高压下CaF_2结构相变和光学性质的第一性原理计算

利用基于密度泛函理论的第一性原理方法,计算了在压力作用下CaF2的结构相变和光学性质。结果证实了CaF2的压致结构转变的顺序是从氟石结构(空间群Fm3m)转变到PbCl2型结构(空间群Pnma),然后继续转变为Ni2In型结构(空间群P63/mmc)。在Fm3m和Pnma两种结构中,电子带隙随着压力的增加而增加,而在P63/mmc结构中,带隙随着压力的增加开始下降。实验结果显示,直到210 GPa,CaF2没有发生由绝缘体到金属的转变。据此推测,CaF2的金属化压力高于300 GPa。还讨论了压力对CaF2光学性质的影响。     

Mg2C高压性质的从头计算法研究

运用基于密度泛函理论的平面波赝势方法,结合广义梯度近似,系统地研究了Mg2C在高压下的结构相变、电子结构和光学性质。计算结果表明Mg2C在高压下将发生两次相变,一次是从反萤石到反氯化铅结构的一阶相变在30.09 GPa,另一次是从反氯化铅结构到Ni2In型结构的二阶相变在260 GPa。此外,对压力下电子结构和光学性质的分析表明,Mg2C的带隙宽度随着压力增加而增加,与Mg2Si在压力下表现出金属性有很大不同。     

ZnS结构相变、电子结构和光学性质的研究

运用第一性原理平面波赝势和广义梯度近似方法, 对闪锌矿结构(ZB)和氯化钠结构(RS) ZnS的状态方程及其在高压下的相变进行计算研究, 分析相变点附近的电子态密度、能带结构和光学性质的变化机理. 结果表明: 通过状态方程得到ZB相到RS相的相变压强值为18.1 GPa, 而利用焓相等原理得到的相变压强值为18.0 GPa; 在结构相变过程中, sp³轨道杂化现象并未消失, RS相ZnS的金属性明显增强; 与ZB相ZnS相比, RS相ZnS的介电常数主峰明显增强, 并向低能方向出现了明显偏移, 使得介电峰向低能方向拓展, 在低能区电子跃迁大大增强. 关键词： 硫化锌 相变 电子结构 光学性质     

ZnTe结构相变、电子结构和光学性质的研究

运用密度泛函理论体系下的投影缀加波方法, 对闪锌矿和朱砂相结构的ZnTe在高压下的状态方程和结构相变进行了研究, 并分析了相变前后的原胞体积、电子结构和光学性质. 结果表明: 闪锌矿结构转变为朱砂相结构的相变压力为8.6 GPa, 并没有出现类似材料高压导致的金属化现象, 而是表现出间接带隙半导体特性. 相变后, 朱砂相结构Zn和Te原子态密度分布均向低能级方向移动, 带隙变小; 轨道杂化增强, 更有利于Te 5p与Zn 3d间的电子跃迁, 介电常数虚部主峰明显增强, 但宏观介电常数不受压力的影响.     

Ce-La-Th合金高压相变的第一性原理计算

采用第一性原理计算对Ce_(0.8)La_(0.1)Th_(0.1)在高压下fcc-bct的结构相变、弹性性质及热力学性质进行了研究讨论.通过对计算结果的分析,发现了合金在压力下的相变规律,压强升高到31.6 GPa附近时fcc相开始向bct相转变,到34.9 GPa时bct相趋于稳定.对弹性模量的计算结果从另一角度反映了结构相变的信息.最后,利用准谐德拜模型对两种结构的高温高压热力学性质进行了理论预测.     

高压下金红石的拉曼光谱分析

常温下利用金刚石压腔装置(DAC)对金红石加压至40GPa,进行拉曼光谱的原位分析.发现压力约为13GPa时,金红石结构转变为斜锆石结构(ZrO2).21.1GPa相变完全.直至实验最高压力,没有进一步相变出现.在卸压中,斜锆石结构转变为α-PbO2结构.实验压力通过红宝石用拉曼光谱测压的计算方法确定,快速方便.     

