First-principles study of structural stability and elastic property of pre-perovskite PbTiO3

The structural stability and the elastic properties of a novel structure of lead titanate,which is named preperovskite PbTiO₃ （PP-PTO） and is constructed with TiO₆ octahedral columns arranged in a one-dimensional manner,are investigated by using first-principles calculations.PP-PTO is energetically unstable compared with conventional perovskite phases,however it is mechanically stable.The equilibrium transition pressures for changing from preperovskite to cubic and tetragonal phases are 0.5 GPa and 1.4 GPa,respectively,with first-order characteristics.Further,the differences in elastic properties between pre-perovskite and conventional perovskite phases are discussed for the covalent bonding network,which shows a highly anisotropic character in PP-PTO.This study provides a crucial insight into the structural stabilities of PP-PTO and conventional perovskite.     

Mg掺杂PST薄膜的溶胶-凝胶法制备及晶相形成研究

利用溶胶-凝胶法成功制备了Mg掺杂Pb_0.4Sr_0.6Mg_xTi< sub>1-xO_3-x薄膜，利用x射线衍射仪对薄膜的物相和结构进行了分析， 用扫描电子显微镜对薄膜的形貌和断面等进行了观察.研究结果表明，薄膜以立方钙钛矿为 晶相，薄膜中晶相以团聚状颗粒存在，晶相含量受热处理条件和Mg的掺杂量所控制.Mg掺杂 对Pb_0.4Sr_0.6Mg_xTi_1-xO_3-x 薄膜晶相含量的影响与钙钛矿中的氧空位缺陷相关.在一定的掺杂范围内，由掺杂引起晶相 的晶格畸变较小时，体系掺Mg平衡了晶体内本征氧空位引入的电荷不平衡，使晶相更为稳定 ，析晶能力提高，晶体形成量随掺杂浓度的提高而提高.当掺杂浓度达到一定量时， 随着Mg 掺杂浓度增加，一方面使形成晶体时杂质浓度增加造成参与形成晶相的组成含量下降，另一 方面使进入钙钛矿结构的Mg增加，氧空位大量增加使畸变程度提高，形成的晶相不稳定，析 晶能力下降，晶体含量随掺杂Mg浓度的增加而不增反降.在相同条件下制备的Pb_{0.4Sr0.6MgxTi1-xO3-x薄膜中Mg掺量约为 x=0.01时，得到的钙钛矿相含量最高，本征氧缺陷所带入的正电荷和Mg引入时带入的负电荷 间达到平衡.此外，Mg的掺入还影响到析晶与热处理过程之间的关系.在高Mg掺量范围，Mg含 量越高，形成的晶相越不稳定，热处理时间越长，使热处理过程中分解的晶相量越多，随Mg 掺量越高和热处理时间越长，薄膜中晶相含量越低. 关键词： 溶胶-凝胶法 PST薄膜 Mg掺杂 晶相形成}     

CdWO4纳米晶形貌可控合成

分别以氯化镉、草酸镉、碳酸镉、柠檬酸镉和钨酸钠为原料,在没有采用模板和表面活性剂的条件下,采用水热法可控制备出了钨酸镉纳米颗粒、纳米棒、纳米束和纳米线.利用X射线衍射(XRD)、透射电镜(TEM)、高分辨透射电子显微镜(HRTEM)和选区电子衍射(SAED)对样品进行了表征.结果表明:[WO42-]/[Cd2+]摩尔比、反应时间及先驱体的类型对产物的形貌和尺寸有着重要的影响.     

一种获得半导体薄膜光学参数的方法

实现薄膜光学参数的简便测量对于薄膜的制备和应用具有重要意义。引入适用于半导体材料的Forouhi_Bloomer模型,用其表征薄膜折射率与色散的关系。考虑到粗糙度的影响,假设薄膜厚度服从正态分布,给出了模拟退火法与迭代法相结合、由可见光光谱测定薄膜光学参数的方法。作为尝试,以硅系薄膜为例进行了计算。结果表明,获得的厚度与用椭偏仪测量的结果较为吻合。该方法适用于研究和测量半导体薄膜的光学性能和膜厚,具有很高的实用价值。     

Strain-induced ferroelectric phase transitions in incipient ferroelectric rutile TiO2

Uniaxial strain induced ferroelectric phase transitions in rutile TiO₂ are investigated by first-principles calculations. The calculated results show that the in-plane tensile strain induces rutile TiO₂, paraelectric phase with P_4-2/mnm (D_4h) space group, to a ferroelectric phase with P_m (C_s) space group,driven by the softening behaviour of the E_u1 mode. In addition, the out-of-plane tensile strain, vertical to the ab plane, leads to a ferroelectric phase with P_42nm (C_4v) space group, driven by the softening behaviour of the A_2u mode. The critical tensile strains are 3.7% in-plane and 4.0% out-of-plane, respectively. In addition, the in-plane compression strain, which has the same structure variation as out-of-plane tensile strain due to Poisson effect, leads the paraelectric rutile TiO₂ to a paraelectric phase with P_nnm (D_2h) space group driven by the softening behaviour of the B_1g mode. These results indicate that the sequence ferroelectric (or paraelectric) phase depends on the strain applied. The origin of ferroelectric stabilization in rutile TiO₂ is also discussed briefly in terms of strain induced Born effective charge transfer.     

