Al2O3:V3+晶体局域结构及其自旋哈密顿参量研究

提出了解释掺杂离子局域结构畸变的配体平面移动模型，建立了此模型下晶体微观结构与自旋哈密顿参量之间的定量关系.在考虑自旋与自旋、自旋与另一电子轨道和轨道与轨道作用等微小磁相互作用的基础上，采用全组态完全对角化方法，对Al₂O₃晶体中V³⁺的局域结构和自旋哈密顿参量进行了系统的研究.结果表明，V³⁺掺入Al₂O₃晶体后，上下配体氧平面间距离增大了0.0060 nm.从而成功地解释了Al₂O₃:V³⁺晶体的自旋哈密顿参量.在此基础上，研究了三角晶场下3d²离子自旋哈密顿参量的微观起源.研究发现，自旋三重态对自旋哈密顿参量的贡献是主要的，微小磁相互作用对自旋哈密顿参量的贡献只与自旋三重态有关.     

BC5力学性质的第一性原理计算

采用平面波赝势密度泛函理论方法对0—60 GPa静水压下BC₅ 六角晶系P3m1和四方晶系I4m2结构的平衡态晶格常数、弹性常数、各向异性以及泊松比与Cauchy扰动进行了研究. 研究结果表明, BC₅的两种结构在高压下是稳定的, 且不可压缩性随着压强的增加而增大. 另外, 对其电子结构也进行了计算, 计算结果表明, BC₅存在一个较宽的带隙, 两种原子间有较强的共价杂化, 材料的性质主要由B的2p¹和C的2p²态电子共同决定. 压强对材料带隙和费米能级附近的态密度几乎没有影响, 只引起微小的漂移, 可推断其很好的高压稳定性.     

准静水压条件下SrF2的X射线衍射

利用同步辐射能量色散X射线衍射技术对SrF₂进行了高压相变实验研究. 实验以Ar作为传压介质, 以获得准静水压条件. 实验用红宝石荧光法标定样品压力, 得到的最高压力为44.2GPa. 在实验压力范围内, SrF₂在11.2GPa时发生了从立方晶系到正交晶系的相变, 通过拟合相变前后的P-V状态 方程, 得到立方SrF₂的体弹模量为70.0(5.0)GPa, 正交SrF₂的体弹模量为198.3(15.2)GPa. 卸压到零压时, SrF₂回到立方相结构.     

掺入Ni2+和V3+对α-Al2O3晶体光谱精细结构、晶体局域结构和零场分裂参量的影响

应用晶体场理论和不可约张量算符方法构造了3d2/3d8态离子在C3v对称晶场中包含自旋-轨道相互作用、自旋-自旋相互作用、自旋-其它轨道相互作用和其它轨道-其它轨道相互作用四种微观磁效应的45阶可完全对角化的能量哈密顿矩阵.利用该矩阵,计算了V³⁺∶α-Al₂O₃和Ni²⁺∶α-Al₂O₃晶体的光谱精细结构、晶体局域结构和零场分裂参量,研究了掺入两种互补态离子Ni²⁺和V³⁺对同种晶体的光谱精细结构、晶体局域结构和零场分裂参量的影响,理论计算值和实验值相符.研究发现：掺杂没有改变晶体的光谱精细结构和能级分裂条数,但改变了能级间距|掺杂也没有改变晶体的对称性,但使晶体局域结构发生了一定程度的畸变| Ni²⁺∶α-Al₂O₃晶体局域结构的伸长畸变量大于V³⁺∶α-Al₂O₃晶体,键角的变化量小于V³⁺∶α-Al₂O₃晶体.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

电子束在VO2薄膜中引起的价态、相结构和光学性能的改变

 利用能量为1.7MeV, 注量分别为1.25×10¹³/cm², 1.25×10¹⁴/cm², 1.25×10¹⁵/cm²的电子束辐照VO₂薄膜,采用XPS, XRD等测试手段对电子辐照前后的样品进行分析,并研究了电子辐照对样品相变过程中光透射特性的影响。结果表明电子辐照引起VO₂薄膜中V离子出现价态变化现象,并使薄膜的X射线衍射峰发生变化。电子辐照在样品中产生的这些变化显著改变了VO₂薄膜的热致相变光学特性。     

Fe3O4/β-CD的高压电学性质研究

 高压下的电学性质测量是获得材料物理性质的有效手段。利用集成在金刚石对顶砧上的薄膜微电路,测量了高压下Fe₃O₄/β-CD（β-糊精）的电导率,并分析了电导率随压力的变化关系。在0～39.9 GPa范围内,Fe₃O₄/β-CD的电导率随压力的增加而逐渐增大,并呈半导体的特征;而在17.0 GPa处其电导率发生突变,表明样品发生了高压相变。在卸压过程中,电导率随压力的变化呈线性关系,并且卸压后样品的电导率不能回到最初的状态,推测这是一个不可逆的高压结构相变。     

