共查询到20条相似文献,搜索用时 31 毫秒
1.
The temperature evolution of the proton spin-lattice relaxation time T1 in p-terphenyl and in p-quaterphenyl around their order-disorder phase transition has been measured. In both cases pretransitional collective fluctuations destroy the high temperature Arrhenius behaviour of the relaxation rate corresponding to a single reorientational jump motion. The spin-lattice relaxation times present then a drastic decrease until the transition temperature (T0 = 193 K in p-terphenyl, T0 = 238 K in p-quaterphenyl). This decrease is associated to the critical slowing down of fluctuations. In the low temperature phase the ordering phenomena lead to a sharp drop of the spin-lattice relaxation rate. 相似文献
2.
The 133Cs spin-lattice relaxation time in a CsHSO4 single crystal was measured in the temperature range from 300 to 450 K. The changes in the 133Cs spin-lattice relaxation rate near Tc1 (=333 K) and Tc2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T1 around Tc2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T1−1=A+BT. In addition, from the stress-strain hysteresis loop and the 133Cs nuclear magnetic resonance, we know that the CsHSO4 crystal has ferroelastic characteristics in phases II and III. 相似文献
3.
The 1H NMR line-width and spin-lattice relaxation time T1 of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T1 near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T1 around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH3 groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the 1H NMR relaxation behavior, and that 1H nuclei play important roles in the phase transitions of the TSCC single crystal. 相似文献
4.
The temperature dependence of the proton spin-lattice relaxation time has been measured at 51, 70 and 300 MHz by different pulse sequences. Besides a strongly frequency dependent background relaxation rate, a frequency independent critical relaxation rate could be resolved particularly in a broad temperature interval above Tc. The temperature dependence of the critical relaxation rate could not be represented by a power law with a single exponent. Rather it most favourably could be fitted by a logarithmic law. The results are discussed with respect to the phase transition mechanism of squaric acid. 相似文献
5.
The 133Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs2CaCl4·2H2O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the 133Cs spin-lattice relaxation rate near 325 K (=Tc1) and 360 K (=Tc2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at Tc1 is small because the crystal lattice does not change very much during this phase transition. The change in near Tc2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures Tc1 and Tc2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K. 相似文献
6.
Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal
Ae Ran Lim Jae Kap JungSe-Young Jeong 《Journal of Physics and Chemistry of Solids》2002,63(4):625-630
The proton NMR line width and spin-lattice relaxation times for LiNH4SO4 single crystal were studied at low temperature range of 6 and 280 K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH4 ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133 K. 相似文献
7.
The effect on the temperature behavior of the spin-lattice relaxation rates in laboratory and rotating frames in presence of extreme slowing-down of the critical fluctuations in an Ising-type system is discussed. Proton spin-lattice relaxation measurements of T1 and T1? in water-deuterated copper formate tetrahydrate are presented. The data shown that the anomalous behavior of the proton T1? in the neighbourhood of the antiferroelectric phase transition recently observed by Zumer and Pir? in the ordinary crystal cannot be ascribed to the critical slowing-down of the water molecules. A possible interpretation on the basis of a mechanism of creation and annihilation of paramagnetic excitons is discussed. 相似文献
8.
It is given the theoretical study of some properties of strongly polarizable dielectric crystals in which off-center impurity ions induce ferroelectric phase transition. The spontaneous polarization, transition temperature, soft mode frequency, dielectric susceptibility, ultrasonic attenuation, nuclear spin-lattice relaxation are analyzed. The theory explains observed in K1?xLixTaO3 saturation of remanent polarization with off-center Li+ concentration increasing, close to dependence of phase transition temperature, the anisotropy of ultrasonic attenuation, the absence of anomalies of Li nuclear spin-lattice relaxation rate near Tc. 相似文献
9.
Solid state 19F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C60F58 have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔEa=1.9 kcal/mol. Neither phase transition nor random rotation of C60F58 have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species. 相似文献
10.
We have studied the organic superconductor (TMTSF)2PF6 using 1H nuclear magnetic resonance. The spin-lattice (T1) and the spin-spin relaxation time (T2) measurements manifested a divergence associated with a structural phase transition at 160 K. 相似文献
11.
Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature TN≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations. 相似文献
12.
