Solid-state 87Rb NMR study in powdered RbMnCl3

Structural phase transition at 290 K and the implication on the intermediate phase above 290 K in powdered RbMnCl₃ are observed by using a solid-state ⁸⁷Rb NMR spectroscopy. Quadrupole coupling constants (e²qQ/h), the asymmetry parameters (η), and the relative peak intensities for two physically nonequivalent Rb sites, Rb(I) and Rb(II), are determined from nonlinear least-squares fits to the ⁸⁷Rb NMR powder patterns in the temperature range from 260 to 330 K. Quadrupole coupling constants and the asymmetry parameters are examined for the detection of the phase transition resulting in a significant structural change in the Rb(II) site. In addition, changes in the relative peak intensity between the Rb(I) and Rb(II) sites seem to suggest the existence of an anomalous intermediate phase, which is complemented by the differential scanning calorimetry and X-ray diffraction studies.     

Phase transitions and127I,133Cs quadrupole couplings in polycrystalline Cesium metaperiodate

The¹²⁷I and¹³³Cs NMR spectra of polycrystalline CsIO₄, a material of potential interest for transformation of laser irradiation, have been measured in the temperature range 170–440 K and the line shape has been analyzed taking into account first and second order quadrupole interactions and chemical shift anisotropy. Quadrupole coupling constants and asymmetry parameters have been determined as a function of temperature.¹²⁷I and¹³³Cs data consistently show two phase transitions in the ranges 243–300 K and 420–440 K with a temperature hysteresis for the first transition and a range for coexistence of two phases. The time evolution of the NMR signal suggests the existence of piezomagnetism, which was believed to exist only in crystals with a magnetic structure.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

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.     

Preface

Phenylethyl ammonium trichloromercurate exhibits a structural phase transition at 402K Phase I—(402K)→ Phase II

This transition has been characterized by differential scanning calorimetry, dielectric measurements and X-ray diffraction. The space groups and the cell parameters of both phases were determined by X-ray diffraction from single crystals and powder samples. Phase I has space group I2 or I2/m, a = 25.88(2) Å, b = 7.792(3) Å; c = 5.971(4) Å; β = 96.14(1)°. Phase II has orthorhombic symmetry, space group Cmm2 or C222 with a = 25.91(1) Å; b = 7.836(5) Å; c = 6.116(4) Å.     

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     

A simple method for analyzing 51V solid-state NMR spectra of complex systems

Five vanadium complexes as models for biological systems were investigated using ⁵¹V-MAS–NMR spectroscopy. All spectra show an uncommon line shape, which can be attributed to a shorter relaxation time of the satellite transition in contrast to the central one. A method for the reliable analysis of such kind of spectra is presented for the first time and the most important NMR parameters of the investigated complexes (quadrupolar coupling constant C_Q, asymmetry of the EFG tensor η_Q, isotropic chemical shift δ_iso, chemical shift anisotropy δ_σ and asymmetry of the CSA tensor η_σ) are presented. These results are of particular importance with respect to the analysis of the ⁵¹V-MAS–NMR spectra of vanadium moieties in biological matrices such as vanadium chloroperoxidase, which show hitherto unexplained low intensity of the satellite sideband pattern.     

Nuclear magnetic resonance studies of the structural phase transitions in RbCaF3

Fourier Transform NMR measurements made on a single RbCaF₃ crystal using Rb indicate a structural phase transition at 198 K. The temperature dependence of the quadrupole coupling constant and hence of the order parameter was studied in the range 130 K – 290 K by measuring the second order shift of the central line of Rb resonance. The critical exponent, β, of the order parameter at the transition is found to be equal to 0.33.     

Structure and Ionic Conductivity of Halocomplexes of Main Group Metallic Elements Studied by NMR and NQR

Li₃InBr₆ undergoes phase transition to a lithium superionic conductor at T _tr = 314 K (σ = 5.0 × 10⁻⁴ S cm⁻¹ at 330 K). The Rietveld analysis and the DSC measurement suggested that the positional disorder is introduced at the cationic sites above T _tr. The X-ray powder diffraction pattern at the superionic phase changes gradually with temperature and finally shows a simple powder pattern at 420 K which is quite similar to that of LiBr. This rock salt structure contains intrinsic vacancies because one In³⁺ and two vacancies substitute for three Li⁺. ⁷Li and ¹¹⁵In NMR support the rapid diffusion of the Li⁺ and the introduction of the In³⁺ into the rock salt structure. On the other hand, the ionic conductivity for Na₃InCl₆ was 10⁻⁵ S cm⁻¹ even at 500 K. Conduction path for the sodium ions could be proposed by means of the Rietveld analysis and the NMR experiment using a single crystal.     

