Theoretical Investigation On Multiferroic Properties And Structural Phase Transitions In ABX₃-type Compounds

ABX₃?X=O,F?compounds have various choices in A or B elements,crystallize in different structures and thus have colorful physical and chemical properties.By performing first-principles calculations,we apply chemical pressure and hydrostatic pressure to study compounds with hexagonal YMnO₃ structure,perovskite,post-perovskite and post-post-perovskite.The selected properties include improper ferroelectricity,magnetoelectrical coupling,Jahn-Teller effect and structural phase transitions.Recent experiments show that hexagonal LuFeO₃ is a new room-temperature multiferroic,which is rare and thus important for fundamental physics and device applications.By means of applying chemical and hydrostatic pressures,the structural,electrical,magnetic and magnetoelectrical properties of hexagonal rare-earth ferrites RFeO₃ are investigated.It is found that increasing chemical pressure can enhance polarization and induce magnetic phase transition and weak ferromagnetism;while increasing hydrostatic pressure can cause an abnormal increase in polarization,and also annihilate weak ferromagnetism.From the view of phonon modes,we propose that it is the improper nature that causes the abnormal increase of polarization.The magnetoelectricity is also discussed.Perovskites are popular due to their colorful physical properties.Post-perovskite,being reported in recent years,also attracts a lot of attentions due to its geologic significance.We perform first-principles genetic algorithm and predict a stable post-postperovskite structure?ppPv-Pnma?,which is proved to be common in ABX₃ and A₂O₃materials under pressure.Such structure has a special one-dimensional octahedral double-chain,which has strong anisotropy and render the ppPv-Pnma phase various electronic gaps and novel magnetic properties.We provide experimental evidences for the existence of ppPv-Pnma phase.Jahn-Teller?JT?effect plays an important role in determining the electronic and crystal structure of materials.We do further study on pPv and ppPv phase of rare-earth manganites RMnO₃ and predict two high pressure phases,ppPv-Pna21 and pPv-Pnma.It is proved that the occurance of these two phases are caused by JT instability of MnO6 octahedral.The JT effects are stable under a pressure up to 80 GPa,which is far beyond the pressure limitation in observing JT effect in perovskites.The JT effect induces improper ferroelectricity,of which we discuss the mechanism.Electronic structures are also studied to reveal the mechanism of such high pressure JT effect.