EPR lineshape study of the incommensurate phase in γ-irradiated K2SeO4

The temperature dependence of the SeO^4-₄ EPR frequencies and the asymmetric broadening of the EPR lines in the incommensurate phase of K₂SeO₄ can be explained by an incommensurate spatial modulation of the g tensors which corresponds to the “broad” phase soliton limit. A comparison between the experimental and calculated lineshape shows a ≈1% volume fraction of commensurate regions in the middle of the incommensurate phase at 110 K.     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

Hydrostatic pressure effect on commensurate—incommensurate transition in K2SeO4-Elastic anomalies studied by Brillouin scattering

A Brillouin scattering study of the commensurate—incommensurate transition in K₂SeO₄ submitted to a hydrostatic pressure was performed. It was found that an increasing pressure lowers the transition temperature (dT_i/dP_i = ?59 K/GPa). The strong anomalies of velocity and damping of the longitudinal acoustic wave propagating along [0 0 1] are weakly affected when pressure acts. The effect of pressure on these anomalies has been calculated on the basis of the lowest order couplings eQ² and e²Q² between strain e and order parameter Q.     

Neutron evidence for an overdamped phason branch in incommensurate K2SeO4

Neutron results on the incommensurate phase of K₂SeO₄ taken near a strong satellite reflection, reveal a heavily damped inelastic feature which is identified as the phase-mode response. The damped-harmonic-oscillator analysis of the experimental spectra leads to mode-parameter values which are consistent with recent optical results.     

Study of commensurate and incommensurate phases in NaNO2 by 23Na NMR

The temperature dependence was determined of the ²³Na NMR spectrum of NaNO₂ with special emphasis on the small incommensurate phase and the surroundings of the phase transition points. Whereas far away from the transitions satellite lines of Gaussian shape with the natural line widths are observed near the transitions broad line distributions are detected showing distinct edge singularities. The results are related to the sinusoidally modulated and homogeneous order parameters of the incommensurate and ferroelectric phases.     

Half-field EPR line in the two-dimensional ferromagnet K2CuF4

Room temperature measurements of the half-field EPR line in the two- dimensional ferromagnet K₂CuF₄ are reported. Changes of the intensity and line-width vs the angle between the applied field and the c-axis indicate the necessity of an additional term in the perturbing Hamiltonian besides the dipolar interaction. Its appearance at half the main Zeeman field at each angle means that there is no observable dynamic effect on the shift of the resonance position in this salt.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

Isomorphism among commensurate and incommensurate high-temperature phases of A2BO4 compounds

The incommensurate and commensurate superstructures of K₂MoO₄, Rb₂WO₄ and K₂WO₄ are shown to have orthorhombic symmetry rather than the hexagonal symmetry, reported earlier. The structures belong essentially to the β-K₂SO₄ type. The β-K₂SO₄ structure can be regarded as a commensurate superstructure of the α-K₂SO₄ structure by doubling of the unit cell. In this sense β-K₂SO₄ and K₂MoO₄ have the same average structures. Rb₂WO₄ displays a lock-in transition at 681 K where the incommensurate distortion wave vector jumps from an incommensurate value to the value 0.5 which results in the β-K₂SO₄ type. The distortion wave vectors of the other compounds range from 0.25 to 0.5.     

Electrical and thermal studies in the commensurate incommensurate phase region of (NH4)2ZnCl4 single crystal

DC electrical conductivity for a virgin and poled annealed (NH₄)₂ZnCl₄b-axis single crystal shows a defect controlled property. A Schottky mechanism is a probable mechanism of conduction in regions of strong structural transitions. The rise of conductivity in the incommensurate and paraelectric phases is linked to an increase in discommensurations density. The activation energies (ΔE) in the three phases region were calculated. DTA measurements shows that the crystal is stable up to 200 °C and the phase transition temperatures were observed at 42, 94.8 and 137 °C. The effective activation energy (E_e) was obtained using Kissinger and Mahadevan equations. It was found to be equal to 0.49 eV. This correlates with the value obtained through DC conductivity.     

Hydrostatic pressure effect on the commensurate—incommensurate phase transition of NiBr2

Pressure effects on the magnetic properties of NiBr₂ have been determined. While the Néel temperature is practically independent of pressure, the temperature of the transition between the incommensurate and the commensurate phases decreases strongly with pressure (9.5% per kbar) and above 10.6 kbar the stability range of the helical state disappears. The susceptibility in the antiferromagnetic phase is depressed by pressure (4.5% per kbar) whereas it remains unchanged in the helical state.     

