NQR Spin-lattice Relaxation Research Articles

Molecular Motion of Azetidinium Ion and Phase Transition in [(CH2)3NH2]2KCo(CN)6 with Double Perovskite Structure

Abstract Molecular dynamics of azetidinium ion (AzH+), a four-membered-ring ammonium, in a cyano-bridged double perovskite compound (AzH)2KCo(CN)6 is studied by spin-lattice relaxation time measurements of 1H NMR as well as 14N NQR in relation to the phase transition associated with considerable change of dielectric property. AzH+ ions are in static and dynamic disorder, respectively, below and above Tc = 199 K. Below Tc, a ring-puckering motion of AzH+ takes place with the activation energy Ea = 17.5 kJ mol−1 and the correlation time τc/s = 2.8 × 10−14 exp(Ea/RT). On the other hand, from sudden decrease of 1H NMR linewidth above Tc, a C3 reorientation of AzH+ ion is shown to be activated in high-temperature phase. The activation energy of the C3 reorientation is estimated to be 18 kJ mol−1 from the temperature dependences of 1H NMR as well as 14N NQR spin-lattice relaxation times T1 and T1Q.

Reoriention of diprotonated DABCO (1,4-Diazabicyclo[2.2.2]octane) cation and proton transfer in organic ferroelectric adduct DABCO-2(2-Chlorobenzoic acid)

Temperature dependences of 1H NMR as well as 35Cl NQR spin-lattice relaxation times T1 were investigated of a ferroelectric molecular adduct with Tc = 323 K, in which 1,4-diazabicyclo[2.2.2]octane (DABCO) is sandwiched between two 2-chlorobenzoic acid (2-ClBA). The NQR frequencies clearly show that proton transfer from 2-ClBA to DABCO is occurred and the molecular adduct consists of diprotonated DABCO cation and two 2-chlorobenzoate anions. The correlation time of reorientational motion of the diprotonated DABCO molecule was determined as a function of temperature. The activation energy Ea of the motion was estimated as 22 kJ mol−1 below Tc. The steep decrease of the NQR T1 with Ea = 50 kJ mol−1, observed above ca. 280 K in the ferroelectric phase, suggests a slow fluctuation of electric field gradient at chlorine nucleus.

35Cl NQR study of deuteration effect on proton dynamics in chlorobenzoic acid

35Cl NQR frequencies and spin-lattice relaxation times T1 were measured of some deuterated chlorobenzoic acid, 3,5-dichlorobenzoic acid-d1 (35ClBA-d1), 2,6-dichlorobenzoic acid-d1 (26ClBA-d1), and 4-amino-3,5- dichlorobenzoic acid-dn (4A35ClBA-dn). The slightly higher frequencies were observed for the deuterated analogues. The temperature dependence of the T1 showed a minimum which can be explained by the fluctuation of electric field gradient (EFG) due to the proton transfer motion in the dimeric chlorobenzoic acid molecules. By the deuteration, the 35Cl T1 minimum shifted to the high-temperature side and showed approximately one order of magnitude shorter value. From comparison with the T1 data of the normal compounds, the tunneling contribution to the proton transfer motion in the dimeric molecular unit of chlorobenzoic acid in the crystal is discussed.

NQR application to the study of hydrogen dynamics in hydrogen-bonded molecular dimers

The temperature dependences of 1H NMR as well as 35Cl NQR spin-lattice relaxation times T1 were investigated in order to study the hydrogen transfer dynamics in carboxylic acid dimers in 3,5-dichloro- and 2,6-dichlorobenzoic acids. The asymmetry energy A/ kB and the activation energy V/ kB for the hydrogen transfer were estimated to be 240 K and 900 K, and 840 K and 2500 K, respectively, for these compounds. In spite of a large asymmetric potential the quantum nature of hydrogen transfer is recognized in the slope of the temperature dependence of T1 on the low-temperature side of the T1 minimum. The NQR T1 measurements was revealed to be a good probe for the hydrogen transfer dynamics.

