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1.
We have investigated crystal structures of martensite and intermediate phases in stoichiometric Ni2MnGa. The neutron diffraction profile of the martensite phase measured at T = 4.2 K exhibits four satellites between [2 0 0]P* and [0 2 0]P* reflections (P stands for the parent phase) at incommensurate positions of [h 2-h 0]P* with h = 0.428, 0.863, 1.136 and 1.572. The profile of the intermediate phase measured at T = 210K exhibits two satellites between [2 0 0]P* and [0 2 0]P* reflections at incommensurate positions of [h 2-h 0]P* with h = 0.343 and 1.657. Although each satellite of the martensite phase moves toward its nearest fundamental reflection as temperature increases, that of the intermediate phase does not move significantly. On the contrary, the intensity of each satellite decreases significantly in the intermediate phase as temperature increases while not in the martensite phase. A synchrotron X-ray diffraction and a Rietveld analysis of the result reveal that, for both the phases, the displacement of atoms from the parent phase are represented by a sine wave whose propagation vector is parallel to [1 1 0]P.  相似文献   

2.
The crystal structure [(C2H5)4N]2CuCl4 has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P42/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at Tc=258K is of improper ferroelastic type and it is associated with the ordering of the CuCl4 2? and a partial ordering of the [(C2H5)4N]+ ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.  相似文献   

3.
The crystal structure [(C2H5)4N]2CuCl4 has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P42/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at Tc=258K is of improper ferroelastic type and it is associated with the ordering of the CuCl4 2− and a partial ordering of the [(C2H5)4N]+ ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.  相似文献   

4.
The first order semiconductor - metall phase transition with the lattice symmetry change from monoclinic to tetragonal and the conductivity jump of several orders of magnitude is observed in pure vanadium dioxide and the dioxide doped by varios impurities [I]. Transition temperature Tt for pure VO2 is 68°C and depends on the impurity. For example, the impurities which enter the insulating phase as pentavalent ions Nb5+ [2] give rise to a decrease in metal-semiconductor transition temperature at low concentrations and the trivalent ions Cr3+, Fe3+ [3,4] lead to an increase in Tt. In the latter case three different monoclinic phases are stabilized at a temperature below Tt. It has been already shown [5] that for double doping of vanadium dioxide by pentavalent Nb5+ and trivalent Fe3+ or Cr3+ ions three semiconductor phases exist (MI, M2, M3) but the transition temperature decreases as the impurity concentration increases.  相似文献   

5.
The triple-axis neutron scattering technique has been used to observe the soft-phonon modes associated with the second-order structural phase transitions in the cubic antifluorite K2ReCl6. The transverse (ReCl6)?2 rotary mode has been found to be soft all along the [ζ00] direction, with the structural distortions being triggered by a condensation first at the zone center and then at the [100] zone boundary. In addition to the soft phonon modes there is a temperature dependent central component to the spectrum along the [ζ00] direction.  相似文献   

6.
The temperature and angular dependences of the EPR spectra of Mg[H2O]6SiF6:Mn2+ crystal were investigated in order to clarify the successive phase transitions and existence of the incommensurate phase. Five successive phase transitions were found to occur, and phase II was found to be incommensurately modulated. The modulated structure is caused mainly by the vibrational displacement of the Mg[H2O]2+ 6 ion along the c-axis. The soliton density of this phase is almost independent of temperature and remains equal to unity.  相似文献   

7.
Single crystal 400 nm thick Laves phase [20 Å?DyFe2/80 Å?YFe2]40 superlattice have been grown by MBE with a (110) growth direction. VSM measurements performed at room temperature with an applied field range of ±1.2×105 Oe, directed along the [001] direction, reveal a unique single-phase-liked ferrimagnetic behavior. A dominant exchange spring behavior is revealed by MOKE measurement along the [–110] direction. Furthermore, for striped arrays patterned along the [001] direction with height-to-width ratio of 0.05, a shape anisotropy of the order of 104 erg/cm3 is induced, resulting into a pronounced change of coercivity due to the comparable magnitude with intrinsic anisotropies. The results demonstrate the feasibility of engineering both single-phase-liked and exchange-spring magnet behavior in Laves phase epitaxial hard/soft superlattices by patterning.  相似文献   

8.
The orientation dependences of the second-order quadrupole shifts of the central component in the 23Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na1/2Bi1/2TiO3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] p direction and a small displacement statistically or dynamically disordered over six equally probable [100] p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.  相似文献   

