Magnetic anisotropy of antiferromagnet (CH3)4NMnCl3

The parameters of the electron paramagnetic resonance (EPR) spectra of S ion pairs in diamagnetic crystals are analyzed. A relation between the spin Hamiltonian constants is established for solitary ions and pairs for (CH₃)₄NCdCl₃: Mn²⁺ crystals. In contrast to solitary ions, an additional contribution (which is a linear function of the exchange field) to the “single-ion” spin Hamiltonian constants appears in the case of pairs. It is shown that anisotropic exchange mechanisms do not play a significant part in the formation of the axial constant of the spin Hamiltonian for this crystal. Some aspects of the method of studying “single-ion” anisotropy predicted by the two-ion model are developed with the help of an isostructural diamagnetic analog with impurity concentration of the paramagnetic ions of a magnetically concentrated substance sufficiently high for observing the EPR spectrum of the pairs. It is found that the microscopic quantities determined partially from the EPR spectra for pairs and solitary Mn²⁺ ions in (CH₃)₄NCdCl₃ are in accord with the experimental value of the effective field for the (CH₃)₄NMnCl₃ crystal anisotropy which can be described primarily by the dipole and “single-ion” mechanisms of the exchange origin.     

Effect of hydrostatic pressure on N(CH3)4+ cation motion in ferroelectric N(CH3)4H(Cl3CCOO)2

The proton spin-lattice relaxation time in ferroelectric N(CH3)4H(Cl3CCOO)2 has been studied under isobaric conditions at pressures 0.1, 200 and 400 MPa over a wide range of temperature. The data indicate that the dominant relaxation mechanism for T1 can be attributed to the classical CH3 group reorientation of N(CH3)4+ cation. The influence of pressure on methyl group reorientation of N(CH3)4+ cation was analysed.     

The thermoelastic properties of [N(CH3)4]2ZnBr4 and [N(CH3)4]2MnBr4

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH₃)₄]₂MnBr₄ and [N(CH₃)₄]₂ZnBr₄ are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.     

NMR determination of dynamic parameters of CH3 groups in P(CH3)4SbCl6

The longitudinal relaxation time T1 and the second moment M2 of 1H NMR line in a wide temperature range have been measured for P(CH3)4SbCl6. It was found that two different methyl groups in each tetramethylphosphonium cation perform two different rates of C3 motions. The reduction of the proton second moment M2 just below the temperature of the phase transition Tc2 = 350 K may suggest that the isotropic tumbling of the whole cation [P(CH3)4]+ is involved in the structural change of the crystal lattice induced by the movements of the [SbCl6]- anion.     

Subthreshold parallel pumping near and above the critical temperature in K2CuF4 and (CH3NH3)2CuCl4

Parallel pumping absorption below the threshold for spin-wave instability was observed near and above the critical temperature T_c in the quasi-two-dimensional ferromagnets K₂CuF₄ and (CH₃NH₃)₂CuCl₄. The absorption shape is well described by the density of states of the two-dimensional magnons at T?T_c, and gradually changes to a more symmetric peak with increasing temperature well above T_c.     

14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

The¹⁴N NMR spectra and spin-lattice relaxation timeT ₁ of [N(CH₃)₄]₂ZnI₄ have been studied between room temperature and 200 K. Two phase transitions atT _c 1=255 K and atT _c 1=217 K are observed. The¹⁴N NMR lineshape andT ₁ data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH₃)₄]₂ZnI₄.     

Ferroelastic domain structure of (CH3)4N CdCl3 (TMCC) crystal

The ferroelastic domain structure and the phase boundaries of TMCC have been studied in the temperature range 114-90 K by direct observation under polarised light. By applying an external, compressive and unidirectional mechanical stress the ferroelastic character of the domain structure has been confirmed. The orientation of the domain walls and phase boundaries are analysed. To characterise quantitatively the observed domain wall distribution the classical symmetry approach, based on the criterion of spontaneous strain compatibility, has to be extended to allow small rotations of the domain walls with respect to their ideal orientation. The observed switching process among the different domains can be understood as a mechanism that minimises the elastic energy. Received 21 July 2000     

Torsional Splitting in the nu(5) Fundamental Infrared Band of CH(3)CD(3) and (13)CH(3)CD(3)

