The EPR of Fe3+ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe2+ ions in an octahedral ferrous complex [Fe(trz)(Htrz)2](BF4). The temperature dependence of the intensity of the Fe3+ EPR line atg=4.3 reveals a spin transition which occurs for the Fe2+ ions, with hysteresis. The transition temperatures areTc↑=374 K in the warming mode andTc↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition. 相似文献
Spin crossover compounds are considered to be a viable alternative for creating display, memory and switching devices due to the bistability of their magnetic, optical, mechanical and electrical properties. This Letter presents the study of the dielectric and transport properties of the [Fe(Htrz)2(trz)](BF4) (Htrz = 1H‐1,2,4‐triazole) complex in a wide temperature and frequency range. Our results reveal a singular behavior of the dielectric modulus upon the spin transition in conjunction with the switching of the conductivity between the high spin and low spin states.
EPR of Mn2+ doped (∼ 1%) in single crystal of zinc fluoroborate hexahydrate has been studied between 94 and 353 K. Angular variation study of the spectra indicated that the crystal field distortion has a trigonal symmetry along the crystallographic c-axis. The D distortion parameter has two linear regions in its thermal variation. It varies above 200 K as D = (149 + 0.107T)G and below 150 K as xzD = (127 + 0.228T)G. A “knee”-like curve links these two linear regions confirming an anomaly observed in a previous work at 181 K due to a phase transition. Other spin Hamiltonian parameters are g = 2.000 ± 0.001, a = (8.9 ± 1.5)G, A = (97.9 ± 0.9)GandB = (97.0 ± 0.9)G. They are constant with temperature. 相似文献
Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P21cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition. 相似文献
The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor ε) accompanying the spin transition is determined. For diluted mixed crystals with x < 0.44 the lattice deformation is directly related to the spin transition, whereas in concentrated mixed crystals with x > 0.44 a first order crystallographic phase transition (R3i → P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm ? 1 for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is ? 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature. 相似文献
The complete set of seven elastic constants of the Fe(II) spincrossover compound [Fe(ptz)6] (BF4)2 (ptz = 1-propyltetrazole) with crystal symmetry R3i was measured by Brillouin spectroscopy at 300K. The measurement on the plate-like single crystals were performed using an oriented sample rotation technique and the combination of two scattering geometries. For comparison the elastic constants of two isostructural compounds were also measured at 300K: the analogous perchlorate spin-crossover compound [Fe(ptz)6] (ClO4)2 and the zinc complex [Zn(ptz)6] (BF4)2. The knowledge of the elastic constants is neccessary for the calculation of the elastic interaction between the spincrossover molecules, and this is the first time that the complete set of elastic constants of a spincrossover compound has been measured directly. 相似文献
19F and 11B spin-lattice relaxation times were measured in [Zn(ptz)6] (BF4)2
and in the spin-crossover compound [Fe(ptz)6] (BF4)2. For both compounds BF4- anion reorientation is active above 50 K. For [Zn(ptz)6] (BF4)2, the anion-reorientation dynamics is different in the temperature regions of 50-90 K, 90-120 K, and above 150 K; between
120 and 150 K it changes rapidly reflecting a structural change. In [Fe(ptz)6] (BF4)2 the mechanism for the paramagnetic relaxation involving the 19F nuclei is found to be of the diffusion-limited type according to the theory of Lowe and Tse. The present results prove that
the spin-crossover takes place in a dynamic surrounding and not in a static crystal lattice.
Received 09 February 1999 and Received in final form 14 June 1999 相似文献
The electron paramagnetic resonance (EPR) spectra of Cu2+-doped RbH2 PO4 at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu2+, and the values of EPR parameters are in accord with the literature on Cu2+-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K. 相似文献
EPR investigations using Cu2+ ion as a probe have been performed on supersaturated sucrose solution with percent concentration c = 66 as a function of temperature T, and at room temperature as a function of c. The motionally averaged spectrum of [Cu(H2O)6]2+ was used to monitor the changes in intermolecular interactions that occur as a function of [c, T]. A drastic increase in the line width, symptomatic of increase in the rotational correlation time of [Cu(H2O)6]2+, is observed between 293 and 288 K. The motionally averaged spectrum disappears below 281 K. The motionally averaged spectrum is also absent in the room temperature spectra of the solution with c= 85. Even in the [c, T] range where [Cu(H2O)6]2 is found to be nearly static, these molecules appear to have an orientational fluctuation manifesting in the m1 dependence of the line width of the parallel component. 相似文献
The 57Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)6](ClO4)2 has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with TC(↓)=223 and TC(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below TC. The present study reveals an increase in high-spin fraction upon heating in the temperature range below TC, and an explanation is provided. 相似文献
