立方相Na_(1/2)Bi_(1/2)TiO_3和K_(1/2)Bi_(1/2)TiO_3的电子结构和结构不稳定性的第一性原理比较研究

采用基于密度泛函理论的第一性原理方法比较研究了Na1/2Bi1/2TiO3和K1/2Bi1/2TiO3的电子结构、离子位移势能面和Γ声子等性质.结果表明,Na1/2Bi1/2TiO3和K1/2Bi1/2TiO3的电子结构很相似,价带由O 2p电子态主导并包含部分Ti 3d和Bi 6p电子态,导带低能部分由Ti 3d空轨道构成;K取代Na后其Ti—O和Bi—O键的键强略有增加.两者的离子位移势能面也很接近,O离子的偏心位移对结构不稳定性起主导作用,且K取代Na后其作用增强.Γ声子都存在3个软模,分析表明软模主要来自O6基团的振动,K取代Na后A2u软模发生硬化.     

Electronic and Optical Properties of KNbO3,NaNbO3 and K0.5 Na0.5 NbO3 in Paraelectric Cubic Phase：a Comparative First-principles Study

The structural,electronic and optical properties of KNbO 3 (KN),NaNbO3(NN)and K05 Na0.5NbO3(KNN) in paraelectric cubic phase were calculated employing the plane-wave pseudopotential method based on density functional theory (DFT).The calculated electronic structures of the three crystals show similar features in the valence bands and the lower conduction bands.However,the structures in higher conduction bands differ markedly due to the effect of Na and K atoms.The calculated optical properties reveal that the features of optical spectrum at low energy are dominated by the transitions from O2p valence bands to Nb 4d conduction bands and those at high energy are related to the transitions to K 4s4p and/or Na 3s3p states.Moreover,the optical constants of KNN are approximately the average of KN and NN at high energy.Therefore,the optical properties of KNN in high energy region can probably be altered by changing the ratio of Na/K.     

The electronic structures,Born effective charge tensors,and phonon properties of cubic,tetragonal,orthorhombic,and rhombohedral K0.5Na0.5NbO3： A first-principles comparative study

The electronic structures, Born effective charges（BECs）, and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5Nb O3 are investigated by the first principles method based on density functional theory.The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb–O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5Nb O3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Γ phonon properties of orthorhombic KNb O3, Na Nb O3, and K0.5Na0.5Nb O3 are also studied in detail.     

分枝结构对以N为耦合中心的多分枝分子的双光子吸收性质的影响

采用ZINDO/SOS方法,研究了以N原子为耦合中心,以二苯乙烯类、噻吩类和芴类分子作为分枝的分子的双光子吸收性质,从而研究了分枝结构对多分枝分子的双光子吸收性质的影响.结果表明分枝结构会影响分枝之间耦合作用的强弱,因而对分子的双光子吸收性质具有重要影响.所设计的分子中以二苯乙烯类和噻吩类分子作为分枝的多分枝分子具有较大的双光子吸收截面.     

取代基对二噻吩并噻吩衍生物的双光子吸收性质的影响

利用ZINDO/SOS方法, 从理论上研究了对称和不对称取代两种情况下, 取代基对二噻吩并噻吩衍生物单双光子吸收性质的影响. 结果表明, 所设计的噻吩类分子具有较大的双光子吸收截面, 且双受体取代比双给体取代更有利于增大分子的双光子吸收截面. 同时发现, 此类分子受体取代可以显著增加波长较短的双光子峰附近的双光子吸收, 而给体取代则可以改善波长较长吸收峰附近的双光子吸收.