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     

The monoclinic/orthorhombic phase transition in Ba2Cu(HCOO)6: An optical and x-ray powder diffraction study

α-Ba₂Cu(HCOO)₆ grown at room temperature crystallizes in space group P2₁. On heating a reversible, hysteretic, equitranslational, first-order phase transition at about 60-90°C takes place. Here we discuss the influence of thermal treatments on the occurrence, coexistence and stability ranges of the two observed phases, as studied by several techniques (optical microscopy, differential scanning calorimetry and X-ray diffraction). On cooling a single set of domain walls parallel to (001) have been observed. Taking into account experimental results and the crystal pseudosymmetry of the α phase we propose that the high-temperature β phase crystallizes in space group Pbnm.     

VO2金属-绝缘体相变机理的研究进展

VO₂是一种热致相变金属氧化物. 在341 K附近, VO₂发生由低温绝缘体相到高温金属相的可逆转变, 同时伴随着光学、电学和磁学等性质的可逆突变, 这种独特的性质使得VO₂在光电开关材料、智能玻璃、存储介质材料等领域有着广阔的应用前景. 因此, VO₂金属-绝缘体可逆相变一直是人们的研究热点, 但其相变机理至今未有定论. 首先, 简要概述了VO₂相变时晶体结构和能带结构的变化情况: 从晶体结构来讲, 相变前后VO₂从低温时的单斜相VO₂(M)转变为高温稳定的金红石相VO₂(R), 在一定条件下此过程也可能伴随着亚稳态单斜相VO₂(B)与四方相VO₂(A)的产生; 从能带结构来看, VO₂处于低温单斜相时, 其d_//能带和π^*能带之间存在一个禁带, 带宽约为0.7 eV, 费米能级恰好落在禁带之间, 表现出绝缘性, 而在高温金红石相时, 其费米能级落在π^*能带与d_//能带之间的重叠部分, 因此表现出金属导电性. 其次, 着重总结了VO₂相变物理机理的研究现状. 主要包括: 电子关联驱动相变、结构驱动相变以及电子关联和结构共同驱动相变的3种理论体系以及支撑这些理论体系的实验结果. 文献报道争论的焦点在于, VO₂是否是Mott绝缘体以及结构相变与MIT相变是否精确同时发生. 最后, 展望了VO₂材料研究的发展方向.     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

The electronic properties tuned by the phase transition between the semiconducting and metallic phase of monolayer MoS2/WS2

Using first-principles methods, we systematically investigate the electronic properties and atomic mechanism of the monolayer MoS₂/WS₂ homo-junction structure, which contains different phase structures, either the semiconducting hexagonal (H) structure or metallic trigonal (T) structure. Through tuning the size of the lateral homo-junction structure of either MoS₂ or WS₂, it can produce different boundaries which induce different phase transferred styles. More interestingly, the electronic structures of homo-junction structures can also be tuned by changing the size of the armchair and zigzag shapes of nanoribbons. The homo-junction structure of either MoS₂ or WS₂ exhibits alterable band structure and band edge position with the changing of the size. The strong dependence of the band offset on the sizes of the homo-junction monolayer also implicates a possible way of patterning quantum structures with size engineering.     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Pressure- and photo-induced phase transition in mixed-valence gold complexes

The pressure- and photo-induced phase transition in mixed-valence gold complexes of Cs₂Au₂X₆ (X = Cl, Br, and I) has been investigated by means of the Raman scattering. The Raman-active Au-X stretching modes were deactivated by the pressure, which indicates a pressure-induced phase transition from the mixed-valence (MV) state to the single-valence (SV) state. The electronic phase diagrams of Cs₂Au₂X₆ (X = Cl and Br) as a function of pressure and temperature have been derived. A photoinduced phase transition from the MV state to the SV state has been found for Cs₂Au₂Br₆. The observed time behavior accompanying this phase transition is successfully interpreted by the Avrami model, indicating the three-dimensional character of the MV cluster growth.     

电场诱导二氧化钒绝缘-金属相变的研究进展

二氧化钒(VO_2)是电子强关联体系的典型代表,其晶体结构在特定阈值的温度、电场、光照和压力等物理场作用下会发生由单斜金红石结构向四方金红石结构的可逆转变,从而引发绝缘-金属相变.其中,电场诱导VO_2绝缘-金属相变后的电导率可提高2-5个数量级,在可重构缝隙天线、太赫兹辐射以及智能电磁防护材料等领域具有广阔的应用前景,成为近年来人们的研究热点.首先,简要概述了VO_2发生绝缘-金属相变时晶体结构和能带结构的变化,进而从电场诱导VO_2绝缘-金属相变的研究方法、响应时间、临界阈值场强调控以及相变机理几个方面系统总结和评述了近年来国内外学者在该领域的重要发现和研究进展.最后,指出了当前VO_2绝缘-金属相变研究存在的问题,并展望了未来的发展方向.     