微区Raman光谱在TiO_2高压结构相变研究中的应用

本文以金红石单晶TiO2和锐钛矿多晶TiO2为研究对象,应用金刚石小压机和原位拉曼光谱测量技术,系统研究了室温高压下TiO2的结构相变。原位拉曼测量表明,金红石单晶TiO2在压力达到12.91GPa时开始发生由金红石结构向斜锆石结构(MI)的相变,当压力达到14.16 GPa时,相变完成;继续加压到21.65 GPa,没有发现进一步的相变;卸压时由斜锆石结构转变为PbO2结构,相变发生在大约7.11 GPa处。锐钛矿多晶TiO2在压力达到4.26 GPa时开始向PbO2结构转变,当压力达到8.34 GPa时相变完成;继续加压到12.94 GPa,样品开始发生由PbO2结构向斜锆石结构的相变,当压力达到18.74 GPa时相变完成;继续加压到21.39 GPa,没有发现进一步的相变;卸压时也由斜锆石结构转变为PbO2结构,起始相变压力点应高于8 GPa。     

掺氧化钙及相变对高压下立方氧化锆电子结构和光吸收的影响

研究表明,立方氧化锆可作为冲击波实验中的窗口材料.为了使得该材料在常态下保持结构稳定,需添加稳定剂——氧化钙.然而,掺杂会导致其在29 GPa的冲击压力下从立方转变为斜方Ⅱ结构相.因此,该材料在冲击压缩下的电子结构和光学吸收性质以及作为光学窗口的适用压力范围是值得研究的重要问题.本文运用第一性原理的方法,分别计算了在100 GPa范围内两种结构氧化锆的电子结构和光学吸收性质.结果表明:(1)在立方结构相区,冲击压力将导致其吸收边蓝移,而在斜方Ⅱ结构相区,却使得其吸收边红移;(2)在立方结构相区,掺杂将引起能隙变窄(吸收边红移),但对于斜方II相区,却导致能隙变宽(吸收边蓝移);(3)冲击结构相变使得能隙变窄,吸收边红移.本文数据建议,掺氧化钙的立方氧化锆在95GPa的冲击压力范围内可作为光学窗口材料.     

In2（MoO4)3晶体的高压拉曼振动研究

钼酸铟(In2(MoO4)3)由于其独特的负热膨胀性质,已被广泛应用于燃料电池、光学器件、激光材料等方面. 为了进一步探讨其晶体结构和物理性质,本工作在金刚石对顶砧上原位测量了In2(MoO4)3 的高压拉曼光谱,最高压力达到18 GPa. 在研究的压力范围内,本工作观察到了两次相变. 首先,In2(MoO4)3在压力为1.2 GPa 时, 发生了 由从P21/a相到低对称结构的相转变. 在压力为5.8 GPa 时, 样品又发生了从晶相到非晶相的第二次相转变. 卸压后样品仍为非晶相,说明In2(MoO4)3 在高压下的非晶相变为不可逆相变.      

高压下TiN的结构转变、弹性和热力学性质的第一性原理计算

 利用基于密度泛函的第一性原理,计算了高压下TiN的结构转变、弹性和热力学性质。计算结果表明：在压力作用下,TiN经历了从NaCl型结构到CsCl型结构的转变,转变压力为348 GPa;TiN的弹性系数随着压力的增加呈线性增加规律。此外,还给出了德拜温度和热容量这两个重要热力学量与温度和（或）压力的依赖关系。     

Structural stabilities, electronic, elastic and optical properties of SrTe under pressure: A first-principles study

The structural, electronic, elastic and optical properties as well as phase transition under pressure of SrTe have been systematically investigated by first-principles pesudopotential calculations. Five possible phases of SrTe have been considered. Our results show that SrTe undergoes a phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 10.9 GPa with a volume collapse of 9.43%, and no further transition is found. We find that SrTe prefer h-MgO instead of wurtzite (B4) structure for its metastable phase because that the ionic compound prefers a high coordination. The elastic moduli, energy band structures, real and imaginary parts of the dielectric functions have been calculated for all considered phases, and we find that a smaller energy gap yields a larger high-frequency dielectric constant. Our calculated results are discussed and compared with the available experimental and theoretical data.     

压力对有机半导体均苯四甲酸晶体结构转变和电子性质的影响

研究高压条件下均苯四甲酸(C₁₀H₆O₈)材料的结构和性质对探索有机半导体材料的应用有积极意义.基于密度泛函理论的第一性原理赝势平面波方法,开展了0-300 GPa压强下C₁₀H₆O₈晶体的结构、电子和光学性质的研究.晶格常数在压强20 GPa和150 GPa下出现了明显跳变,且原子之间随着压强变化反复地出现成键/断键现象,表明压强可诱导晶体结构变化.电子结构的性质表明,0 GPa的C₁₀H₆O₈晶体是带隙为3.1 eV的直接带隙半导体,而压强增加到150 GPa时,带隙突变为0 eV,表明了晶体由半导体转变为导体.当压强为160 GPa时,晶体又变成了能隙约为1eV的间接带隙半导体,这可能是费米能级附近仅受O-2p轨道电子影响所导致.通过对C₁₀H₆O₈晶体介电函数的分析,再次验证了晶体在150 GPa时发生了结构相变.同时...     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

