Organic metal (EDO-TTF)2PF6 with multi-instability

Abstract

The multi-instability of the electronic structure of (EDO-TTF)₂PF₆, where EDO-TTF means ethylene-dioxytetrathiafulvalene, is reviewed. This complex showed the metal–insulator transition at 280 K associated with distinct molecular deformations. The mechanism is interpreted as the cooperation of Peierls transition, charge ordering, and the order–disorder transition of the countercomponent. The charge ordering pattern in the low-temperature phase is of the novel [0, 0, 1, 1] type. The sensitivity of the electronic state to external perturbations is demonstrated applying not only static but also instantaneous stimuli. In the latter case, the photo-induced phase transition is ultrafast and highly efficient. One photon causes the transition of several hundreds of donor molecules in the low-temperature phase to relax into a highly conducting metastable state within about 1.5 ps. In the early stage of the transient state, the charge ordering of the [1, 0, 1, 0] type occurs. As for the chemical modifications of this material, the partial deuteration of this complex increases the metal–insulator transition temperature. The introduction of a methyl group greatly modulates the electronic structure of the complex, i.e. (methyl-EDO-TTF)₂X (X=BF₄, ClO₄) shows a two-dimensional electronic structure. The working hypotheses for developing the systems with multi-instability are described.     

Charge Order in (TMTTF)2PF6 Investigated by Infrared Spectroscopy

To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)₂FeBr₄ under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near T_min ∼ 170 K followed by a gradual increase below it. With increasing pressure, T_min abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT _min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.     

(La_(1-x)Yb_x)_2Mo_2O_9(x=0.06)陶瓷的制备及其电性能

采用XRD、SEM、电化学工作站等方法对高温固相反应法合成的(La1-xYbx)2Mo2O9(x=0.06)固体电解质材料的电性能进行了研究.粉末XRD结果表明,该样品为单一立方相La2Mo2O9结构.在测定温度范围内,不同气氛下的电导率随温度的升高基本呈线性增大,干燥氧气气氛下表现出较高的电导率,900℃时达到最大值0.066S/cm.(La1-xYbx)2Mo2O9(x=0.06)能完全抑制La2Mo2O9母体的相变.氧分压与电导率的关系表明样品在高氧分压气氛中是离子导体,在低氧分压气氛中是离子与电子的混合导体.     

Na3＋xZ2—xYbxSi2PO12系统的相关系电导和结晶化学

研究了 Na_(3 x)Zr_(2-x)Yb_xSi_2PO_(12)系统的相关系和电导。此系统形成有结构连续转变的固溶体,在 x=0.5和 x=0.8时分别完成从单斜至三方和从三方至四方的结构转变。电导率开始随 x 的增加而缓慢降低,至 x=0.8后剧烈下降。比较了 Na_(3 x)Zr_(2-x)Yb_xSi_2PO_(12)与 Na_3Zr_(2-x)Yb_xSi_(2-x)P_(1 x)O_(12)以及 Na_(3 x)Zr_2Si_(2 x)P_(1-x)O_(12)系统,讨论了在此类系统中影响钠离子迁移的结晶化学因素。     

Spatial mapping of electronic states in κ-(BEDT-TTF)2X using infrared reflectivity

We review our recent work on spatial inhomogeneity of the electronic states in the strongly correlated molecular conductors κ-(BEDT-TTF)₂X. Spatial mapping of infrared spectra (SMIS) is used for imaging the distribution of the local electronic states. In molecular materials, the infrared response of the specific molecular vibration mode with a strong electron–molecular vibration coupling can reflect the electronic states via the change in the vibration frequency. By spatially mapping the frequency shift of the molecular vibration mode, an electronic phase separation has been visualized near the first-order Mott transition in the bandwidth-controlled organic conductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br. In addition to reviewing SMIS of the phase separation, we briefly mention the electronic and optical properties of κ-(BEDT-TTF)₂X.     

Rapid Oscillations in (TMTSF)2PF6

In order to clarify the origin of the “Rapid Oscillation” (RO) in (TMTSF)₂PF₆, we studied the magnetoresistance anisotropy in the Field-Induced Spin Density Wave (FISDW) phase. We have found that in the FISDW insulating state, the Fermi surface is not totally gapped; the remaining 2D metallic pockets are quantized in magnetic field and give rise to the RO. Decreasing temperature does not change the size and orientation of the closed pockets, rather, it causes depopulation of the delocalized states in favor of the localized ones, resulting in the disappearance of the RO.     

