Critical slowing down of fluctuations in p-terphenyl and p-quaterphenyl observed by proton spin-lattice relaxation

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.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

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 T_c1 (=333 K) and T_c2 (=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 T_c2 (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.     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

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.     

Anomalous behaviour of the proton spin-lattice relaxation time near the phase transition of the layered antiferroelectric squaric acid

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 T_c. 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.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

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 (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 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 T_c1 and T_c2 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.     

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

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.     

Spin-lattice relaxation in the rotating frame and extreme slowing-down at the antiferroelectric transition in copper formate tetrahydrate

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 T_1? in water-deuterated copper formate tetrahydrate are presented. The data shown that the anomalous behavior of the proton T_1? 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.     

Anomalous properties of crystals with ferroelectric phase transition induced by off-center ions

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 K_1?xLi_xTaO₃ saturation of remanent polarization with off-center Li⁺ concentration increasing, close to $\text{x}$ dependence of phase transition temperature, the anisotropy of ultrasonic attenuation, the absence of anomalies of Li nuclear spin-lattice relaxation rate near T_c.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

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 ΔE_a=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.     

NMR observation of critical dynamics in the (TMTSF)2PF6 organic superconductor

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.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 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 T_N≈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-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

Cs nuclear magnetic resonance study in CsZnCl3 single crystals of perovskite ABX3 type

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 T_C (=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.     

Soliton dynamics and nuclear spin-lattice relaxation in incommensurately modulated crystals

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     

Structure, phase transitions and molecular dynamics in 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4]

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.     

Effect of the magnetic field on the 3d ordering and on the spin correlations of the linear antiferromagnet TMMC

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 T_N for fields up to 47 kOe have been determined.     

Fluctuation processes in paraelectric phase of chromium ammonium alum

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.     

Proton NMR study of pure liquid crystal exhibiting re-entrant phase transition

Proton NMR experiment was carried out on a pure re-entrant nematic liquid crystal OBBC in nematic (N), smectic A(S_A), 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?S_A?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.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c 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 T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

T1 as a function of larmor frequency in solid CH4 at 4.2 K: An experiment on recent theories

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.     

Charge density wave transition in Na2Ti2Sb2O probed by 23Na NMR

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.