CsZnCl3 single crystals were grown by the slow evaporation method, and the spin-lattice relaxation rates and resonance lines of the 133Cs nuclei in the resulting crystals were investigated using FT NMR spectrometry. The temperature dependence of the relaxation rate of the 133Cs nuclei in the CsZnCl3 crystals was found to be continuous near TC (=366 K), and was not affected by this phase transition. Our results for CsZnCl3 are compared with those obtained previously for other CsBCl3 (B=Mn, Cu, and Cd) perovskite crystals. The Cs relaxation time of CsCdCl3 is longer than that of CsMnCl3. The differences between the atomic weights of Mn, Cu, Zn, and Cd are responsible for the differences between the spin-lattice relaxation times of these single crystals. The influence of paramagnetic ions is also important in these crystals. The differences between the spin-lattice relaxation times of these crystals could also be due to differences between the electron structures of their metal ions, in particular the structures of the d electrons. 相似文献
13.
U. Hacker J. Petersson R. Walisch D. Michele 《Zeitschrift für Physik B Condensed Matter》1997,100(3):441-446
The nuclear spin-lattice relaxation rate in incommensurate systems is analysed for the the so-called soliton limit which often can be applied near the transition to a subsequent commensurate phase. Previous calculations are corrected by taking into account adequately the eigenfunctions of the relevant fluctuations. In contrast to previous conclusions, it is shown that for nuclei in the discommensurations the spin-lattice relaxation rate is expected to increase on approaching the phase transition. The theoretical predictions are confirmed by experimental data obtained from NMR measurements on the prototype incommensurate systems Rb2ZnCl4 and BCCD. b]References 相似文献
14.
B. Kulicka Z. Ciunik W. Medycki J. Baran 《Journal of Physics and Chemistry of Solids》2004,65(5):871-879
Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P21/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol−1 K−1, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the 1H NMR spin-lattice relaxation time T1 and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order-disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The 1H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH3 groups. 相似文献
15.
The effect of an external magnetic field in the range 6–47 kOe on the low temperature proton spin-lattice relaxation rate in TMMC has been investigated. A peak in T1?1 at the 3d ordering temperature has been detected. Values of TN for fields up to 47 kOe have been determined. 相似文献
16.
The electron spin-lattice relaxation times T 1, linewidths, and the hyperfine structure parameters of the ESR spectra have been measured for chromium ammonium alum and aluminum-chromium ammonium alum at temperatures above the phase transition point. The correlation times characterizing the fast fluctuation processes in the temperature range 84–360 K are determined. It is demonstrated that the correlation times are described by two exponents with different activation energies. It is established that the fluctuations are most probably brought about by vibrations of water molecules with anomalously large amplitudes in the environment of NH 4 + ions at high temperatures and also by the reorientations of the SO 4 2? groups at low temperatures. The dehydrated alum samples with partly removed crystallization water have been studied. It is shown that neither fast fluctuation processes nor phase transition are observed in these compounds. 相似文献
17.
Proton NMR experiment was carried out on a pure re-entrant nematic liquid crystal OBBC in nematic (N), smectic A(SA), and re-entrant nematic (RN) phases. The re-entrant phase transition was detected by measuring the rotation pattern (θ0-dependence) of the spectra in the external field. The dipolar splitting at θ0=0 showed smooth temperature-dependence through the phase changes, RN?SA?N. Orientational order of the molecular core is thus hardly affected by the formation of 1-D density modulation. The temperature-dependence of the spin-lattice relaxation rate showed that translational self-diffusion is the predominant mechanism of relaxation in RN, but the director fluctuation is quite significant in N. 相似文献
18.
The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature Tc and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above Tc in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities. 相似文献
19.
The spin-lattice relaxation time T1 has been measured in partially converted samples at NMR frequencies ranging from 4 to 55 MHz. Results allow to check in detail the existing models for relaxation in solid CH4. 相似文献
20.
Herein we investigated the electronic properties of layered transition-metal oxides Na2Ti2Sb2O by23Na nuclear magnetic resonance(NMR)measurement.The resistivity,susceptibility and specific heat measurements show a phase transition at approximately 114 K(TA).No splitting or broadening in the central line of23Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected.The spin-lattice relaxation rate divided by T(1/T1T)shows a sharp drop at about 110 K which suggests a gap opening behavior.Below the phase transition temperature zone,1/T1T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states(DOS)because of the gap.No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature.These results suggest a commensurate charge-density-wave(CDW)phase transition occurring. 相似文献