X-ray Investigation for α′-(BEDT-TTF)2BrICl at Low Temperature

The organic semiconductor α′-(BEDT-TTF)₂BrICl which exhibits an abrupt decrease of static magnetic susceptivity at Tc (approximately 200 K) with decreasing temperature was investigated by X-ray diffraction experiments. The drastic first-order structural phase transition at Tc was found. The unitcell volume became approximately half of that above Tc. The result of crystal structure analysis at 25 K indicated that the donor molecular arrangement was no longer “α′-type” but “β″-type” below Tc.     

Anisotropic SDW Dynamics in (TMTSF)2PF6

The anisotropic transport properties of the spin-density-wave model-com- pound (TMTSF)₂PF₆ have been studied by dc and microwave methods. According to mean-field theory, the activation energy in the SDW state below T_SDW = 12 K is approximately 20–25 K in all directions when measured by dc methods. Microwave experiments along the a, b′ and c* axes reveal that the collective transport, which is considered to be the fingerprint of the spin-density-wave condensate, is present in the a and b′ directions, but not along the least conducting c* axis. In contrast to common quasi one-dimensional models, the density wave also slides in the perpendicular b′ direction. Can this behavior be explained by the nesting properties of the quasi one-dimensional conductor?     

(Nd0.75Na0.25)1-x(Nd0.5Ca0.5)xMnO3中电荷有序稳定性关系的超声研究

系统研究了（Nd0.75Na0.25）1-x（Nd0.5Ca0.5）xMnO3（x=0、0．25、0．5、0．75、1）单相多晶样品在低温下的电磁输运性质和超声特性．电阻和磁化率的测量表明所有样品均发生了电荷有序相变．随着钠掺杂量的增加，电荷有序相变温度（Tco）向低温移动同时低温端磁化强度增大，并且电荷有序态趋向于不稳定和短程化．超声纵波声速从室温开始随着温度的降低逐渐减小，在Tco之后声速急剧硬化．这种超声异常表明体系中存在着强烈的电-声子相互作用，该电-声子耦合来源于Mn^3＋的Jahn-Teller效应．对纵波模量软化部分的拟合显示，随着钠的掺入，反映Jahn-Teller效应大小的Jahn-Teller耦合能EJT变小．分析认为电荷失配效应是导致电荷有序被抑制和Jahn-Teller耦合能EJT变小的主要因素．     

快离子导体Na2＋xZr2—xYbxSiP2O12系统的研究

以高温固相反应制备了 Na_(2 x)Zr_(2-x)Yb_xSiP_2O_(12)系统的合成物,确定了它们的相组成以及 Nasicon 单纯相的范围。计算了系统合成物的晶胞参数,测定了它们从室温至400℃的电导率。x=1.5的合成物具有最好的导电性,在300℃时其电导率为3.65×10~(-2)(Ω·cm)(-1),在200～400℃温区内其活化能为26.36 kJ/mol。     

锂快离子导体Li_(1 2x y)Al_xYb_yTi_(2-x-y)Si_xP_(3-x)O_(12)系统的研究

以ＬｉＴｉ２（ＰＯ４）３为基以天然高岭石为起始原料,经高温固相反应（９５０～１１５０℃）制得了 一系列锂快离子导体材料Ｌｉ１＋２ｘ＋ｙＡｌｘＹｂｙＴｉ２－ｘ－ｙＳｉｘＰ３－ｘＯ１２（以下简称Ａｌ－Ｙｂ－Ｌｉｓｉｃｏｎ）．系统 的合成温度随ｘ和ｙ值的增大而降低．应用交流阻抗技术测定的电导率数据结果表明ｙ＝０．３, ｘ＝０．１的合成物的电导率最好,４００℃时电导率达２．４５×１０－２Ｓ／ｃｍ, ２００～４００℃内的电导激 活能为３８．３ｋＪ／ｍｏｌ．ＸＲＤ分析结果表明在ｙ＝０．３,ｘ≤０４及ｙ＝０．５,ｘ≤０．３的组成范围内 均能得到空间群为Ｒ３ｃ的合成物．     

Microscopic Theory of Organic Superconductors

A microscopic theory of organic superconductors based on the concept of partial electron dielectrization is developed from first principles. Self-consistent equations for the superconducting order parameter () and insulating order parameter (D) are derived using a Green's function technique and equation of motion method. The theory is applied to explain the experimental results in the two-dimensional organic superconductor k-(BEDT-TTF)₂ Cu(NCS)₂. The present model explains coexistence of spin density wave (SDW) state and superconductivity state in the system. The behavior of superconducting order parameter (), insulating order parameter (D), specific heat, density of state, free energy, and critical field is also studied for the system k-(BEDT-TTF)₂ Cu(NCS)₂.     