GaN结构相变、电子结构和光学性质

运用第一性原理平面波赝势和广义梯度近似方法,对纤锌矿结构和氯化钠结构GaN的状态方程及其在高压下的相变进行计算研究,分析相变点附近的电子态密度、能带结构和光学性质的变化机制.通过状态方程和焓相等原理得到GaN从纤锌矿到氯化钠结构的相变压强分别为43.9 Gpa和46.0 Gpa;在相变的过程中,GaN由典型的直接带隙半导体转变为间接带隙半导体材料;氯化钠结构GaN相比于纤锌矿结构,介电函数主峰值增强,本征吸收边明显往高能方向移动,氯化钠结构GaN在低能区域的光学性质差于纤锌矿结构.     

Contribution to the study of structural and elastic properties of wüstite under pressure up to 140 GPa by pseudopotential calculations

Using first-principles calculations based on the density functional theory and the generalized gradient approximations, we have studied the effect of high pressures up to 140 GPa on the structural and elastic properties of wüstite. Our results indicate that FeO undergoes a structural phase transition from NaCl-type (B1) to NiAs-type (B8) almost at the pressure of 77 GPa. The density increases across this transition by about 5%, which is a higher value than that obtained in other researches. We can clearly present the wüstite elastic properties and isotropic wave velocities which are not already studied in this range of pressure, and we could compare these results with the available experiment data, especially with that of PREM model.     

高压下稳态六方Ge2Sb2Te5结构、电子和光学性质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法研究了稳态六方petrov原子序列结构Ge2Sb2Te5的结构、电子和光学性质。计算所得的平衡态晶格参数与实验数据和先前的理论结果吻合很好。基态的能带结构和态密度表明了稳态六方petrov原子序列结构的Ge2Sb2Te5持有金属性。从压强影响下体积的变化趋势发现稳态六方Ge2Sb2Te5在17 GPa和34 GPa 出现不稳定,暗示在此压强下的相变发生,这与2009年Krbal等人的实验结果相吻合。同时,还系统地研究了稳态六方petrov原子序列结构的Ge2Sb2Te5高压下的光学性质,得到了高压下介电函数、吸收率、光反射率、折射率、消光系数和电子能量损失谱在20 eV内的变化情况。     

First-principles calculations of the structural, electronic, and optical properties of LiF up to 300 GPa

The structural, electronic, and optical properties of LiF are investigated at high pressures using the plane-wave pseudo-potential density functional method (DFT) within the generalized gradient approximation (GGA). From the analysis of Gibbs free energies, we find that no phase transition takes place for LiF in the presented pressure range from 0 to 300 GPa. The result is consistent with the theoretical prediction obtained from the ab initio calculations [N.A. Smirnov, Phys. Rev. B 83 (2011) 014109] that the rock-salt structure is thermodynamically stable up to 1000 GPa. Meanwhile, good agreement between the calculated equation of state parameters and the experimental results is obtained, and a direct energy gap of 8.65 eV is estimated in the DFT-GGA for LiF with rock-salt structure. In addition, the dielectric function and optical properties such as reflectivity, absorption coefficient, and refractive index dependence of the photon energy from 0 to 50 eV and wavelength from 0 to 200 nm at different pressures are also calculated and analyzed. It is found that the rock-salt LiF is transparent from the partially ultra-violet to the visible light area and hardly is the transparence affected by the pressure. Furthermore, the curve of optical spectrum will shift to high energy area with increasing pressure.     

High Pressure effects on electrical resistivity and dielectric properties of nanocrystalline SnO2

Nanocrystalline tin oxide (SnO₂) was prepared by chemical precipitation method with different grain sizes. The X-ray diffraction studies showed the structural stability of nanocrystalline SnO₂ under high-pressure. Electrical and dielectric properties were studied on these samples using complex impedance spectroscopy under different hydrostatic pressures. Electrical resistivity and dielectric studies showed a transition in nanocrystalline SnO₂ when it was subjected to high-pressure. The transition pressures obtained from both the resistivity and dielectric measurements agree with each other. The transition pressures were found to increase considerably with the decrease in grain size. Dielectric constant was found to decrease with the reduction of grain size. In order to find whether the transition with pressure is structure- related or not, Raman spectroscopy was done at normal temperature and pressure (NTP) and as a function of pressure at room temperature. Raman modes at NTP showed lines which correspond to tetragonal rutile structure of SnO₂. In situ high-pressure Raman measurements were carried out up to 3.38 GPa. No structural change was found with pressure.