9.
Gas‐phase structure, hydrogen bonding, and cation–anion interactions of a series of 1‐(2‐hydroxyethyl)‐3‐methylimidazolium ([HOEMIm]+)‐based ionic liquids (hereafter called hydroxyl ILs) with different anions (X = [NTf2], [PF6], [ClO4], [BF4], [DCA], [NO3], [AC] and [Cl]), as well as 1‐ethyl‐3‐methylimizolium ([EMIm]+)‐based ionic liquids (hereafter called nonhydroxyl ILs), were investigated by density functional theory calculations and experiments. Electrostatic potential surfaces and optimized structures of isolated ions, and ion pairs of all ILs have been obtained through calculations at the Becke, three‐parameter, Lee–Yang–Parr/6‐31 + G(d,p) level and their hydrogen bonding behavior was further studied by the polarity and Kamlet–Taft Parameters, and 1H‐NMR analysis. In [EMIm]+‐based nonhydroxyl ILs, hydrogen bonding preferred to be formed between anions and C2–H on the imidazolium ring, while in [HOEMIm]+‐based hydroxyl ILs, it was replaced by a much stronger one that preferably formed between anions and OH. The O–H···X hydrogen bonding is much more anion‐dependent than the C2–H···X, and it is weakened when the anion is changed from [AC] to [NTf2]. The different interaction between [HOEMIm]+ and variable anion involving O–H···X hydrogen bonding resulted in significant effect on their bulk phase properties such as 1H‐NMR shift, polarity and hydrogen‐bond donor ability (acidity, α). Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

10.
In order to assess the localization in k-space of the secondary valence band extrema, we have measured the stress dependence of the indirect transition of TiO2, at 1.6 K, for static uniaxial compression along the [100] and [110] directions. From the stress-induced splittings we localize the secondary valence-band extrema in [100] direction (Δ direction) and associate the indirect gap of TiO2 with a Δ4vΓ1c phonon-assisted transition. The shear deformation potential of the Δ4v valence band has been determined from the stress-induced splitting of the TA phonon-assited transition. We find |CΔ4v| = 0.75 eV.  相似文献   

11.
崔万秋  周晶 《物理学报》1988,37(3):430-438
本文用透射法EXAFS和IR,Raman光谱等手段研究了以P2O5,为基体的非晶态铜离子导体的微观结构,得出了非晶结构网络中存在[PO4]3-,[CuO4]6-,[CuO3I]5-,[PO3I]2-等四元骨架结构单元,Cu+在结构网络空隙中有两种不同的分布状况。并在此基础上对材料在50℃左右存在的二级相变及Cu+的其它传导性能进行了定性的描述。 关键词:  相似文献   

12.
Pulse radiolysis with optical absorption detection has been used to study the reactions of hydroxyl radical (OH?) with 4‐thiouracil (4TU) in aqueous medium. The transient absorption spectrum for the reaction of OH? with 4TU is characterized by λmax 460 nm at pH 7. A second‐order rate constant k(4TU+OH) of 1.7 × 1010 M?1 s?1 is determined via competition kinetics method. The transient is envisaged as a dimer radical cation [4TU]2?+, formed via the reaction of an initially formed radical cation [4TU]?+ with another 4TU. The formation constant of [4TU]2?+ is 1.8 × 104 M?1. The reactions of dibromine radical ion (Br2??) at pH 7, dichlorine radical ion (Cl2??) at pH 1, and azide radical (N3?) at pH 7 with 4TU have also produced transient with λmax 460 nm. Density functional theory (DFT) studies at BHandHLYP/6–311 + G(d,p) level in aqueous phase showed that [4TU]2?+ is characterized by a two‐centerthree electron (2c‐3e) [?S∴S?] bond. The interaction energy of [?S∴S?] bond in [4TU]2?+ is ?13.01 kcal mol?1. The predicted λmax 457 nm by using the time‐dependent DFT method for [4TU]2?+ is in agreement with experimental λmax. Theoretical calculations also predicted that compared with [4TU]2?+, 4‐thiouridine dimer is more stable, whereas 4‐thiothymine dimer is less stable. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
本文通过对11B核磁共振(11B-NMR)、红外光谱等实验方法,研究了LiF-LiCl-B2O3三元系统玻璃的结构和离子导电性,着重于F-离子在玻璃网络中所起的作用,以及F-,Cl-和Li+离子对导电率的影响。LiF-LiCl-B2O3三元系统玻璃,随LiF含量的增加,B由三角体向四面体变化,从而F-离子进入网络,使玻璃结构由[B2O3]三角体层状结构向三维空间延展,形成了含有[BO3F]基团的三维空间网络,Cl-离子以游离的离子存在于网络中,起着松散网络的作用,对提高电导率有利,而Li+离子作为传导离子,对电导率的贡献是主要的。本系统玻璃的电导率是随LiF,LiCl含量的增加而增大,在300℃时测得电导率σ=6.12×10-4Ω-1·cm-1关键词:  相似文献   