N,N-二甲基硫代乙酰胺S3(ππ*)态的衰变动力学 (cited: 1)

采用共振拉曼光谱和完全活化空间自洽场方法研究了N,N-二甲基硫代乙酰胺在被激发至S₃(ππ^*)态后的衰减动力学. 指认了紫外吸收光谱和振动光谱. 获得了乙腈、甲醇和水溶剂中不同激发波长下的A带共振拉曼光谱,以探测Franck-Condon区域的结构动力学. 开展了CASSCF计算以确定低能单重激发态和锥形交叉点的电子激发能和优化几何结构. 通过共振拉曼强度分析和CASSCF计算获得了结构参数、A带结构动力学和S₃(ππ^*)态衰减机制. 提出了主要衰减通道为_3,FC(ππ^*)→S₃(ππ^*)/S₁(nπ^*)→_1(nπ^*).     

量子群Uqp(u2)模型对A～190区超形变带研究

用双参数量子群U_qp(u₂)理论模型公式对A～190区47条超形变(SD)转动带进行了系统分析.计算得到的Eγ跃迁谱与实验较好地吻合;按转动带自旋指定的3种方案确定¹⁹⁴Hg(1),¹⁹⁴Pb(1)的带首自旋,结果与实验一致.此外,进一步讨论了核软度参数σ1的物理意义,发现一对旋称对偶带的σ1几乎全等.     

Nd掺杂对Bi4Ti3O12铁电薄膜的微结构和铁电性能的影响

利用溶胶-凝胶法在Pt/Ti/SiO₂/Si(100)衬底上制备了Nd掺杂Bi₄Ti₃O₁₂(Bi_4-xNd_xTi₃O₁₂, x=0.00,0.30,0.45,0.75,0.85,1.00,1.50)铁电薄膜样品.研究了Nd掺杂对Bi₄Ti₃O₁₂薄膜的微结构和铁电性能的影响.研究结果表明:Nd掺杂未改变Bi₄Ti₃O₁₂薄膜的基本晶体结构.在掺杂量x<0.45时,Nd³⁺只取代类钙钛矿层中的A位Bi³⁺.当x=0.45时,样品剩余极化强度达最大值,在270kV·cm^-1的电场下为32.7μC·cm^-2.掺杂量进一步增加时,结构无序度开始明显增大,Nd³⁺开始进入(Bi₂O₂)²⁺层,削弱其绝缘层和空间电荷库的作用,导致材料剩余极化逐渐下降.当掺杂量x达到1.50时,掺杂离子最终破坏(Bi₂O₂)²⁺层的结构,材料发生铁电-顺电相变.     

Ab initio study on phase transition and magnetism of BiFeO3 under pressure

In this paper the first-principles calculations within local spin density approximation (LSDA)+U show that BiFeO3 experiences a mixed phase state with P4mm structure being the intermediate phase before the pressure of phase transition is reached. The critical pressure for the insulator-metal transition (IMT) is found to be about 50 GPa. A pressure induced crossover of high-spin states and low-spin states is observed close to the IMT pressure in R3c structure. The LSDA+U calculations account well for the mechanism of the IMT and crossover of spin states predicted in recent experiment (Ref.[1]).     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

Prediction of the phase transition from ferromagnetic perovskite to non-magnetic post-perovskite in SrRuO3 : A first-principles study

The phase transition in the perovskite (Pv) SrRuO₃ under pressure has been studied by using first-principles calculations based on density functional theory. The post-perovskite (Ppv) phase transition of SrRuO₃ will take place under hydrostatic pressure of about 40 GPa. The stability of Ppv- SrRuO₃ is justified by the enthalpy calculations, and this phase transition accompanies volume disconnection and magnetic moment collapses. The crystallographic data and the electronic structure of Ppv- SrRuO₃ are also predicted. The crystal structure of Ppv- SrRuO₃ is similar to that of Ppv- CaRuO₃. A non-magnetic ground state is found in Ppv- SrRuO₃. The strong hybridization of Ru and O is evident in the electronic structures of both phases. We expect that these results will help further understanding of SrRuO₃ under high pressure.     

Pressure dependence of electronic structure and magnetic properties in Fe16N2

The electronic structures and magnetic properties of Fe₁₆N₂ system and their pressure dependence were investigated by using first-principles calculations based on the density functional theory. It has been found that the total magnetic moment in Fe₁₆N₂ system decreases monotonically as increasing pressure from 0 to 14.6 GPa. A phase transition from ferromagnetic (FM) to non-magnetic (NM) occurs with a volume collapse of around 0.008  at 14.6 GPa, The lattice constants a and c for magnetic results decrease monotonically as pressure increasing from 0 to 14.6 GPa, at 14.6 GPa, the lattice constant a decreases sharply, on the contrary, the lattice constant c increases abruptly. We think that the change of microscopic structure of Fe₁₆N₂ is responsible for the phase transition from FM to NM.     