Isotope ratio of Cl NQR spin-lattice relaxation times in 1D hydrogen-bonding system of tetramethylpyrazine-chloranilic acid at high temperatures

The temperature dependences of spin-lattice relaxation time T1 of 35Cl and 37Cl NQR were studied for the co-crystal of tetramethylpyrazine (TMP) with chloranilic acid (H2ca), TMP-H2ca, in which one-dimensional hydrogen bonding is formed by alternate arrangement of TMP and H2ca. The isotope ratio 37Cl T1 / 35Cl T1 was determined to be 1.0 ± 0.1 above ca. 290 K where a steep decrease of spin-lattice relaxation time T1 with increasing temperature was observed. In this temperature range it is suggested that the relaxation is originated from the slow fluctuation of electric field gradient (EFG). Beside EFG fluctuation due to the external-charge-density fluctuation, the small angle reorientation of the quantization axis triggered by a proton transfer motion between N...H-O and N-H...O hydrogen bonding states is proposed.

Proton dynamics in one-dimensional hydrogen-bonding system in molecular co-crystals TMP-D2ca and DMP-H2ca

The proton dynamics in one-dimensional hydrogen-bonding system in molecular co-crystals of tetramethylpyrazine (TMP) with chloranilic acid (H2ca), as well as 2,6-dimethylpyrazine (DMP) with H2ca is studied by 35Cl NQR and 2H NMR spin-lattice relaxation measurements. No transfer motion of proton between the acid and base molecules is observed in DMP-H2ca, while the motion of the acid proton is excited in TMP-H2ca and the activation energy for the motion increases from 35 kJ mol − 1 to 50 kJ mol − 1 by the deuteration.

Relation between 35Cl NQR frequencies and spin-lattice relaxation times in HGeCl3

An equation relating 35Cl NQR frequencies toinverse spin-lattice relaxation times was found for HGeCl3 crystalline phases. The equation was used to identify shortand long-range order contributions to the NQR frequency in the environment of the resonance chlorine atom. Changes in the strength of local electric fields caused by the crystal field were determined.

35Cl NQR for the SbCl3 complex with aniline, SbCl3·NH2C6H5

The temperature dependence of the 35Cl NQR frequencies and spin-lattice relaxation times has been investigated for a trigonal-bipyramidal vn complex SbCl3·NH2C6H5. Thermally activated motion of chlorine atoms (pseudorotation) was not revealed in the complex, in contrast to the vπ complexes of SbCl3 with related molecular structures. The high potential barrier of pseudorotation in the aniline complex is likely to be due to the unusually high nonequivalence of Sb-Cl chemical bonds.

Anisotropic Superconducting Gap in Transuranium Superconductor PuRhGa5: Ga NQR Study on a Single Crystal

69,71 Ga NMR/NQR studies have been performed on a single crystal of the transuranium superconductor PuRhGa 5 with T c ≃9 K. We have observed a 69 Ga NQR line at ∼29.15 MHz, and assigned it to the 4 i Ga site using the NMR results. The 69 Ga NQR spin–lattice relaxation rate 1/ T 1 shows no coherence peak just below T c , but obeys T 3 behavior below T c . This result strongly suggests that PuRhGa 5 is an unconventional superconductor having an anisotropic superconducting gap. The gap amplitude 2Δ 0 ( T →0)≃5 k B T c and the residual density of states N res ( T =0)/ N 0 ( T = T c )≃0.25 have been determined assuming a simple polar function of the form Δ(θ,φ)=Δ 0 cos θ, where θ and φ are angular parameters on the Fermi surface.

A 35Cl NQR Study on Exchange Interactions between Paramagnetic [IrCl6]2– Ions

The 35Cl and 37Cl NQR frequencies and spin-lattice relaxation times T1Q in paramagnetic M2IrCl6 (M = NH4, Cs) were measured at 4 - 350 K. The observed temperature dependences were attributed to EFG fluctuations caused by lattice vibrations and magnetic field fluctuations caused by paramagnetic ions. The exchange parameters in the NH4 and Cs salts were calculated from 35Cl NQR T1Q to be 8.6 K and 1.8 K respectively. 37Cl data yielded 9.1 K and 2.1 K respectively. The obtained lattice constant dependence of J values was explained by considering Ir-Cl Cl-Ir superexchange interaction

