分子磁体NaFe^Ⅱ₂（PO₄）（HPO₄）的磁性及电导性的第一性原理研究

【作者】 王燕；

【导师】 孙玉明；

【作者基本信息】 烟台大学 ， 理论物理， 2008， 硕士

【摘要】 本文采用基于密度泛函理论（DFT）的全势线性缀加平面波（FP-LAPW）法,对非纯有机分子磁体NaFe₂^Ⅱ（PO₄）（HPO₄）的电子结构、磁性质以及电导性质进行了第一性原理研究。为了测试计算方法的正确性和精确性,我们先研究了包含原子数目较少的铁电体BaTiO₃的电子结构,并分析了其与铁电性的关系。对部分原子的电子态密度的分析显示,Ti的3d轨道和O的2p轨道的电子态密度有着非常相似的特征,这表明它们之间存在着强烈的轨道杂化,或许正是由于在八面体中原子轨道的这种杂化导致了BaTiO₃中的自发极化,自发极化的结果是使晶胞中的电偶极子有序排列从而出现铁电性。由于外部压力和温度可以导致BaTiO₃八面体中原子轨道之间的杂化更加明显,所以会引起铁电相变。对于金属磷酸盐NaFe₂^Ⅱ（PO₄）（HPO₄）,我们算得得结果与实验中所测到的结论是一致的:反铁磁（AFM）态是该体系的基态,铁磁（FM）态是其亚稳态。计算得到的每个分子的磁矩为7.00μB,主要来源于FeⅡ离子。对态密度的分析显示,NaFe₂^Ⅱ（PO₄）（HPO₄）体系在AFM基态具有半导体的性质,在FM亚稳态具有导体的性质。这可以从其能带结构中进一步得到证实,对于FM亚稳态,自旋向上和自旋向下的能带的最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）之间存在着明显的交叠,而对于AFM基态则存在着一个约为0.2eV的能隙,由此我们可以断定NaFe₂（PO₄）（HPO₄）的FM亚稳态确实具有导体的性质,AFM基态确实具有半导体的性质。更多还原

【Abstract】 In this thesis, first-principle calculations have been performed to study the electronic structure and the magnetic properties of non-pure organic magnet NaFe₂^Ⅱ（PO₄）（HPO₄）. All calculations were performed using a full potential linearized augmented plane wave （FP-LAPW） method based on the density functional theory （DFT）.To test the accuracy and precision of FP-LAPW method, the electronic structure and ferroelectric properties of the simple bulk BaTiO₃ were first studied. The analysis of the partial density of states indicates that Ti 3d states and O 2p states are greatly overlapped, showing strong hybridization between them. This orbital hybridization may lead to the spontaneous polarization in octahedron, and it results in aligned electric dipoles in bulk BaTiO₃, presenting ferroelectric property. External pressure and temperature can induce ferroelectric phase transition of BaTiO₃ because of changed orbital hybridization.For metal phosphonates NaFe₂^Ⅱ（PO₄）（HPO₄）, a stable antiferromagnetic （AFM） ground state and a ferromagnetic （FM） metastable state were found in agreement with the experimental results. The calculated spin magnetic moment per molecule is 7.00μB, mainly from the FeⅡ. Density of states shows that NaFe₂^Ⅱ（PO₄）（HPO₄） is characteristic of semiconductor and conductor in AFM and FM states respectively. In order to further investigate the conductivity properties in FM and AFM states, we also give the electronic band structure, it is found that the bands have significant overlaps between the highest occupied molecule orbital （HOMO） and lowest unoccupied molecule orbital （LUMO） of spin-up and spin-down in FM states, while an energy gap of about 0.2eV in AFM states, based on this we can conclude that the FM metastable state has conductor properties, but AFM ground state has semiconductor properties.更多还原