Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

Dielectric dispersion in the incommensurate phases of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4

The complex dielectric permittivity ?(ω) of [N(CH₃)₄]₂CoCl₄ and [N(CH₃)₄]₂ZnCl₄ along the a-axis was measured between 0.35 MHz and 100 MHz. It has been found that for both substances the relaxation frequencies are about 5 MHz at T_c. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of T_c. However, for the temperature region of (T?T_c) > 0.6 for [N(CH₃)₄]₂CoCl₄ the dielectric absorption seems to be rather monodispersive.     

Proton magnetic resonance study of the phase transitions of [N(CH3)4]2BCl4 (B=Co, Cu, Zn, and Cd) single crystals

The proton spin-lattice relaxation rates in [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals grown using the slow evaporation method were investigated over the temperature range 120-400 K. It was found that the relaxation processes of ¹H for all the [N(CH₃)₄]₂BCl₄ crystals can be described with single exponential functions. The changes in the ¹H relaxation behavior in the neighborhood of the phase transition temperatures are used to detect changes in the state of internal motion. From the ¹H spin-lattice relaxation rate measurements for [N(CH₃)₄]₂BCl₄ crystals, the activation energies were calculated for each phase. The large values of the activation energies indicate that the N(CH₃)₄ groups are significantly affected during the transitions. Although these [N(CH₃)₄]₂BCl₄ crystals all belong to the group of A₂BX₄-type crystals, their ¹H spin-lattice relaxation rates have different temperature dependences and indicate the occurrence of different molecular motions within the crystals. We additionally show for the first time that the differences in ¹H spin-lattice relaxation rates among the [N(CH₃)₄]₂BCl₄ (B=⁵⁹Co, ⁶³Cu, ⁶⁷Zn, and ¹¹³Cd) single crystals arise from differences in the electron structures of the metal ions within the series.     

Crystal growth, spectroscopic and crystal field studies of [N(CH3)4]2MnCl4 and [N(CH3)4]2CoCl4 single crystals in the paraelectric phase

Single crystals [N(CH₃)₄]₂MnCl₄ and [N(CH₃)₄]₂CoCl₄ were grown by the slow evaporation technique from the super-saturated solutions. The samples obtained were undergone the X-ray and spectroscopic studies. Absorption spectra in the paraelectric phase at T=303 K have been recorded using the Shimadzu 160A double beam automatic scanning spectrophotometers. On the basis of the exchange charge model and Racah theory the crystal field parameters and Racah parameters have been calculated; all absorption bands for both crystals were given an assignment.     

Electron paramagnetic resonance of gamma irradiated [(CH3)4N]InCl4 and [(CH3)4N]2CdCl4 single crystals

Gamma irradiated [(CH₃)₄N]InCl₄ and [(CH₃)₄N]₂CdCl₄ single crystals were investigated by electron paramagnetic resonance at ambient temperature, and it has been found that both compounds indicate the existence of (CH₃)₃N⁺ radicals. The g factors were found to be isotropic, and the hyperfine constant for H atoms was measured as 2.86 mT and is isotropic for this radical in these substances. The hyperfine coupling constant of the N nucleus with the hole in (CH₃)₃N⁺ in [(CH₃)₄N]InCl₄ was found to be anisotropic with the A_zz=2.92, A_yy=1.62 and A_xx=1.40 mT. From these, it has been revealed that the C_3v-axis of (CH₃)₃N⁺ radical performs rotational or jumping reorientational motions around a fixed axis, in addition to the rotations of protons in CH₃ groups and the rotational motions of CH₃ groups around the C_3v-axis of the radical. The g, and the hyperfine coupling factors of the N nucleus were isotropic in (CH₃)₃N⁺ in [(CH₃)₄N]₂CdCl₄. This indicates the motional behaviour of the radical in this compound is as in a liquid. This isotropic behaviour of the hyperfine coupling constants was found to be same until the attainable lowest temperature of 113 K in our laboratory.     