Incommensurability and Domain Structure of K2SbF5

Phase transitions and incommensurability in K2SbF5 have been studied by means of 123Sb NQR spectra and spin-lattice relaxation measurements. The phase transitions occur at 117, 135 and 260 K. The line shape and temperature dependence of the spin-lattice relaxation time T1 at 135 to 260 K are characteristic for an incommensurate state with a plane wave modulation regime. At 117 to 135 K a distinct fine structure of the NQR spectra has been observed. The X-ray diffraction pattern of this phase is interpreted as a coexistence of two modulation waves along the a and b axis with wave vectors (a*/6 + b*/6) and (a*/2 + b*/2), respectively. The best interpretation that fits our NQR data is a coexistence of two domains, the structures of which are modulated with different periods in such a manner that each domain exhibits only one of the aforementioned modulation waves. Redistribution of line intensities with the variation of temperature shows that one of the domains becomes energetically preferable on cooling and is transformed into the low temperature phase at 117 K. The 123SbNQR measurements in K2SbF5 show unusually short values of T1, which become close to the spin-spin relaxation time T2 with increasing temperature. - Pacs: 61.44.Fw, 64.60, 64.70, 64.70.Rh, 76.60

Nuclearquadrupole resonance studies on weak exchange interactionsbetween paramagnetic ions in M(II)(H2O)6SnCl6(M(II) = Mn, Co, and Ni)

The 35Cl NQR spin-lattice relaxation times T1Qin paramagnetic Mn(H2O)6SnCl6 andCo(H2O)6SnCl6 crystals were measured as a function oftemperature. Using the T1Q values observed, thecorrelation times τf of electron-spin flip-flops andexchange parameters J were estimated. The J value in Mn saltwas found to be smaller than those found for Co and Ni salts. It isinferred from the J values that the indirect interactionwhere some chemical bonds (including OH···Cl hydrogen bonds)intervene is dominant in the exchange interactions between theparamagnetic ions in M(II)(H2O)6SnCl6 (M(II) = Mn, Coand Ni).

Effects of nonsinusoidal character of atomic modulation on NQR spin-lattice relaxation time of incommensurate phases

The present work is an extention of the theoretical calculation developed by Blinc to explain the temperature and frequency dependence of the spin-lattice relaxation time in incommensurate phases. We have evaluated the influence of the nonsinusoidal character of the atomic modulation, in the linear approximation, over the NQR spectra and over the spin-lattice relaxation due to direct and Raman processes. It is shown that the peak with lower intensity in the NQR spectra always has a larger T{sub 1} and viceversa. The results have been applied to bis(4-chlorophenyl)sulfone T{sub 1} and line-shape data. The temperature and frequency dependence of T{sub 1} are well reproduced if Raman processes are considered.

35 Cl NQR and 2 H NMR analysis of the critical dynamics by approaching from above the normal-incommensurate phase transition of Bis-(4-chlorophenyl)-sulfone (BCPS)

The inverse of the NQR or NMR spin lattice relaxation time of BCPS exhibits a Landau like critical behaviour on approaching the Normal/Incommensurate phase transition from above. This critical behaviour is interpreted as the contribution from the central peak observed in neutron experiments within the framework of the model of a coupling of the soft mode to pure relaxation modes. This result would be consistent with a central peak of dynamical nature.

Nuclear quadrupole resonance study of phase transitions andincommensurability in K2SbF5

Phase transitions and incommensurability in K2SbF5 has been studied bymeans of 123Sb NQR spectra and spin-lattice relaxation measurements. Thephase transitions have been obtained at 117, 135 and 260 K. Line shape andtemperature dependence of the spin-lattice relaxation time T1 in thetemperature range from 135 to 260 K are characteristic for the incommensuratestate with the plane wave modulation regime. At 117 to 135 K, a distinct finestructure of NQR spectra has been observed. Redistribution of line intensitieswith the variation of temperature allows us to suggest that this phaseexhibits domain structure, and that one of the domains becomes energeticallybeneficial on cooling and transforms into the low temperature phase at 117 K.The 123Sb NQR measurements in K2SbF5 show unusually short values ofT1, which become close to the spin-spin relaxation time T2 withincreasing temperature.

Hydrogen transfer in hydrogen-bonded chloranilic acid studied by 35Cl NQR I – a 1:2 complex with 1,4-diazine

35Cl NQR frequencies and spin-lattice relaxation time ( T 1Q) and 1H NMR relaxation times ( T 1H) in a H-bonded three molecular system, chloranilic acid–1,4-diazine (1:2) were measured to reveal H-motions in a symmetric two H-bonds in solid. A single 35Cl NQR frequency observed implies that the time-averaged structure of chloranilic acid is roughly monovalent in accordance with p K a values in both acid and base. T 1H temperature dependence was explained by a single relaxation mechanism due to the correlated H-transfer in two H-bonds, while that of T 1Q yielded two relaxation processes. One of these undetected by 1H NMR was explained by the uncorrelated H-transfer.