Nuclear Spin–Lattice Relaxation of Single Crystal Sr14Cu24O41

Nuclear spin–lattice relaxation rate T ₁ ⁻¹ has been measured for the ladder sites of two single crystals Sr₁₄Cu₂₄O₄₁ (Sr14-A,B) by ⁶³Cu NMR/NQR. The hole localization around 100 K appears as a peak in the T variation of T ₁ ⁻¹(NQR). On the other hand, it is suppressed in the T ₁ ⁻¹ (NMR) data under the magnetic field H ∼ 11 T, and a new peak appears around 20 K. T ₁ ⁻¹(NMR) around the peak is more enlarged for Sr14-B than for Sr14-A. Hence, holes on the ladders of Sr14-B tend to be more localized. This is considered to be an origin for the occurrence of the magnetic order in Sr14-B under H ∼ 11 T.     

Structure determination of Cs0.864Rb1.136SeO4

The present structure determination of Cs _x Rb_2-x SeO₄ shows that the crystal belongs to the β - K₂SO₄ family, like the related compounds Cs₂SeO₄ and Rb₂SeO₄. In β - K₂SO₄ compounds the cations occupy two different sites; one of these sites, which is surrounded by 11 anionic atoms, is occupied preferentially by Cs, while the other one with 9 neighbours by Rb. A slight excess of Rb has been determined by X-ray structure analysis in a single crystal prepared at 292 K; its formula is Cs_0.864(9)Rb_1.136(9)SeO₄.

Crystals were grown by evaporation from equimolar water solutions of Cs₂SeO₄ and Rb₂SeO₄ at 292 and 301 K. No phase transition was detected by DSC in the range 103-323 K for either of the samples. Lattice parameters of different crystals isolated at various stages of crystallization indicate formation of crystals with a different proportion of Cs/Rb cations. The lattice parameters as well as their ratios differed significantly from those given by Endo et al., (1983).     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

Deuterium Nuclear Spin–Lattice Relaxation Times and Quadrupolar Coupling Constants in Isotopically Labeled Saccharides

¹³C and ²H spin–lattice relaxation times have been determined by inversion recovery in a range of site-specific ¹³C- and ²H-labeled saccharides under identical solution conditions, and the data were used to calculate deuterium nuclear quadrupolar coupling constants (²H NQCC) at specific sites within cyclic and acyclic forms in solution. ¹³C T₁ values ranged from 0.6 to 8.2 s, and ²H T₁ values ranged from 79 to 450 ms, depending on molecular structure (0.4 M sugar in 5 mM EDTA (disodium salt) in ²H₂O-depleted H₂O, pH 4.8, 30°C). In addition to providing new information on ¹³C and ²H relaxation behavior of saccharides in solution, the resulting ²H1 NQCC values reveal a dependency on anomeric configuration within aldopyranose rings, whereas ²H NQCC values at other ring sites appear less sensitive to configuration at C1. In contrast, ²H NQCC values at both anomeric and nonanomeric sites within aldofuranose rings appear to be influenced by anomeric configuration. These experimental observations were confirmed by density functional theory (DFT) calculations of ²H NQCC values in model aldopyranosyl and aldofuranosyl rings.     

La2-xNdxCuO4+δ(0.1≤x≤1.2)体系中滞弹性弛豫与相变内耗研究

通过对La_2-xNd_xCuO_4+δ(0.1≤x≤1.2)体系中滞弹性弛豫与相变内耗性能的研究发现,当0.1≤x≤1.0时,在250K左右存在一个与间隙氧有关的弛豫内耗峰,并且当0.1≤x≤0.4时,弛豫内耗峰峰高随着x值的增大而升高,此时体系为正交结构;当0.5≤x≤1.0时,体系在宏观上呈现四方结构,此时内耗峰峰高随着x< 关键词： 2-xNd_<i>xCuO_4+δ')" href="#">La_2-xNd_<i>xCuO_4+δ 间隙氧 弛豫内耗峰 相变内耗峰     

More line narrowing in TROSY by decoupling of long-range couplings: shift correlation and JNC′ coupling constant measurements

Since the introduction of RDCs in high-resolution NMR studies of macromolecules, there is a growing interest in the development of accurate, and sensitive methods for determining coupling constants. Most methods for extracting these couplings are based on the measurement of the splitting between multiplet components in J-coupled spectra. However, these methods are often unreliable since undesired multiple-bond couplings can considerably broaden the multiplet components and consequently make accurate determination of their position difficult. To demonstrate one approach to this problem, G-BIRD^(r) decoupled TROSY sequences are proposed for the measurement of ¹J_NH and ¹J_NC′ coupling constants. Resolved or unresolved splittings due to remote protons are removed by a G-BIRD^(r) module employed during t₁ and as a result, spectra with narrow, well-resolved peaks are obtained from which heteronuclear one-bond couplings can be accurately measured. Moreover, introduction of a spin-state-selective α/β-filter in the TROSY sequence allows the separation of the ¹J_NC′ doublet components into two subspectra which contain the same number of peaks as the regular TROSY spectrum. The ¹J_NC′ couplings are obtained from the displacement between the corresponding peaks in the subspectra.     

