【作者】 杨辉；

【导师】 黄忠兵；

【作者基本信息】 湖北大学 ， 材料物理与化学， 2017， 博士

【摘要】 多铁材料是指同时具有两种或两种以上铁性特征（如铁电、铁磁、及铁弹等）的材料。由多种铁性的耦合而产生的如磁电效应,磁弹效应,压电效应等引起了人们的广泛关注。本论文采用基于密度泛函理论的第一性原理计算方法系统地研究了两类重要的多铁性材料,即三角阻挫体系MCrS₂和富瓦烯分子。主要内容如下:首先,对三角阻挫材料AgCrS₂在低温区的磁性和铁电性进行了系统的理论分析。在低温CM结构下,通过对七种不同的磁结构进行能量比较,确定出与实验观测完全一致的Cr-S层内共线双条纹反铁磁结构。根据海森堡模型近似计算给出的磁交换常数,发现沿条纹方向的铁磁交换作用与垂直条纹方向的反铁磁交换作用对磁基态的形成具有关键的影响。由于层间磁交换作用不能忽略,可以得出AgCrS₂具有三维磁性特征。根据离子的玻恩有效电荷和晶体的结构,发现AgCrS₂的铁电性主要来源于AgS₄四面体的非中心对称性,即它属于几何铁电类。通过对比低温区和室温区体系总能量与Ag-S键长的依赖关系,发现低温区的磁性对AgS₄四面体的非中心对称结构有着重要的稳定作用。该结果成功地说明了低温区实验观测到铁电极化的原因。此外,通过对比磁序以及Ag-S键长对材料电极化的影响,发现前者的影响远小于后者,更进一步证实了AgCrS₂的几何铁电特征。在论文的第二部分,探究了MCrS₂（M=Ag,Au,Na）的晶体结构、磁性、以及铁电性在0-100GPa范围内随压力的演化特征。采用USPEX晶体结构预测软件包,预测到以下结果:AgCrS₂有两个高压相,即0-38GPa压力下的R3M相和38-100GPa压力下的P-6m2相;AuCrS₂有两个高压相,即0-8GPa压力区间的R-3m相和8-100GPa压力区间的P-6m2相;NaCrS₂有3个高压相,分别为R-3m相（0-36GPa）,P4/MMM相（36-70GPa）、以及AMM2相（70-100GPa）。进一步研究发现AgCrS₂的磁结构在20GPa时发生由铁磁到120^o反铁磁的转变,并伴随着铁电极化的显著变化,表明压力可以增强AgCrS₂的磁电耦合。在38GPa发生结构相变后,AgCrS₂又转变为铁磁态,且具有弱极化的空间群P-6m2。最后,结合第一性原理和约束路径量子蒙特卡罗方法,对三五富瓦烯和五七富瓦烯低聚物（聚合分子数n=2-6）的磁性和铁电性质进行了系统地研究。两种方法的计算结果一致地表明三五富瓦烯低聚物左端或两端碳原子的氢化可导致自旋三重态（即铁磁态）在室温区的稳定存在,这个结论同样适用于五七富瓦烯低聚物右端或两端碳原子被氢化的情况。由于低聚物的中心反演对称性被破坏,使其具有自发铁电极化。同时还发现在两端氢化的低聚分子里,铁电极化随着n的增加不断增强,而在一端氢化的低聚分子里,铁电极化随n的变化很小。以上结果预测了在氢化三五和五七富瓦烯低聚分子中可以实现室温多铁性。更多还原

【Abstract】 Multiferroics,in which two or three ferroic orders（such as ferroelectric,ferromagnetic and ferroelastic）coexist in the same phase,have attracted great interests recently,due to the effects induced by coupling of various ferroic orders,such as magnetoelectric effect,magnetoelastic effect,and piezoelectric effect.In this thesis,based on the density functional theory,we perform a systematic study of two important multiferroic materials,i.e.triangular frustration system MCrS2 and fulvenene molecules.First,we performed a systematic study of the magnetic and ferroelectric properties in triangular frustrated material AgCrS₂.By comparing the energies of seven different magnetic structures in the low-temperature CM phase,we found that the structure with a collinear double-striped antiferromagnetic ordering in Cr-S layer is the magnetic ground state,which is consistent with the experimental finding.In terms of the exchange constants extracted from the Heisenberg model,we found that the ferromagnetic interaction along the parallel spin stripes and antiferromagnetic interaction across the stripes play an important role in stabilizing the unusual magnetic structure.Moreover,because the interlayer exchange is comparable to the intra-layer ones,AgCrS₂ belongs to a three-dimensional magnetic structure.From the results of born effective charge and crystal structure,it is revealed that the ferroelectricity is produced mainly by the non-central symmetry of AgS₄ tetrahedrons,suggesting a“geometric”ferroelectric class.We have examined the dependence of total energy on the Ag-S distance,and the results show that the magnetism at low temperature plays an important role in stabilizing the AgS₄ tetrahedrons.Furthermore,by comparing the effects of the magnetic ordering and the Ag-S bond length on the electric polarization,we found that the influence of the former is much smaller than that of the latter,which confirms the geometric ferroelectric characteristics of AgCrS₂.In the second part of the thesis,we explored the evolution of crystal structure,magnetism,and ferroelectricity of MCrS₂（M=Ag,Au,Na）in the range of 0-100GPa.Using the USPEX crystal structure prediction software package,we predicted the following results.AgCrS2 has two high-pressure phases:R3M phase at 0-38 GPa pressure and P-6m2 phase at 38-100 GPa pressure;AuCrS₂ has two high-pressure phases:R-3m phase in the 0-8GPa region and P-6m2phase in the 8-100GPa region;NaCrS2 has three high-pressure phases,namely R-3m phase（0-36GPa）,P4/MMM phase（36-70GPa）,and AMM2 phase（70-100 GPa）.It is found that the magnetic structure of AgCrS₂ is changed from ferromagnetism to 120^o antiferromagnetism at20GPa,accompanied by a significant change of ferroelectricity,suggesting that pressure can enhance the magnetoelectric coupling of AgCrS₂.After the structural phase transition at 38GPa,AgCrS2 is transformed into the ferromagnetic state and has a weakly polarized space group P-6m2.Finally,we performed a systematic study on the magnetic and ferroelectric properties of fused triapentafulvalene and pentaheptafulvalene oligomers（n=2-6）,by using the density functional theory and quantum Monte Carlo method.The results of the two methods consistently show that selective hydrogenation of fused triapentafulvalene oligomers at the left end or at both ends（right and left）can result in the room-temperature spin-triplet（ferromagnetic）state.This is also found in fused pentaheptafulvalene oligomers with hydrogenation at the right end or at both ends.Simultaneously,there exists a finite polarization in the ferromagnetic state,due to the breaking of the inversion symmetry of oligomers.We also found that the ferroicity increases continuously with the increase of n for the triapentafulvalene and pentaheptafulvalene oligomers with hydrogenation at both ends.Thees results show that the hydrogenated triapentafulvalene and pentaheptafulvalene oligomers could be the good candidates of room temperature organic multiferroics.更多还原