蓝宝石(α-Al_2O_3:Fe~(3+))体系基态分裂与局域晶格畸变的研究

2002年Scholz和Buzaré对蓝宝石晶体中Fe3+离子的基态分裂重新进行了EPR实验测量和研究,他们的初步分析表明在蓝宝石晶体中Fe3+离子的6A1基态分裂有可能同时与两个方向的畸变角(θ和φ)有关.本文采用对角化d5组态在C3点群对称下的252×252完全能量矩阵的方法,对蓝宝石晶体中Fe3+离子的光谱和EPR谱进行了系统的研究.计算结果表明蓝宝石体系中Fe3+离子的6A1基态分裂确实将明显依赖于两个方向的畸变角θ和φ,这一理论结果与Scholz和Buzaré等的实验相符合.同时,通过拟合Fe3+离子在蓝宝石体系中的实验光谱和EPR参量,确定了蓝宝石晶体中(FeO6)9团簇局域晶格畸变角θi的范围.     

Low-frequency critical dynamics in (CnH2n+1NH3)2SnCl6 model biomembrane systems

Nuclear magnetic resonance has been employed as a probe for the collective hydrocarbon chain dynamics in the organic–inorganic model biomembranes (C_nH_2n+1NH₃)₂SnCl₆, undergoing order–disorder and conformational phase transitions. No anomalies were observed in the laboratory-frame spin–lattice relaxation measurements at the order–disorder phase transitions, whereas a discontinuity was manifest at the conformational phase transitions characteristic of a first-order phase transition. On the other hand, our rotating frame spin–lattice relaxation measurements revealed a low-frequency critical collective chain dynamics in the kilohertz regime associated with the order–disorder phase transition.     

The Peierls phase transition in <Emphasis Type="Italic">n</Emphasis>-Ge caused by impurity interaction

The study of the electron paramagnetic resonance in Ge:As has revealed that the insulating state in uncompensated semiconductors is preserved near the insulator-metal phase transition because of the appearance of lattice distortions. The latter are caused by the interaction of the spins localized on impurity atoms due to the spin-Peierls transition. In Ge:As, this effect manifests itself in the concentration range n = n = 3 × 10¹⁷–3.7 × 10¹⁷ cm⁻³.     

Deep UV resonance Raman spectroscopic study of CnF2n+2 molecules: the excitation of C–C σ bond

The σ–σ* transition of C–C bond in C_nF_2n+2 molecules was studied by deep UV resonance Raman spectroscopy. With the C–C σ bond selectively excited by the deep UV laser at 177.3 nm, the resonance Raman spectra of C_nF_2n+2 molecules were obtained on our home‐assembled deep UV Raman spectrograph. The Raman bands at 1299, 1380 and 2586 cm⁻¹ due to the C–C skeletal stretching modes are evidently enhanced owing to the resonance Raman effect. Based on the resonance Raman spectra and theoretical calculation results, it is proposed that the electronic geometry of C_nF_2n+2 molecules at the σσ* excited state is displaced along the directions perpendicular and parallel to the C–C skeleton, and the excited C–C bond is not dissociative due to the delocalization of the excited electron in σ* orbital. Copyright © 2012 John Wiley & Sons, Ltd.     

Magnetic and structural anomalies of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (n = 2, 3)

Sodium fullerides Na _n C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na₂C₆₀ at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na₂C₆₀ fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na₃C₆₀ is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na₃C₆₀ fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the ²³Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.     

Comparative study of ESR spectra in incommensurate antiferromagnets

The electron spin resonance is studied for noncollinear low-dimensional antiferromagnets RbMnBr₃ and RbFe(MoO₄)₂ in a wide range of frequencies and fields. Both compounds have incommensurate spin structures appearing due to a low-symmetry distortion of an ideal hexagonal crystal lattice. Magnetic field applied in the spin plane induces a first-order transition into the commensurate phase. The low-energy resonance branch corresponding to a uniform oscillation of the spin system in the easy plane is observed in the two compounds in both incommensurate and commensurate phases, with a dramatic change of the spectra taking place near the transition field. The resonance spectrum of a nearly commensurate spin structure with long-wave modulations is analyzed in clean and dirty limits in the framework of a hydrodynamic approach. The resonance branch with steep field dependence in the incommensurate state is attributed to the acoustic mode with the gap resulted from pinning of local domain walls (discommensurations) on defects of the crystal structure.     