Proton Dynamics in Letovicite, (NH4)3H(SO4)2: A H and N NMR Spectroscopic Study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.     

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σ_ac) and dielectric permittivity (?) are determined in the temperature range 300?K?T3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T _c?=?(486?±?1)?K. A second structural phase transition was observed around 345?K. The conductivity varies with temperature range and for T?>?428?K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) ^s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.     

Magnetic resonance investigations of phase transitions and modulation wave motions in incommensurate solids

We measured⁸⁷Rb nuclear magnetic resonance (NMR) and³⁵Cl nuclear quadrupole resonance (NQR) Hahn spin-echo magnetization decays in the incommensurate (I) phase of Rb₂ZnCl₄ and, in each case, obtained a Hahn echo decay that was shorter than the Carr-Purcell-Meiboom-Gill decay and one which decayed with a time constant proportional to the cube of the echo time. From these measurements we obtained from both the⁸⁷Rb NMR and³⁵Cl NQR measurements values for the diffusion coefficients that are comparable in magnitude, a fact that strongly supports the existence of slow modulation wave diffusionlike motions in the I phase, since such motions should affect both Rb and Cl ions similarly. In addition, we used⁸⁷Rb two-dimensional exchange-difference NMR to study atomic motions in the incommensurate (I) and paraelectric (P) phases to elucidate the nature of the I-P transition. We measured as a function of the mixing time the frequency shifts of the cross peaks from the main diagonal and observed a gradual increase towards an asymptotic value in the I phase but a sudden jump to the final value in the P phase. We interpreted the motions observed in the P phase as normal modes arising from simultaneous reorientations of ZnCl₄ tetrahedra and corresponding Rb ions displacements between two sites. These normal modes freeze out in the I phase and change to the diffusionlike motion of the modulation wave. We also performed³⁵Cl NQR lineshape andT ₁ measurements in K₂ZnCl₄ and obtained conclusive evidence for the presence of a narrow 1q (singly modulated) I phase between 146 and 149 K.     

High‐resolution Brillouin scattering and heat capacity studies of bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate, [C(NH2)3]2Zr[N(CH2COO)3]2·H2O

High‐resolution micro‐Brillouin scattering and heat capacitiy studies of polar bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate were performed in the temperature range 230–305 K. Strong anomalies in the Brillouin shift and the attenuation were observed near 290 K upon cooling for the sound waves corresponding to the c₂₂ elastic constant. For this elastic constant, the coupling between the order parameter and the elastic strains is linear quadratic. In case of the c₄₄ and c₆₆, the coupling is biquadratic, giving rise to an increase of these parameters below 290 K. The analysis of the acoustic anomalies and heat capacity results near 290 K revealed that they correspond to a second‐order, cell‐multiplying, displacive phase transition. The second phase transition leads to weak acoustic anomalies at approximately 284 K. Copyright © 2009 John Wiley & Sons, Ltd.     

Substitutional site of Co2+ ion in RbMgF3 crystal

The spin-Hamiltonian (SH) parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) for Co²⁺ ions at the trigonal Mg²⁺ (I) and Mg²⁺ (II) sites of RbMgF₃ crystal are calculated from the second-order perturbation formulas based on the cluster approach for 3d⁷ ions in trigonal symmetry. From the calculations, it is found that the calculated SH parameters for Co²⁺ ion at the Mg²⁺ (I) site are in poor agreement with, but those for Co²⁺ at the Mg²⁺ (II) site are close to, the experimental values. Therefore, we suggest that Co²⁺ in RbMgF₃ crystal substitutes for Mg²⁺ (II) ion. The results are discussed.     

Statics and dynamics of the modulation in incommensurate Rb2ZnBr4 as detected by NMR

The dynamics of the incommensurate modulation of Rb₂ZnBr₄ is investigated near the transition to the normal high-temperature phase using first-order quadrupole effects in nuclear magnetic resonance (NMR).⁸⁷Rb NMR spectra and two-dimensional⁸⁷Rb NMR exchange spectra are reported. All results can be described consistently in terms of a static modulation in the incommensurate phase without any indication for “floating” or large-scale fluctuations of the modulation wave. The spectra taken about 135 K below T_i in the lower incommensurate phase well above the soliton regime show no indication for the existence of a higher-order commensurate modulation in Rb₂ZnBr₄.