The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Vibrational study of layered ZnPS3 compounds intercalated with [Co(η5 -C5H5)2+] and [Cr(η6 -C6H6)2+] CATIONS

Infrared and Raman spectra (10–3100 cm^?1) of the layered ZnPSs compound and of intercalates with [Co(η⁵ ? C₅H₅)₂⁺] and [Cr(η⁶ ? C₆H₆)₂⁺] cations in the polycrystalline state have been recorded 300–10 K temperature range. A complete assignment of the spectra is proposed in terms of PSs group motions, Zn²⁺ ion translations and [Co(η⁵ ? C₅H₅)₂⁺] or [Cr(η⁶ ? C₆H₆)₂⁺] internal vibrations. New low frequency for the [Co(η⁵-C₅H₅)₂⁺] intercalate at low temperature are assigned to librational and torsional modes of C₅H₅ rings. Moreover, the preresonance Raman spectra of this intercalate show a selective enhancement for the metal-ligand vibrations when the charge transfer band of the cobalticenium is approached. One concludes that guest molecules are intercalated under their cationic form, are weakly interacting with the host lattice and seem to be dynamically disordered at room temperature.     

Theoretical investigation of EPR and optical spectra of Mo(V) in [Mo6O19][N(C4H9)4]3 salt

The electron paramagnetic resonance (EPR) spectral data (the g factors and hyperfine structure constants) and d–d transition spectra for the tetragonal Mo⁵⁺ centre in [Mo₆O₁₉][N(C₄H₉)₄]₃ salt are theoretically investigated from the complete diagonalization method (CDM) for a 4d¹ ion in tetragonally compressed octahedron. The theoretical results are in good agreement with the experimental data. The dependency of the g factors of the ground state on the R_∥(MoO bond length) has been studied. It is shown that the g factors varied with the R_∥ approximately in a linear way.     

A study on the structural phase transitions of ferroelastic [N(CH3)4]2CuCl4 from microhardness measurements

Measurements of Vickers microhardness (MHv) as a function of temperature has been carried out on [N(CH₃)₄]₂CuCl₄ single crystals. The suitability of this technique for the determination of transition temperatures in the paraelastic-incommensurate→ferroelastic (commensurate) phase transitions sequence is shown.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

Iron containing vitamins and dietary supplements: control of the iron state using Mössbauer spectroscopy with high velocity resolution

Partially inverted (Co_0.185Fe_0.815)[Co_0.815Fe_1.185]O₄ spinel submicron particles with spherical shape are studied at low temperature by Mössbauer spectroscopy. Canted magnetic structures are evidenced. The hyperfine magnetic field distributions P(H _hf ) have been simulated by model with two polynomial cation distributions for A- and B-sites. The influence of the second nearest-neighbours and perturbation effect of the third nearest-neighbours are taken into account. The synthesized magnetic field distributions cannot fit with high precision the experimentally observed one. Better explanation is possible if clustering, predominantly occupation of some crystallographic planes with Co^2?+? ions, formation of cobalt super lattice and submicron surface layer with spin pinning effects are taken into account.     

Influence of isomorphous cation and anion substitution on optical spectra of cobalt alkylaminehalide crystals

Based on data of spectral investigations of crystals with the group A₂CoCl₄ (A=NH₂(C₂H₅) ₂ ⁺ , N(C₂H₅) ₄ ⁺ , N(CH₃) ₄ ⁺ ) and [N(C₂H₅)₄]₂CoCl₂Br₂, the identification of absorption bands corresponding to internal electron transitions in a Co²⁺ ion has been carried out. The values of the crystalline field force Dq for the above-indicated materials at room temperature have been determined. The regularities of the change in the given parameter with change in the temperature and isomorphous cation-anion substitution have been established. It is shown that substitution of chlorine ions by bromine ions in the [N(C₂H₅)₄]₂CoCl₂Br₂ solid solution leads to the formation of distorted tetrahedral complexes CoCl₂Br₂. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 91–95, January–February, 2000.     

Nuclear spin-lattice relaxation of protons in low-dimensional Ising-like system: [(CH3)3NH]CoCl3 · 2H2O

The nuclear spin-lattice relaxation times of protons in a low-dimensional Ising-like system [(CH₃)₃NH]CoCl₃ · 2H₂O were measured from 1.2 to 4.2 K in zero field and in an external magnetic field applied along the spin easy-axis. The calculation for the two-magnon Raman process was carried out with respect to a ferromagnetic layer of the bc-plane. By taking the gap energy to be 14.0 K, the best fit of the theoretical curves with the data was obtained from 1.2 K to about 2.0 K. The experimental results at high temperatures deviate seriously from the prediction of this process, which is discussed in terms of a tentative model for the nuclear relaxation process associated with magnon bound states.     