14.
By using electrospray ionisation mass spectrometry, it was proven experimentally that the cesium cation (Cs+) forms with [2.2.2]paracyclophane (C24H24) the cationic complex [Cs(C24H24)]+. Further, applying quantum chemical calculations, the most probable structure of the [Cs(C24H24)]+ complex was derived. In the resulting complex with a symmetry very close to C3, the ‘central’ cation Cs+, fully located in the cavity of the parent [2.2.2]paracyclophane ligand, is bound to all three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, E(int), of the considered cation–π complex [Cs(C24H24)]+ was found to be ?73.2 kJ/mol, confirming the formation of this fascinating complex species as well. This means that [2.2.2]paracyclophane can be considered as a receptor for the Cs+ cation in the gas phase.  相似文献   

15.
Lithium niobate (LiNbO3) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg2+, Sc3+ and W6+, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]Nb centres coexisting with the [Cr]Li centres.In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO3 to tetravalent cation such as GeO2. A singularity in the relative intensity of the 4A24T1 and 4A24T2 absorption band was observed for a concentration of ~1.5 mol%, compared with 4.5 mol% for Mg2+. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.  相似文献   

16.
The present paper analyzes the valence-coordination state of iron atoms in polymer composite materials fabricated from polystyrene and iron-ore concentrates – magnetite and hematite – upon exposure to fast electrons with energy of 6.2 MeV and fluence of 1018 electrons/cm2. Changes in the phase content and valence-coordination and magnetic states of iron atoms are established. After irradiation, the hematite phase is transformed into the magnetite phase with a predominance of Fe3+ ions with [FeO4] coordination, and the magnetite phase forms the wüstite-type (FeO) phase in which Fe2+ ions are arranged in [FeO6] coordination.  相似文献   

17.
In this paper we investigate the solvation of silver bis(trifluoromethylsulfonyl)imide salt (AgTFSI) in 1‐ethyl‐3‐methylimidazolium TFSI [EMI][TFSI] ionic liquid by combining Raman and infrared (IR) spectroscopies with density functional theory (DFT) calculations. The IR and Raman spectra were measured in the 200–4000 cm−1 spectral region for AgTFSI/[EMI][TFSI] solutions with different concentrations ([AgTFSI] <0.2 mole fraction). The analysis of the spectra shows that the spectral features observed by dissolution of AgTFSI in [EMI][TFSI] solution originate from interactions between the Ag+ cation and the first neighboring TFSI anions to form relatively stable Ag complexes. The ‘gas phase’ interaction energy of a type [Ag(TFSI)3]2− complex was evaluated by DFT calculations and compared with other interionic interaction energy contributions. The predicted spectral signatures because of the [Ag(TFSI)3]2− complex were assessed in order to interpret the main IR and Raman spectral features observed. The formation of such complexes leads to the appearance of new interaction‐induced bands situated at 753 cm−1 in Raman and at 1015 and 1371 cm−1 in IR, respectively. These specific spectral signatures are associated with the ‘breathing’ mode and the S–N–S and S–O stretching modes of the TFSI anions engaged in the complex. Finally, all these findings are discussed in terms of interaction mechanisms enabling the electrodeposition characteristics of silver from AgTFSI/[EMI][TFSI] IL‐based electrolytic solutions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

18.
钟立军  陶瑞宝 《物理学报》1992,41(12):2003-2007
应用第二类相变的对称理论于磁性体系,导出了不同于幺正群情形的共表示的Molien函数;讨论了FeRh合金的对称破缺方向;发现由于磁群的共表示非磁群的真正同态,共表示意义下的等价不可约表示可导致不同的对称破缺;存在Oh1⊕θOh1→C4v1⊕θ[D4h1-C4v1]的连续磁相变,与实验结果一 关键词:  相似文献   

19.
20.
Magnetization measurements have been made on a Sm2Fe14B single crystal in magnetic fields up to 140 kOe. The easy direction of the magnetization lies along [100] in the tetragonal structure P42/mnm. Magnetic anisotropy energies at 290K along [110] and [001] have been estimated to be 5.8×106 and 1.1×108 erg/cm3, respectively, both becoming much larger at lower temperature. No evidence of the spin canting of Nd2Fe14B type is observed even at 4.2K.  相似文献   

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