Hydrostatic pressure driven spin,volume and band gap collapses in SmFeO3: a GGA + U study

The crystal structure, magnetic and electronic properties of SmFeO₃ under hydrostatic pressure have been studied by first-principles calculations within the generalized gradient approximation plus Hubbard U (GGA + U). The iso-structural phase transition with spin, volume and band gap collapses can be induced by a large enough hydrostatic pressure. The high-spin (HS) state of Fe³⁺, with the magnetic moment of ~4 μ_B, is retained at low pressure. The spin crossover occurs at a transition pressure (~68 GPa) with the magnetic moment of Fe³⁺ decreasing to ~1 μ_B in low-spin (LS) state. Meanwhile, the reductions of cell volume (by ~?5.43%) and band gap (from >2 eV to ~1.6 eV) of SmFeO₃ are obtained when the HS–LS transition happens. Finally, the critical pressure of HS–LS transition, magnetic and electronic properties are found to be Hubbard U dependent.     

自旋-Peierls化合物GeCuO3电子结构的第一性原理研究

采用平面波赝势方法对自旋-Peierls化合物GeCuO₃的电子结构进行了第一性原理研究.计算结果表明：Cu²⁺的3d轨道自由度被冻结,未配对电子填充d_x²-y²轨道.自旋向上和向下的d_x²-y²轨道间的交换劈裂导致了体系的绝缘性.费米能级附近的Cu 3d态与O(2) 2p态存 关键词： 3')" href="#">GeCuO₃ 自旋-Peierls相变 第一性原理计算 共价绝缘体     

强冲击压缩下LiF，Al2O3和LiTaO3单晶的透光性

在二级轻气炮上，用高速电子相机扫描照相技术和改进的Mallory实验装置，对z切LiF，Al₂O₃(蓝宝石)和LiTaO₃单晶材料的冲击透光性进行了对比测量，并用黑密度计提取出动态图像定量化的光强对比度变化曲线.结果表明，LiF单晶在102 GPa压强下能够保持长时间的初始透光性不变，与公认的LiF具有优良的高压下透明性的认识一致.LiTaO₃单晶在实验压力(139GPa)下变成基本不透明.而Al_{关键词： 2}O₃')" href="#">Al₂O₃ LiF 3')" href="#">LiTaO₃ 光学透明性     

Structural, electronic and thermodynamic properties of cubic Zn3N2 under high pressure from first-principles calculations

The structural, electronic and thermodynamic properties of cubic Zn₃N₂ under hydrostatic pressure up to 80 GPa are investigated using the local density approximation method with pseudopotentials of the ab initio norm-conserving full separable Troullier-Martin scheme in the frame of density functional theory. The structural parameters obtained at ambient pressure are in agreement with experimental data and other theoretical results. The change of bond lengths of two different types of Zn-N bond with pressure suggests that the tetrahedral Zn-N bond is slightly less compressible than the octahedral bond. By fitting the calculated band gap, the first and second order pressure coefficients for the direct band gap ofthe Zn₃N₂ were determined to be 1.18×10⁻² eV/GPa and −2.4×10⁻⁴ eV/(GPa)², respectively. Based on the Mulliken population analysis, Zn₃N₂ was found to have a higher covalent character with increasing pressure. As temperature increases, heat capacity, enthalpy, product of temperature and entropy increase, whereas the Debye temperature and free energy decrease. The present study leads to a better understanding of how Zn₃N₂ materials respond to compression.     

氧化钒晶体的半导体至金属相变的理论研究

本文利用第一性原理方法研究了金红石相和单斜相VO₂晶体的电子结构和热力学性质.在计算中采用局域密度近似结合Hubbard U模型(LDA+U)描述电子的局域强关联效应,同时也利用微扰密度泛函方法计算了两种相结构的声子谱.计算结果表明V原子3d电子轨道中x²-y²轨道能级分裂决定了VO₂晶体在不同相结构下的金属和绝缘体特性.零温状态方程计算揭示了在68 GPa时可以发生从单斜结构 关键词： 2')" href="#">VO₂ 相变 第一性原理     

立方(Ba0.5Sr0.5)TiO3高压诱导带隙变化的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算研究了 (Ba_0.5Sr_0.5)TiO₃ (BST) 晶体在高压下的电子结构及能带变化行为. 研究结果发现,随着压强的增加,BST能带间隙先增加,在压强为55 GPa时达到最大值,然后减小,这些有趣的结果将有助于开发与设计新的BST铁电器件. 进一步地,通过电子态密度和密度分布图的研究分析可知:在低压区域(055 GPa),则是出现的离域现象占主导(电子的离域作用超过键态的作用),从而使带隙减小. 关键词： 钛酸锶钡 第一性原理 高压 能带间隙