Raman spectra of K2CuF4

The crystal structure of K₂CuF₄ has been studied by means of Raman scattering. The structure is found to belong to the space group D_4h⁵ rather than D_4h¹⁷ of K₂NiF₄ between 2 and 800 K because of the distortion arising from the “orbital ordering effect”.     

Theoretical examination of optical and EPR spectra for Cu ion in K2PdCl4 crystal

In this work, two d-d transition spectra and four EPR parameters g_∥, g_⊥, A_∥, A_⊥ of K₂PdCl₄/Cu²⁺ are uniformly interpreted based on Zhao's crystal-field model. The calculation result is in good agreement with the experiment findings. The ligand spin-orbit coupling is neglected on the calculation, which is consistent with the ab initio result by Hillier et al. [J. Am. Chem. Soc. 98 (1976) 95]     

The commensurate and incommensurate TGBC^* liquid crystal phases

We propose complements to the Renn model of the liquid crystal TGBC^* phase. We argue that the rotation angle per helislab is spontaneously in the radian range, not too small to limit the energy cost of the twist grain boundaries between the helislabs, not too large to preserve the double twisting efficiency. Taking the elastic interactions between the helislabs into account, we show that the structure may undergo two different lock-ins, provided that the uncontrolled interactions at the sample surfaces are small enough. First, for appropriate values of the elastic constants, an angular lock-in may fix the rotation angle per helislab at values exactly commensurate to π. Three characteristic lengths of the TGBC^* phase, the TGB period and the thicknesses of both the smectic blocks and of the helislabs are then commensurate to one another, with moreover, integer ratios at the most efficient commensurabilities. The TGBC^* phase could thus exist in several versions, incommensurate and commensurate, according to the steps of a restricted Devil's staircase. A second elastic lock-in should then arise in the commensurate TGBC^* phases, to set the arrays of disclination lines between the helislabs in simple rectangular lattices, arranged in a helical manner. Being placed right behind one another, the disclination lines then coincide when observed along the TGB axis. This could explain the typical textures with square or hexagonal grids, observed in oriented samples. The commensurability of the TGBC^* phases could be analyzed with X-ray scattering experiments in the same manner as already reported for the TGBC phase. Received 30 November 1999 and Received in final form 5 May 2000     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Metastable chaotic state in K2ZnCl4

The ³⁵Cl NQR frequencies of K₂ZnCl₄ have been measured close to the incommensurate-commensurate transition. A calculation of the temperature dependence of the soliton density n_s from the NQR data showed that in K₂ZnCl₄ close to T_c exists a metastable chaotic state.     

Experimental determination of the spin-wave spectrum of the two-dimensional ferromagnet K2CuF4

K₂CuF₄ is a quasi-bidimensional ferromagnet with an easy magnetization plane. The Curie temperature is 6.25 K. We have measured the spin-wave spectrum from 2 to 14 K, and found an exchange integral of 11.4 K between the first neighbours in the basal plane. No measurable dispersion was found along the axis perpendicular to that plane. We have observed a shift in the energy of the magnons, and a broadening of the magnon peak both of which behave differently below and above the Curie temperature.     

EPR evidence of local lattice mode in K3H(SO4)2 and Rb3H(SO4)2 fast-proton conductors

The observation of an anomalous temperature dependence of Mn²⁺ EPR spectra linewidth and nonaxial crystal-field parameter in K₃H(SO₄)₂ and Rb₃H(SO₄)₂ allows one to suggest the presence of “local mode” predicted by Yamada (Ferroelectrics 170 (1995) 23). The activation energy for this kind of excitation was found and equals 11.3 (0.5) and 7.4 (0.3) meV for Mn²⁺ doped K₃H(SO₄)₂ and Rb₃H(SO₄)₂, respectively.     

Hyperfine interactions in Zn-doped K2CuF4

The hyperfine fields of Cu²⁺ and F^- ions nearest to a Zn²⁺ ion are measured by a spin echo method in a single crystal of K₂Cu_0.95Zn_0.05F₄. It is shown that the local symmetry due to the Jahn-Teller distortion is practically unaffected by the proximity of Zn²⁺ ions.     

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.     

EPR in the impurity-doped two-dimensional ferromagnetic system K2CucZn1−cF4

EPR measurements of the half-field sideband as well as of the main line are reported in the impure two-dimensional ferromagnetic system K₂Cu_cZn_1?cF₄. A drastic change of the decay rate of spin correlation near the percolation limit and direct evidence for a correlation between these two lines have been observed.