Charge and Sodium Ordering in β-Na0.33V2O3

Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate -Na_0.33V₂O₃ are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.     

Superplasticity in Aluminium Brass(HAl 66-6-3-2)

1.IntroductionSuperplasticity is not merely a specialphenomenon for certain specific alley,but isone of the intrinsic properties of metallicmaterials[1].When the internal and externalconditions are suitable,superplasticity ofmetal materials will be presented.     

溶胶-凝胶法制备Sr掺杂La_(9.33-2x/3)Sr_x(SiO_4)_6O_2及其导电性研究

采用溶胶-凝胶法于900℃成功合成了Sr掺杂的La9.33-2x/3Srx(SiO4)6O2前驱粉体,然后于1450℃烧结成陶瓷样品。利用X射线粉末衍射和环境扫描电镜对材料的物相和形貌进行了分析和表征,结果表明合成产物为磷灰石相,烧结后样品晶体颗粒比较均匀,有较好的致密度。采用交流阻抗谱研究了材料的导电性,发现Ln位阳离子空位比Ln位阳离子掺杂对材料电导率的影响更大,合适的Ln位阳离子空位更有利于提高材料的电导率。     

Na_3Hf_(2-x)TixSi_2PO_(12)系统快离子导体的研究

以 Na_3PO_4、HfO_2、SiO_2为主要原料,铂舟为反应器皿,在900～1200℃的高温下,加热数小时至20h(少数样品达40h),合成了一系列合成物。多数反应在1100～1150℃进行。x=0～0.5的合成物为 NASICON和 HfO_2的二相区。x=0.6～1.0时为 NASICON 单纯相。x>1.0开始出现玻璃态。系统合成物的电导性测定结果发现除母体以外最好的是 x=0.6的合成物。其电导率在400℃时为2.76×10~(-2)(Ω·cm)~(-1)。它的活化能在200～400℃温区里为25.5kJ/mol。     

Electrochemical fabrication of (TMTSF)2X (X = PF6, BF4, CIO4) nanowires

TMTSF-based (TMTSF = tetramethyltetraselenafulvalene = C10H12Se4) charge-transfer salt nanowires were fabricated using the galvanostatic deposition technique that was assisted by an anodic aluminum oxide (AAO) template. By applying a low current density of 1-2 microA/cm2 for more than one month, nanowire arrays with diameters of approximately 150 nm and lengths of approximately 6 microm were obtained. The length of nanowires can be controlled by the duration of the constant current application. Energy-dispersive X-ray spectroscopic (EDX) analysis confirmed that selenium is one of the main components of the nanowires. The micro-Raman (v3C == C) and FT-IR spectra (v3PF6-, v3BF4-, v3CIO4-) indicated that the nanowire arrays had the (TMTSF)2X (X = PF6, BF4, CIO4) phase. The TEM images and the selected area electron diffraction (SAED) patterns indicate that the nanowires were not single crystals, but the current-voltage characteristic that was measured with the four-terminal method showed the conductivity of the (TMTSF)2PF6 single crystals (sigmaRT = 1.6 S/cm) at room temperature.     

DTA measurements of normal-incommensurate phase transition in (NH4)2ZnCl4 and K2ZnCl4 crystals

Differential thermal analysis (DTA) was applied to determine the changes in enthalpy and entropy of (NH₄)₂ZnCl₄ and K₂ZnCl₄ crystals at their phase transition from the orthorhombic normal phase to the incommensurate phase. The temperature of this transition, T _i , is 406 K for (NH₄)₂ZnCl₄ and 555 K for K₂ZnCl₄ and the entropy changes (S/R) are 0.053 and 0.035, respectively. The low value obtained for S/R is characteristic of incommensurate phase transitions. The results were compared with the data reported for other crystals of the A₂BX₄ family. Thermal properties of the crystals of the A₂ZnCl₄ subgroup were found to the correlated with the length of A-Cl bonds.