13C-NMR study of the phase transition in the layered antiferroelectric squaric acid

The temperature dependence of the¹³C-NMR spectra of undoped crystals of the layered antiferroelectric squaric acid (C₄H₂O₄; H₂SQ was measured at a resonance frequency of about 75 MHz by means of the¹H–¹³C double resonance technique. The relation between the order parameter and the chemical shift tensors at the¹³C nuclei is discussed in detail. The line splitting in the ordered phase belowT _c100°C is shown to be proportional to the order parameter. The phase transition is of first order and can be well described by a Landau expansion of the free energy. The results are compared to previous determinations of the temperature dependence of the order parameter. The spin lattice relaxation time of the¹³C system is estimated to be 5000–7500 s at 5°C and the given magnetic field.     

Antiferromagnetic spin glass in doped Ge near insulator-metal transition

A low-temperature (3–100 K) electron spin resonance (ESR) study of the spin system of neutral As donors in Ge showed that on the insulator side of the insulator-metal transition the single-spin density exponentially disappears as T → 0. Such spins are bound into pairs to give an antiferromagnetic (AF) phase. Upon increasing the temperature the AF phase is destroyed, the single-spin density and, as a result, the ESR absorption signal becomes stronger. The temperature dependences of the densities of the pairs and single spins are typical for a chaotic distribution of neutral donors. In this case, there is no Néel temperature. For a low degree of compensation, the crystal lattice of Ge with the AF phase is actually a nanostructured system characterized by anisotropic internal stresses that are the strongest along one of the [110] directions. These stresses give rise to the anisotropy of the g-factor which is responsible for experimentally observed splitting of the ESR line. The compensating impurities destroy the AF phase and reduce this splitting. Local stresses are present in this case, too, but now they appear because of the Coulomb interaction of oppositely charged impurities and have no preferred orientation.     

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.     

Spin splitting in HgTe/CdHgTe (013) quantum well heterostructures

The electron transport and cyclotron resonance in a one-sided selectively doped HgTe/CdHgTe (013) heterostructure with a 15-nm quantum well with an inverted band structure have been investigated. The modulation of the Shubnikov-de Haas oscillations has been observed, and the spin splitting in zero magnetic field has been found to be about 30 meV. A large Δm _c/m _c ≃ 0.12 splitting of the cyclotron resonance line has been discovered and shown to be due to both the spin splitting and the strong nonparabolicity of the dispersion relation in the conduction band.     

Electron spin resonance and Rashba field in GaN-based materials

We discuss problem of Rashba field in bulk GaN and in GaN/Al_xGa_1−xN two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin-orbit coupling between bulk material and heterostructures. We observe coupled plasmon-cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in the literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin-orbit coupling in bulk material, not exceed 400 G, α_BIA<4×10⁻¹³ eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/Al_xGa_1−xN heterostructures originates from properties of the interface.     

Features of structure and properties of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2, 3) fullerides synthesized in toluene

The structure and electron properties of Na _n C₆₀ (n = 2, 3) sodium fullerides synthesized from simple compounds in toluene were studied. It was shown that Na₂C₆₀ fulleride forms a face-centered cubic lattice at temperatures above 300 K. As the temperature is lowered, the phase transition to a structure with a simple cubic lattice takes place. The temperature dependences of the properties of Na₃C₆₀ with a more complex structure exhibit features that are presumably due to sodium atom redistribution in the Na₃C₆₀ fulleride lattice and the formation of sodium ion clusters.     

Mössbauer studies related to the solid-state dynamics of C60

Summary C₆₀ (Buckminsterfullerene) is isolated from organic solvents above 260 K as an f.c.c. solid with complete orientational disorder. The rotational dynamics are diffuse and isotropic, with a short (12 ps) correlation time at 300 K. Below 260 K, the solid forms in a simple cubic lattice withPa3 symmetry. The librational motion occurs by activated jumps with a correlation time of about 60 ns at 200 K. To gain further insight into the dynamics of C₆₀, the compound C₆₀Fe(CO)₄ (I) (generously made available to us by A. Stephens and M. Green of Oxford University) has been studied by⁵⁷Fe M?ssbauer spectroscopy. As expected, the resonance at 90 K consists of a doublet with an isomer shift (relative to metallic iron) of 0.034(1) mm s⁻¹ and a quadrupole splitting of 1.607(5) mm s⁻¹. There is no significant temperature-dependent, intensity asymmetry of the doublet. The temperature dependence of the effect magnitude is well fit by a linear regression over the range 85<T<210 K. To elucidate the relationship of these results to the effect of the C₆₀ moiety in I, further lattice dynamical M?ssbauer studies have been carried out on (maleic anhydride) Fe(CO)₄ (II), one of the few isolatable tetracarbonyl complexes of iron. For (II), the quadrupole splitting is 1.371(5) mm s⁻¹ at 90 K, and the temperature, dependence of the isomer shift is smaller than it is for (I), while the temperature dependence of the recoil-free fraction is significantly larger in (I) than in (II). Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Still more on the reaction of the intercalation compound C8K with water

