| Ferroelectric materials possess two or more stable polar states that can be switched under external electric fields.In 1920,the French scientist Joseph Valasek discovered ferroelectricity in Rochelle salt,which opened a new era in the ferroelectricity research.Over the past century,ferroelectricity has been found in different types of materials such as ceramics,single crystals,polymers,and thin films.The coupling of the polarization with external electric fields has potential application in nonvolatile storage.Compared with volatile memories,ferroelectric non-volatile memory has the advantages of faster reading and writing speed,and lower power consumption.Therefore,it is of great significance to search high-performance ferroelectric materials.Meanwhile,the CALYPSO（Crystal structure Ana LYsis by Particle Swarm Optimization）structure prediction method based on the group intelligence theory can theoretically predict the crystal structure efficiently and the results obtained by the method have been frequently confirmed in experiments.Thus,combining the CALYPSO method and first-principles calculation,this thesis predicts ferroelectricity in quasi-ionic and cluster-ionic systems,and performs detailed studies.（1）We investigated ferroelectricity in quasi-ionic crystals-silver and copper monohalides.In binary compounds,the electrical polarization of polar covalent crystal systems such as BN and Zn O is difficult to reverse owing to the brittleness of covalent bonds.However,this switching barrier can be reduced in ionic crystals due to the long-range Coulomb interactions of ionic bonds,but ionic systems such as Na Cl and Cs Cl are non-polar.We found that silver and copper monohalides at the transition region between covalent and ionic systems have electrical polarizations with moderate switching barriers.In this work,we predicted that silver/copper monohalides have coupled ferroelectricity and ferroelasticity.Through extensive structural searches,we have designed a variety of 2D monohalide structures with low exfoliation energies,suggesting that it is experimentally possible to obtain such 2D materials by the mechanical exfoliation.（2）We predicted a family of ionic supersalts with covalent-like directionality.We show that ionic crystals can be polar by changing their building blocks from elemental ions to cluster-ions.Due to the non-spherical geometries of these cluster ions,the corresponding supersalts form anisotropic polar structures with ionic bonding.In this work,we designed a series of stable ferroelectric/ferroelastic supersalts PnH4MX4（Pn=N,P;M=B,Al,Fe;X=Cl,Br）composed of superalkali,PnH4 and superhalogen,MX4.The cluster-ion based supersalts possess ultra-low switching barrier and can endure large ion displacements and reversible strain.In particular,PH4Fe Br4 exhibits triferroic coupling of ferroelectricity,ferroelasticity,and antiferromagnetism with controllable spin directions via either ferroelastic or 90-degree ferroelectric switching.（3）We showed that a mixed valence supercrystal with ferroelectricity can be built by using only superhalogen clusters,namely,SbCl4.In general,conjugation of two superhalogen clusters yields nonpolar symmetrical clusters,akin to diatomic halogen molecules.Herein,we found that SbCl4 superhalogen is an exception:its dimerization yields a polar cluster that can be viewed as aquasi-bonded[SbCl5]δ-and[SbCl3]δ+clusters.The symmetry breaking arises from the valence stratification of Sbinto Sb5+and Sb3+as well as their lone pair electrons.When assembled,SbCl4 clusters form a supercrystal that is thermodynamically stable up to 600 K and possess three different modes of ferroelectricity with distinct magnitudes and Curie temperature. |