The paper presents the EPR evidence for the occurrence of phase transitions in (NH4)4ThF8 and (NH4)3ThF7. In both cases the Cu2+ probe occupies a NH4+ site and predominantly experiences a dynamic coordination, either due to dynamic Jahn-Teller effect, or due to fluxional behaviour of surrounding (ThF8)4- units in (NH4)4ThF8. It is proposed that the structural phase transition in this compound at 214 K is associated with the transition of the dynamic coordination of Cu2- into a static one, probably due to freezing of motion of (ThF8)4- units below this temperature. In (NH4)3ThF7 the dynamic features of the Cu2+ EPR spectrum are absent and characteristics of a local orthorhombic symmetry are seen down to 77 K. However, in the high temperature range a change from orthorhombic to axial symmetry is observed at 524 K, possibly due to a phase transition. 相似文献
Paramagnetic crystal of hexaimidazole nickel nitrate is studied by EPR at Q-band frequency (35.768 GHz). Well defined temperature dependence of the zero-field splitting is obtained at the perpendicular orientation of the trigonal axis,cΛB. An inversed EPR signal is observed in the powder spectrum of the compound. 相似文献
[Fe(mtz)6](CiO4)2 (mtz=1-methyltetrazole) is a spin crossover compound with two different iron(II) lattice sites. Only one of them (lattice site A) shows thermally induced high spin (HS) low spin (LS) spin transition. The LIESST effect (Light-Induced Excited Spin State Trapping) can be observed below 50 K. Complex molecules in B-sites remain in HS state at all temperatures. At 20 K irradiation with red light causes a partial conversion to another HS species, HS(C), with also practically infinite lifetime.In partial fulfilment of the Ph.D. thesis. 相似文献
An initial virgin plasticine-like poly(3-octylthiophene) (P3OT) and this sample modified by annealing and recrystallization
were investigated at X-band (10 GHz) and mainly at D-band (140 GHz) electron paramagnetic resonance (EPR) in a wide temperature
region. Paramagnetic centers with anisotropic magnetic-resonance parameters were proved to exist in all polymers, namely,
mobile polarons whose concentration and susceptibility depend on the temperature and the polymer treatment. Superslow torsional
motion of the polymer chains and layers was studied by the saturation transfer method at D-band EPR. Spin-spin and spin-lattice
relaxation times were measured separately by the steady-state saturation method at the same waveband EPR. Intrachain and interchain
spin diffusion coefficients and conductivity due to polaron dynamics were calculated. It was shown that the charge transport
in P3OT is determined by the strong spin-spin interaction and is stimulated by torsion motion of the polymer chains. The total
conductivity of P3OT is determined mainly by dynamics of paramagnetic charge carriers. Magnetic, relaxation and dynamics parameters
of P3OT were also shown to change during the polymer treatment. 相似文献
The Jahn-Teller effect in the ZnGa2O4 spinel single crystal has been investigated using electron paramagnetic resonance of Cu2+ ions in the temperature range 110–560 K. It has been shown that copper ions occupy octahedral sites 16d in the ZnGa2O4 crystal with cubic symmetry Oh7 (Fd-3m). At T < 560 K, the octahedra undergo tetragonal distortions (predominantly tension) and rotation around the fourfold axes by the
angle θ ≈ 2.6°. The parameters of the spin Hamiltonian, which characterize the prolate (g‖ = 2.355, g⊥ = 2.077, A‖ = 116 Oe, Atp = 12 Oe) and oblate (g‖ = 2.018, g⊥ = 2.246, A‖ = 75 Oe, A⊥ = 44 Oe) octahedra, have been determined. At temperatures above 560 K, the static Jahn-Teller effect transforms into the
dynamic effect and the spectrum of the magnetic resonance becomes isotropic with g = 2.116 (the experimental frequency corresponds to the X band). 相似文献
The complex [Fe(H2O)6]SiF6 is one of the most stable and best characterized high-spin Fe(II) salts and as such, is a paradigm for the study of this important transition metal ion. We describe high-frequency and -field electron paramagnetic resonance studies of both pure [Fe(H2O)6]SiF6 and [Zn(H2O)6]SiF6 doped with 8% of Fe(II). In addition, frequency domain magnetic resonance spectroscopy was applied to these samples. High signal-to-noise, high resolution spectra were recorded which allowed an accurate determination of spin Hamiltonian parameters for Fe(II) in each of these two, related, environments. For pure [Fe(H2O)6]SiF6, the following parameters were obtained: D=+11.95(1) cm(-1), E=0.658(4) cm(-1), g=[2.099(4),2.151(5),1.997(3)], along with fourth-order zero-field splitting parameters: B4(0)=17(1)×10(-4) cm(-1) and B4(4)=18(4)×10(-4) cm(-1), which are rarely obtainable by any technique. For the doped complex, D=+13.42(1) cm(-1), E=0.05(1) cm(-1), g=[2.25(1),2.22(1),2.23(1)]. These parameters are in good agreement with those obtained using other techniques. Ligand-field theory was used to analyze the electronic absorption data for [Fe(H2O)6]SiF6 and suggests that the ground state is 5A1, which allows successful use of a spin Hamiltonian model. Density functional theory and unrestricted Hartree-Fock calculations were performed which, in the case of latter, reproduced the spin Hamiltonian parameters very well for the doped complex. 相似文献