Theoretical Calculation Study On The Polarization Properties Of (K_0.5Na_0.5)NbO₃,Croconic Acid And Trichloroacetamide Ferroelectrics

Ferroelectric materials exhibit typical spontaneous polarization properties,which have been widely applied in electronics,ultrasonic,memory device,sensor,laser and biological fields.At present,the research and development of ferroelectric materials encounter with new opportunities and challenges.On one hands,organic ferroelectrics have exhibited great potential in biology and bionics fields.However,the organic ferroelectric materials are rare and still suffering from poor ferroelectric performance,which encourage the researchers to explore novel types of organic ferroelectrics and study the mechanism of their ferroelectric properties.On the other hand,the lead-based perovskite ferroelectric oxides with outstanding ferroelectric performance are limited in many fields owing to their toxic property and high-pollution to the environment.The investigations on the lead-free ferroelectrics such as potassium sodium niobate(KNN)-based materials and the performance improvement of them have attracted great attentions.The calculating and analyzing about the ferroelectric properties and polarization mechanism of the novel organic ferroelectrics and lead-free materials can provide the theoretical basis for not only the performance improvement of the materials,but also the design of the ferroelectric devices.In this thesis,the first-principles calculation based on the density functional theory was performed to investigate the energy band structure,charge distribution and polarization properties of the croconic acid,trichloroacetamide and KNN,which belong to the proton transfer,order-disorder and displacement type of the ferroelectric materials,respectively.The ferroelectric phase transition temperature of these materials are also simulated by using the Monte Carlo method.The major results are listed as follow:(1)Using the density functional theory,the lattice and atomic positions of orthorhombic(K0.5Na0.5)NbO3 are modeled and optimized.The calculation of band and density of states show that the 2p orbitals of O atoms and the uncompleted 4d orbitals of Nb atoms mix heavily,and the octahedrals are formed.The electron density and deformation density can interpret the direction of polarization along[110]orientation,which is in good agreement with the experiments.By Berry phase method,the polarization of(K0.5Na0.5)NbO3 is 0.39C·m-2.The polarization originates mainly from the distortion of oxygen octahedrals.In virtue of the guidance of calculation,the(K0.5Na0.5)NbO3 nanorods are synthesized in the experiment.The experimental results indicate that with orthorhombic perovskite structure the nanorods have higher piezoelectric constant along[110]orientation,which is consistent with previous calculation.The polarization properties of tetragonal(K0.5Na0.5)NbO3 are also calculated by methods similar.The results interpret that its direction of polarization is along[001]orientation,and its polarization is highly improved compared with that of orthorhombic structure.(2)In croconic acid molecule,the 2p orbitals of the C and O atoms mix heavily and form the strong covalent bonds,and the H atoms and the nearest O atoms form moderate intermolecular hydrogen bonds.It is found that the origin of its large polarization is the charge transfer due to the strong "push-pull" effect of electron-releasing and-withdrawing groups along the hydrogen bond.The proton in the moderate hydrogen bond can transfer when external electric field applied,and the polarization reversal is realized through the transfer of the proton.The moderate hydrogen bond should be the key clue to discover new organic ferroelectrics of proton transfer type.The spontaneous polarization of croconic acid has been obtained by Berry phase method,and the calculated value is in good agreement with the experiments.By using one-dimensional ferroelectric Hamiltonian model and Monte Carlo method,the simulated polarizability and the temperature dependence of the spontaneous polarization are in good agreement with the experimental data.(3)Based on the analysis of the electronic structure and molecular structure of trichloroacetamide,the origin of polarization is speculated to be from the charge transfer due to the strong "push-pull" effect of electron-releasing and-withdrawing groups,the intermolecular hydrogen bond favours the molecular dipole moments in the same direction.The trichloromethyl can be rotated when external electric field applied,and the polarization reversal is realized through the rotation of the trichloromethyl.The ferroelectrics which undergo polarization reversal though the rotation of certain groups often have a low coercive field.By using Berry Phase method,the spontaneous polarization of trichloroacetamide have been obtained from the wave functions,and these calculated values(about 0.2 μC·cm-2)are in good agreement with the experimental data.It is found that although trichloroacetamide has a large molecule dipole moment(about 3.31D),while their opposing orientations of molecules in crystal make most of the dipole moments counter balanced.To find new organic ferroelectrics with excellent polarization properties should avoid the materials with almost perfect structural symmetry,which have low polarization.By using the dipole-dipole interaction model supposed to be induced by hydrogen bond and Monte Carlo method,the calculated Curie temperature value is in good agreement with the experiments.