The nu(5) fundamental (C-C stretching) of CH(3)CD(3) shows a resolved torsional structure, caused by perturbations due mainly to the linear dependence of the torsional potential barrier on the normal coordinate Q(5). We were able to analyze this structure and to assign vibration-rotation transition wavenumbers for all five torsional components, classified according to the symmetry species of the G(18)((3)) extended molecular group. The torsional splitting pattern is qualitatively similar to that of a nondegenerate vibrational state with an even number of excited torsional quanta v(6). Explorative calculations show that the main perturber system should consist of the torsional components of the vibrational ground state correlating with v(6)=4 in the high barrier limit. The strength of the perturbation on the E(r0) torsional components of nu(5) increases rapidly with r, the E(40) component being the most affected. The observed transition wavenumbers can be reasonably fitted by a simplified model containing independent effective vibration-rotation parameters for the five different torsional components of nu(5), for both CH(3)CD(3) and (13)CH(3)CD(3). The trend of the determined values of the effective vibrational wavenumbers and rotational parameters over the torsional components supports the proposed vibration-torsion interaction mechanism, responsible for the observed torsional splittings. A strong anomaly observed in the rotational intensity distribution of nu(5) is discussed. Copyright 2001 Academic Press.     

CH3CH2+N(4S)反应机理的理论研究

利用abinitio方法对CH3CH2+N(4S)反应进行了理论研究,在MP2/6-311+G(d,p)水平上优化得到了反应途径上的反应物、中间体、过渡态和产物的几何构型和谐振频率,并在QCISD(T)/6-311+G(d,p)水平上进行单点能计算.计算结果表明,CH2CH2+3NH和H2CN+CH3是此反应主要产物,CH3CHN+H是此反应次要产物.产物CH2CH2+3NH主要来自直接氢抽提反应通道,H2CN+CH3来自加成-解离反应通道,CH3CHN+H来自加成-解离反应通道.     

Manifestation of Spatial Modulation in the Vibration Spectra of (N(CH3)4)2CuCl4 Crystals

The specific manifestations of the spatial modulation in the vibration spectra of (N(CH₃)₄)₂CuCl₄ crystals have been investigated. It has been shown that because of the narrow temperature range of existence of the incommensurate phase and the complexity of the spectra being analyzed, reliable resolution of additional modes arising under the influence of both incommensurate and commensurate modulation is only possible using a computer approximation of the spectra obtained. In accordance with the theoretical models considered, additional modes have been observed mainly in the region of low-frequency skeletal vibrations exhibiting a marked dispersion.     

81Br NQR Study of [NH3(CH2) n NH3]CdBr4 (n = 4 and 5) and [NH3(CH2) n NH3]ZnBr4 (n = 5 and 6)

⁸¹Br NQR frequencies and differential scanning calorimetry (DSC) were measured as a function of temperature. [NH₃(CH₂)₄ NH₃]CdBr₄ (1) and [NH₃(CH₂)₅NH₃]CdBr₄ (2) showed a doublet and quartet ⁸¹Br NQR spectrum, respectively. [NH₃(CH₂)₅NH₃]ZnBr₄ (3) and [NH₃(CH₂)₆NH₃]ZnBr₄ (4) exhibited a four-line ⁸¹Br NQR spectrum. From the NQR results, it is inferred that (1) and (2) consist of infinite two-dimensional sheets of corner-sharing CdBr₆ octahedra, whereas (3) and (4) have isolated [ZnBr₄]²⁻ tetrahedra. All of the crystals except (1) showed at least one structural phase transition above 380 K.     

Analysis of incommensurate structure in [N(CH3)4]2ZnCL4 crystal

We analyzed the temperature dependence of the amplitude of modulated structures of [N(CH₃)₄]₂ZnCl₄ crystal in the incommensurate (INC) phase by measuring the EPR spectra for Mn²⁺ doped single crystal. The amplitude of the vibrational modulation linearly increased with decreasing temperature. Applying Frank and Van der Merwe model, we simulated the EPR spectra in the INC phase. The simulated spectra agreed with the observed one very well. We, therefore, revealed that the vibrational modulation in this INC phase is due to the interaction between the harmonic chain of inter-particles and the modulation due to underlying potential which comes from the commensurate structure.