Fe-Ni-BaTiO3复合材料的介电行为及其机理研究

利用金属铁、镍(Fe与Ni保持mol比为22:78不变)与钛酸钡复合,在保护气氛下成功烧结制备了高介电常数Fe-Ni-BaTiO3复合陶瓷材料,并研究了该复合材料的电导和介电性能及其物理机理.分析结果表明,由于渗流效应,随着陶瓷中金属含量的增加,材料经历了绝缘体-导体突变.同时,在渗流阈值附近,材料的介电常数有了极大的提高.当金属体积含量为0.23时,即在绝缘体向导体转变的渗流阈值附近,复合材料的介电常数达到了22000,为同条件下制备的纯钛酸钡陶瓷体介电常数的12倍,同时材料的介电损耗仍保持为较低值.材料具有明显的Maxwell-Wagner效应特征,这表明传导机理对材料的介电常数的提高起到了很重要的作用.分析了传导机理对介电的影响.     

薄膜生长初期多原子移动影响的模拟

模拟了在低温底上沉积单层薄膜初期的表面形貌．提出了新的算法，实现了多个原子同时在沉积基底上移动的过程，克服了以往只能模拟基底上仅有一个运动原于的缺点，同时引入了沉积流量，并在此基础上讨论了移动步数、沉积流量等因素对生长的影响作用．模拟结果与实验相近，获得的花样具有分形特征．     

First-principles study of structural stability and elastic property of pre-perovskite PbTiO3

The structural stability and the elastic properties of a novel structure of lead titanate, which is named pre-perovskite PbTiO₃ (PP-PTO) and is constructed with TiO₆ octahedral columns arranged in a one-dimensional manner, are investigated by using first-principles calculations. PP-PTO is energetically unstable compared with conventional perovskite phases, however it is mechanically stable. The equilibrium transition pressures for changing from pre-perovskite to cubic and tetragonal phases are -0.5 GPa and -1.4 GPa, respectively, with first-order characteristics. Further, the differences in elastic properties between pre-perovskite and conventional perovskite phases are discussed for the covalent bonding network, which shows a highly anisotropic character in PP-PTO. This study provides a crucial insight into the structural stabilities of PP-PTO and conventional perovskite.     

First-principles study of the influences of oxygen defects upon the electronic properties of Nb-doped TiO_2 by GGA + U methods

The influence of oxygen defects upon the electronic properties of Nb-doped TiO2has been studied by using the general gradient approximation(GGA)+U method. Four independent models(i.e., an undoped anatase cell, an anatase cell with a Nb dopant at Ti site(NbTi), an anatase cell with a Nb-dopant and an oxygen vacancy(NbTi+VO), and an anatase cell with a Nb-dopant and an interstitial oxygen(NbTi+Oi)) were considered. The density of states, effective mass, Bader charge, charge density, and electron localization function were calculated. The results show that in the NbTi+VOcell both eg and t2glevels of Ti 3d orbits make contributions to the electronic conductivity, and the oxygen vacancies(VO) collaborate with Nb-dopants to favor the high electrical conductivity by inducing the Nb-dopants to release more excess charges. In NbTi+Oi, an unoccupied impurity level appears in the band gap, which served as an acceptor level and suppressed the electronic conductivity. The results qualitatively coincide with experimental results and possibly provide insights into the preparation of TCOs with desirable conductivity.     

锐钛矿相和金红石相Nb:TiO_2电学性质的GGA(+U)法研究

采用基于密度泛函理论第一性原理GGA和GGA+U相结合的方法研究了不同掺杂浓度下锐钛矿相和金红石相Nb:TiO2的晶体结构、电子结构以及稳定性.结果表明:锐钛矿相Nb:TiO2能带结构与简并半导体类似,呈类金属导电机理.金红石相Nb:TiO2呈半导体导电机理.Nb原子比Ti原子电离产生出更多的电子.锐钛矿相Nb:TiO2中Nb原子的电离率比金红石相Nb:TiO2的大.以上结果说明锐钛矿相Nb:TiO2比金红石相Nb:TiO2更适宜用作TCO材料;掺杂浓度对其杂质能级,费米能级和有效质量都有影响.Nb原子掺杂浓度越高,材料电离率呈降低趋势;形成能计算结果显示:在富钛条件下不利于Nb原子的掺杂,而在富氧条件下有利于Nb原子的掺杂.对于金红石相和锐钛矿相Nb:TiO2,不论是在贫氧或富氧条件下,随着Nb原子掺杂浓度的提高,形成能均增大.