated, and half-metallic La2FeMnO6is obtained. The results show that La2FeMnO6exhibits ferromagnetic semiconductor at ambient condition and turns into ferromagnetic half-metal under the whole tensile strain applied in this work, while the compound transfers into ferromagnetic metal under the compressive strain within-8%and into ferrimagnetic semiconductor with the compressive strain beyond-9%. For both ferromagnetic half-metallic and metallic La2FeMnO6, they exhibit very slight change in the electronic states and magnetic moments of the ions, comparing with those of the compound at ambient condition. The electronic configurations of Fe and Mn originating from the high-spin state Fe3+and intermediate-spin state Mn3+in ferromagnetic La2FeMnO6emerge in the low-spin state and high-spin state in ferrimagnetic La2FeMnO6, respectively, when the compressive strain beyond-9%. The strain can also significantly influence the optical properties of La2FeMnO6, the wavelength of the light absorbed by La2FeMnO6stretches with the increase of tensile strain, while the increase of the refractive index of La2FeMnO6is caused by compressive strain.The effect of high-pressure on the electronic and magnetic properties in perovskite oxide NiCrO3is explored and antiferromagnetic half-metallic NiCrO3is achieved under high pressure. The results show that NiCrO3exhibits antiferromagnetic semiconductor when the external hydrostatic pressure is below-80GPa, and then behaves as antiferromagnetic half-metal on increasing the pressure. The band gaps in the spin-up and spin-down channels decrease from3.9to2.5and0.35to zero eV, respectively. The magnetic moments of Ni, Cr, and O ions are slightly changed in the whole range of the pressure, and the electronic configurations of both Ni and Cr ions are in the high-spin state. Based on the mean-field theory, the estimated Neel temperature of semiconducting NiCrO3increases from270K and reaches the maximum of321K around60GPa with increasing the pressure, and the Neel temperature of the half-metallic one is of about309K. In the whole pressure range, the frequency of the light absorbed by NiCrO3moves to the low frequency band with the increase of pressure. When the pressure is80GPa, the peak of absorption spectrum and the refractive index of NiCrO3are saltatory.The research results will provide theoretical guide to using materails under external pressure and tuning the phyical properties of perovskite oxides.