35Cl NQR Studies of Hydrogen Transfer in Crystalline p-Chlorobenzoic Acid

35Cl NQR frequencies and spin-lattice relaxation times T1Q were measured in p-ClC6H4CO2H(PCB A) and p-ClC6H4CO2D(PCBA-d1) at 77-333 K. T1Q in PCB A gave a shallow minimum of 8.0 ms at ca. 110 K, which could be explained by a double proton transfer mechanism in the carboxylic acid dimer referring to 1H NMR data giving a T1H minimum at almost the same temperature. PCBA-d1, showed temperature dependent NQR frequencies quite analogous to those in PCBA, whereas their T1Q behaviour was quite different in its minimum value and its temperature as well as temperature gradient. These results were explained by suppressed deuteron tunnelling and the Ubbelohde effect.

Unique1−qand3−qincommensurate phases in proustite:75AsNQR line-shape and spin-lattice relaxation study

${}^{75}\mathrm{As}$ NQR spectra and spin-lattice relaxation time ${T}_{1}$ have been measured between room temperature and 4.2 K in a proustite $({\mathrm{Ag}}_{3}{\mathrm{AsS}}_{3})$ single crystal. In agreement with x-ray scattering data we find that the phase between ${T}_{I}=60\mathrm{K}$ and ${T}_{L}=49\mathrm{K}$ is triple-q $(3\ensuremath{-}q)$ incommensurably modulated. Our results show unambiguously that we deal here with three independent noncoplanar incommensurate modulation wave vectors. Such a phase seems to be unique in a sense that other phases with three incommensurate modulation waves known so far (e.g., in charge-density-wave systems) are either a superposition of differently oriented $1\ensuremath{-}q$ modulated domains, or the three modulation waves are confined to a plane and are thus not independent. In addition the ${}^{75}\mathrm{As}$ NQR line shape suggests that the phase just below ${T}_{I}$ is a single-q $(1\ensuremath{-}q)$ modulated stripe phase. This is confirmed by the variation of ${T}_{1}$ over the NQR line in the $1\ensuremath{-}q$ and $3\ensuremath{-}q$ phases. On further cooling further into the incommensurate phase the volume fraction of the $3\ensuremath{-}q$ phase gradually increases and the crystal becomes fully $3\ensuremath{-}q$ modulated about 2 K below ${T}_{I}.$ The nonclassical critical exponents for the amplitude of the order parameter were determined to be ${\ensuremath{\beta}}_{1}=0.3\ifmmode\pm\else\textpm\fi{}0.02$ in the $1\ensuremath{-}q$ stripe phase and ${\ensuremath{\beta}}_{3}=0.4\ifmmode\pm\else\textpm\fi{}0.02$ in the $3\ensuremath{-}q$ phase. On approaching the lock-in transition temperature in the low-temperature part of the $3\ensuremath{-}q$ incommensurate phase the phases of the modulation waves become nonlinear functions of the corresponding spatial coordinates, resulting in sharp peaks superimposed on the broad bell-shape frequency distribution. A comparison between experimental and theoretical line shapes allowed for a quantitative determination of the temperature dependence of the soliton density.

Temperature dependence study of chlorine NQR frequency and spin-lattice relaxation time in 2-amino,-3,5-dichloropyridine

Chlorine-35 NQR frequency and spin–lattice relaxation time measurements as a function of temperature in the range 77–300 K were carried out on 2-amino-3,5-dichloropyridine. Two NQR signals were observed and were assigned to the two chlorines present in the molecule using the additive model for substituent effects. The temperature dependence of the NQR frequency was analysed in terms of the torsional oscillations of the molecule and the torsional frequencies and their temperature dependence were calculated numerically using a two-mode approximation.The temperature dependence of the NQR spin–lattice relaxation time was found to be mainly due to the torsional oscillations of the molecule, with anharmonicity effects showing up at higher temperatures. Copyright © 1999 John Wiley & Sons, Ltd.

Spin dynamics in hole-doped two-dimensional S = 1/2 Heisenberg antiferromagnets: Cu NQR relaxation in La Sr CuO for

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.