The reaction of C₈K made from polycrystalline Ultracarbon with D₂O leads to a solid phase product containing both potassium and water. The absence of quadrupole splitting in the ²D NMR of the product argues against the presence of crystalline water of hydration, as in potassium hydroxide monohydrate. X-ray diffraction and electron spin resonance show the carbon skeleton to be inequivalent to the initial graphite. Hydrogen evolution in the reductive protonation of the dianion of perylene is shown to arise from an unstable intermediate hydroaromatic species.     

Structural Phase Transition of Perovskite-Type N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>CdBr<Subscript>3</Subscript> Studied by MAS NMR and Static NMR

The structural geometry change in the perovskite-type N(CH₃)₄CdBr₃ single crystal near the phase transition temperature of T _C = 390 K was investigated using magic angle spinning nuclear magnetic resonance techniques. For ¹H and ¹³C nuclei, the temperature dependences of their chemical shift, spectral intensity, and spin–lattice relaxation time (T _1ρ) in the rotating frame were obtained and analyzed. While the chemical shift and T _1ρ of ¹H showed change near T _C, those of ¹³C did not. In addition, the ¹¹³Cd spin–lattice relaxation time T ₁ in the laboratory frame near T _C show no evidence of anomalous change near the phase transition temperature, which coincides with the measured changes in the ¹H T _1ρ. The driving force for this phase transition was connected to the ¹H in the CH₃ groups.     

Magnetic resonance study of the two-dimensional spin diffusion in the Heisenberg paramaget (C18H37NH3)2MnCl4

¹H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) techniques were employed to study the perovskite-type layered structure compound (C₁₈H₃₇NH₃)₂MnCl₄ undergoing structural phase transitions. The spin relaxation was found to sensitively reflect the two-dimensional electron spin diffusion.     

The fragmentation of negative ions of fullerene C60 trifluoromethyl derivatives

The formation and decay of gas-phase negative ions of trifluoromethylated fullerenes C₆₀(CF₃) _n (n = 2–10) were studied. The resonance electron capture mass spectra were measured to find that the main fragmentation channel of negative ions was the detachment of trifluoromethyl groups. The degree of fragmentation directly depended on the energy of electrons and reached the complete splitting off of all the CF₃ addends with the formation of C₆₀⁻ ions. The observed metastable ion signals were analyzed to determine the scheme of sequential fragmentation of negative ions.     

Soliton lattice to metal: A first order phase transition

$\text{In-situ}$ measurements of the electron spin resonance of [Na⁺_y (CH)^?y]_x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.     

Study on ethyl groups with two different orientations in [N(C2H5)4]2CuBr4

The crystal structure and phase transition temperature of [N(C₂H₅)₄]₂CuBr₄ are studied using X-ray diffraction and differential scanning calorimetry (DSC); measurements revealed a tetragonal structure and the two phase transition temperatures T_C of 204 K and 255.5 K. The structural geometry near T_C is discussed in terms of the chemical shifts for ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups are distinguishable by the ¹³C NMR spectrum. The molecular motions are discussed in terms of the spin–lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The T_1ρ results reveal that the ethyl groups undergo tumbling motion, and furthermore that the ethyl groups are highly mobile.     

Nuclear magnetic resonance of 55Mn and 75As in MnAs

Four sets of NMR signals, two each, from ⁵⁵Mn and ⁷⁵As nuclei have been observed. The temperature dependences of ⁵⁵Mn resonances have been studied from 77 to 311 K and that of ⁷⁵As, from 77 K to about 250 K. The results show that there is a phase transition at T₁ ≈ 220 K. This transition may be due to introduction of a local spontaneous distortion in the region of the domain walls in the lattice, resulting in lowering of symmetry at low temperatures. Another possibility is the canting of spins which would lower the magnetic group symmetry. The observed resonances have been assigned to arise from the nuclei at the edge and the centre of the domain walls at temperatures T >T₁ and from two types of wall edges with inequivalent orientation of atomic spins at T < T₁. The isotropic hyperfine field at 0 K obtained by extrapolating the resonance frequencies are 227 and 285.1 kOe at ⁵⁵As nuclei, respectively. The anisotropy in the hyperfine field is nearly zero at ⁵⁵Mn nuclei and about 5.8 kOe at ⁷⁵As nuclei at 0 K.