The electronic properties of [CH(FeCl4)0.061]x

The temperature dependence of d.c. conductivity, thermopower and electron paramagnetic resonance (EPR) results on [CH(FeCl₄)_0.061]_x are reported. The d.c. conductivity and thermopower measurements indicate metallic charge transport along the polyacetylene chain interrupted by the interfibril contact resistances and also ‘dragged’ primarily by the dopant ions. The initial measurements of EPR show Dysonian lineshape with very broad linewidth (ΔH ≈ 600 G at room temperature). The temperature dependence of EPR absorption intensity implies that there exist localized magnetic moments. The observed g value (g ≈ 2.03) suggests the dopant anion is in a form of (FeCl₄)^-.     

PMR spin lattice relaxation by reorientational motions of NH3 and CH3 groups in some organic compounds

The spin-lattice relaxation times, T₁, of protons in o, m, p-phenylene-diamine dihydrochlorides C₆H₄(NH₂)₂·2HCl, phenylhydrazinium chloride C₆H₅NHNH₃Cl, hexaethylbenzene C₆(CH₂CH₃)₆, tetrabutylammonium bromide [CH₃(CH₂)₃]₄NBr, iodide [CH₃(CH₂)₃]₄NI, tetraheptylammonium bromide [CH₃(CH₂)₆]₄NBr and iodide [CH₃(CH₂)₆]₄NI powders have been measured between 400 and 100 K at 60MHz. The experimental results have been explained by considering the reorientational motions of ?NH₃⁺ and ?CH₃ groups about C₃ axes and their role of behaving as sinks to rapid spin diffusion of the ring protons of the phenylene and the methylene protons. The observed T₁, minima in all these substances turn out to be the measures of the ratios between the total number of protons and the number of reorienting ?NH₃⁺ or ?CH₃ protons. Therefore it has been concluded that the T₁, minima of ?NH₃⁺ and ?CH₃ groups, when obtainable can indicate their number present in a solid sample.     

Spin dynamics in the impurity-doped one-dimensional heisenberg paramagnet (CH3)4NMn(1-x)CuXCl3: Nuclear magnetic relaxation

Proton spin-lattice relaxation rate has been measured at room temperature in the impurity-doped (CH₃)₄NMn_(1-x)Cu_xCl₃ for X = 0.04, 0.1 and 0.17. The result for Hz. snfc;chain-axis as a function of resonance frequency clearly shows that the spectral density of the spin fluctuations in the impure system remains to have the characteristics 1-D diffusive term (ω^{-$\text{1}\text{2}$}), with a slower rate of the spin diffusion in accordance with the theory by Richards. The result for H⊥ chain-axis indicates, however, the existence of a singularity of the fluctuations near ω = 0.     

Estimation of T1 for Co2+ ions from the temperature variation of the ESR linewidths for VO2+ in Cs2Co(SeO4)2·6H2O single crystals

The electron spin resonance of VO²⁺ is studied in single crystals of Cs₂M″ (SeO₄)₂·6H₂O (M″ = Zn, Co) from 290 to 77 K at ～ 9.45 GHz. The line broadening of VO²⁺ spectra on cooling the Cs₂Co(SeO₄)₂·6H₂O crystal is explained on the basis of host spin-lattice relaxation narrowing. T₁ for Co²⁺ is estimated to be ≈ 1.7 × 10^?12 sec. at 290 K.     

Nuclear magnetic resonance and transferred hyperfine interaction study of the crystallographically inequivalent Cs(I) and Cs(II) in a Cs2CuCl4 single crystal

¹³³Cs (I=7/2) nuclear magnetic resonance in a Cs₂CuCl₄ single crystal grown by using the slow evaporation method was measured in its three mutually perpendicular crystal planes. The ¹³³Cs resonances of two different groups with two crystallographically inequivalent cesium nuclei, Cs(I) and Cs(II), in the unit cell were recorded. The transferred hyperfine fields for Cs(I) and Cs(II) calculated from the paramagnetic shift and the molecular susceptibility measurements could be expressed by the linear equation H_hf=AT+B. The angular dependence of the ¹³³Cs nuclear magnetic resonance spectra showed that the Cs(I) and the Cs(II) nuclei had different values for the quadrupole coupling constant. The electric field gradient tensors of Cs(I) and Cs(II) were symmetric, and the orientations of their principal axes did not coincide. The Cs(I) ion surrounded by 11 chlorine ions had a small quadrupole parameter, a smaller charge distribution, and a small value for the transferred hyperfine field. However, the Cs(II) ion surrounded by nine chlorine ions had a larger quadrupole parameter, a larger charge distribution, and a larger value for the transferred hyperfine field.