First-principles Study Of The Stress And Strain Induced Ferroelectric Phase Transitions In Perovskite And Perovskite-like Materials

The first-principles method, which is the main method used in this thesis, has been reviewed as well as the relative theories. Using a density functional theory in the framework of first-principles method, we calculated the ferroelectric phase transitions in perovskite SrTiO3, rutile TiO2and mutiferroic BiFeO3, induced by stress and strain. The Raman spectrum and dielectric property of pre-perovskite PbTiO3were also investigated.First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO3. The P4mm-to-P4/mmm-to-4mm2phase transitions in SrTiO3with the epitaxial strain range from compressive to expansive have been identified by analysis of the soft-mode eigen-displacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2phase transitions are-0.72%and0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behavior of the A1and B2modes in the Amm2phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies.The negative pressure and epitaxial strain induced phase transitions in rutile TiO2were investigated. The softening behavior of the A2u (TO) modes at the Γ point following the decreasing pressure leads to a ferroelectric phase transition from P42/mnm (rutile) space group to P42nm(ferro) space group, with a critical pressure of-10GPa. The dramatic increase of the c-axial dielectric tensor in the vicinity of the phase transition indicates it to be a typical ferroelectric phase transition. The changing of unit cell volume, spontaneous polarization and structure parameter under the pressure show that, the phase transition displays a second order character. The in-plane twinaxial tensile strain induces rutile TiO2to a ferroelectric phase with Pm (Cs) space group, driven by the softening behavior of the Eu1mode. While the out-of-plane uniaxial tensile strain, vertical to the ab plane, induces rutile TiO2to a ferroelectric phase with P42nm (C4v) space group, driven by the softening behavior of the A2u mode. The critical tensile strains are3.7%in-plane and4.0%out-of-plane, respectively. In addition, the in-plane twinaxial compression strain, which has the same structure variation as out-of-plane tensile strain due to Poisson effect, leads the paraelectric rutile TiO2to a paraelectric phase with Pnnm(D2h) space group, driven by the softening behavior of the B1g mode with a critical strain of-3.6%. These results indicate that the sequence ferroelectric (or paraelectric) phase depends on the strain applied. The origin of ferroelectric stabilization in rutile TiO2is also discussed briefly in terms of strain induced Born effective charge transfer.The phase transition of BiFeO3(BFO) from tetragonal to monoclinic, corresponding to the softening behavior of the E mode, was investigated using the density-functional theory with local spin density approximation (LSDA). The sequential monoclinic phase, Ma, which is favorable during low compression with respect to the tetragonal phase, was characterized. The transition occurred at a pressure of about13.3GPa or at an epitaxial strain of-9.7%. Above the critical compression conditions, the tetragonal BFO became dynamically stable. The spontaneous polarization, polarization angle, magneton, and volume of the unit cell were calculated as order parameters in the range of-5GPa to27GPa. The results showed that tetragonal-to-monoclinic phase transition has a second-order character. Although the antiferromagnetic spin order or oxygen octahedra tilts were not considered in the present calculations, the results demonstrated that the compression-induced tetragonal-to-monoclinic phase transition in BFO is related to the softening behavior of vibration mode. Moreover, the results showed the structure and property evolutions of BFO during phase transition, which are very helpful in further investigations of the morphotropic phase boundary (MPB) in lead-free materials.The assignment of microscopic Raman spectra on powdered samples of pre-perovskite PbTiO3(space group I4/m) has been established according to the correspondence between the experimental and theoretical frequency and relative intensity data. We find no giant LO-TO splittings in pre-perovskite PbTiO3. In contrast to conventional perovskite, the largest pre-perovskite LO-TO splitting comes from its stiffest Au mode instead of from its softest mode. The pre-perovskite’s Born effective charges, dielectric tensors, and infrared frequencies are also calculated to determine its lattice dynamics and optical properties, and compared with the conventional perovskite PbTiO3. The results provide invaluable information for use in further research on PbTiO3phase transition.