Influence of crystal synthesis on the incommensurate phase and lock-in transition of Rb2ZnCl4 crystals

A comparative study of several crystals of Rb₂ZnCl₄, obtained by different crystal growing methods, has allowed us to determine the influence of growth defects on the incommensurate phase and on the lock-in transition of these samples. X-ray diffraction has allowed us to complete previous dielectric measurements realized on the same samples and to relate the crystalline quality to the evolution of the modulation as a function of the temperature. The principal influence of an increasing defect density seems to be a stronger pinning of the modulated phase and this induces a lower lock-in temperature and a wider hysteresis.     

Non-hysteretic metal-insulator transition of VO2 films grown by excimer-laser-assisted metal organic deposition process

We examined the correlation between thickness of an epitaxial VO₂ phase grown on a TiO₂ (0 0 1) substrate by the excimer-laser-assisted metal organic deposition (ELAMOD) process and the metal-insulator transition (MIT) property of it. The abrupt and hysteretic MIT was observed for the epitaxial films (thickness: t ≥ 6 nm), and the epitaxial film (t ≤ 4 nm) showed semiconductor behavior. When an amorphous VO_x layer was prepared on the ultrathin epitaxial phase (t ≤ 4 nm) by the ELAMOD, a non-hysteretic MIT was successfully observed. The non-hysteretic MIT was found to be owing to roughened interface between the epitaxial phase and the amorphous phase, where there would be a number of structural defects.     

A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract

High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD₃ND₃)₂MnCl₄. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) Å; b = 7.409 (8) Å c = 17.126 (12) Å and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (?10%) of the inter-layer distance. Space groups P2/c or P2₁/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.     

(H2O)6的稳定结构及异构过程研究

(H₂O)₆是形成三维立体结构的最小水分子团簇并具有能量较低的多个稳定异构体.本文利用从头计算方法研究了各稳定结构的异构化过程.(H₂O)₆的环状结构与最稳定结构的能量差0.31 eV为一个氢键的键能.水分子团簇的异构化是分子间氢键打开或重组的过程,不同异构体之间的转化每次只涉及一个氢键的打开或重组,异构化的能垒高度在0.07—0.21 eV之间. 关键词： 水分子团簇 2O)₆')" href="#">(H₂O)₆ 异构化过程 从头计算     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

Probing the metal-insulator phase transition in the (DMEDO-EBDT)2PF6 single crystal by optical measurements

The temperature and polarization dependence of the optical reflectivity spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT)(2)PF(6), have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (E(g)?>?0.1?eV) and transfer of the spectral weight accompanied by the metal-insulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase.     

A genetic approach to the dielectric investigation of the high-temperature phase transition of TEA2MnCl4 crystals

The dielectric function of TEA₂MnCl₄ crystals is studied for a temperature range of 203–263?K in a frequency range of 1 kHz to 10?MHz. The measurement results are processed in the framework of the Cole–Cole model. An influence of the phase transition on the results is widely discussed. A genetic approach is successfully applied to the measured data and it is explained in detail. The method renders it possible to calculate the temperature dependence of dielectric properties in the time domain. The calculation method (the genetic algorithm) proved to be trustworthy and it is recommended to be employed in studying dielectric properties. A structural properties of the high temperature phase (i.e. above the phase transition temperature) will be also discussed and certain presumptions on the low temperature phase will be given.     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

Elastic,electronic and thermal properties of topological insulator SmB6: first-principles calculations

Abstract

The pressure dependence of the structural, elastic, electronic and thermal properties of Kondo insulator SmB₆ have been systematically studied by density functional theory combined with the quasi-harmonic Debye model. The calculated structure at zero pressure is in good agreement with the available experimental results at low temperature. The obtained elastic constants, bulk modulus and shear modulus indicate that SmB₆ is mechanically stable and behaves in a brittle manner under the applied pressure 0–20 GPa, consistent with available experimental data. In addition, the elastic-relevant properties, Young’s modulus and the Poisson ratio manifest that increasing pressure results in an enhancement in the stiffness of the compound. It is found that unlike temperature, pressure has little effect on the heat capacity of SmB_6. What more important is that we observed an insulator to metal phase transition at about 5.5 GPa through the disappearance of the band gap, well consistent with the experimental data. This transition has little effect on the physical properties of SmB₆.     

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO₂ crystal were investigated. At the MIT, the VO₂ crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ～10⁵, and a small transition width (i.e. temperature difference before and after MIT) as small as 10^?3°C. These MIT characteristics of the VO₂ crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.