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The ENDOR technique is applied to study the structure of the free radical N(CH3)3, found in the high temperature phase in the incommensurate crystal BCCD after γ-irradiation. The radical possesses a planar structure with strong deviation from the C3 symmetry. The hybridization of the nitrogen atom is sp2. Each of the three methyl groups exhibits a single ENDOR line, indicating the rotation of groups at room temperature. The deviation of the1H hyperfine tensors of the methyl groups from axial symmetry is not a hint as to a hindered rotation of the groups. This is predicted by the C2 symmetry of the system consisting of a π orbital and a rotating CH3 group. 相似文献
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The electron nuclear double resonance (ENDOR) technique is applied to study the influence of the incommensurate modulation on the proton hyperfine structure (hfs) tensors of the N(CH3)3 radical in betaine calcium chloride dihydrate (BCCD). The temperature dependence of the edge singularities due to the increasing of the incommensurate (IC) modulation amplitude could be observed in the1H ENDOR of the CH3 groups for the first time. The rotation pattern of the edge singularities observed in the1H spectra in the IC phase of BCCD can be explained by a libration around the C1-N axis with a tilt angle of about ±7°. 相似文献
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L.G. Vieira O. Hernandez A. Almeida M. Quilichini J.L. Ribeiro M.R. Chaves A. Klöpperpieper 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):447-456
We report a combined experimental study by means of elastic neutron scattering and dielectric measurements of a partially
deuterated and brominated BCCD (Betaine Calcium Chloride Dihydrate) crystal. The lowest-temperature phase is one-dimensional modulated and
characterized by the coexistence of different commensurate domains (with = 1/4, 4/17, 2/9 and 1/5 on cooling), but with a clear predominance of the five-fold phase. A huge global thermal hysteresis
of the wave-vector of the modulation, attaining values of about 9 K in the incommensurate phase and up to 15 K in the “harmless”
low temperature part of the phase diagram, is observed up to . The role of lattice defects on this phenomenon is discussed. Similarly to the behaviour of the pure compound, the structural
modulation evolves on cooling towards a soliton regime (growth of third and fifth-order satellite peaks), probably with respect
to a non-stabilized non-modulated ferroelectric phase. The critical temperatures deduced from dielectric constant and pyroelectric
current measurements are in very good agreement with those obtained from neutron scattering. The dielectric anomaly observed
in at K, and known as the “-anomaly”, could not be related with any special feature detected in the neutron data, and in particular no correlation between
this anomaly and the appearance of the soliton regime can be established.
Received 26 October 1998 相似文献
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E. V. Balashova B. B. Krichevtsov F. B. Svinarev N. V. Zaitseva 《Physics of the Solid State》2016,58(7):1397-1406
Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al2O3(110) and NdGaO3(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films D varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature Tcafe = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance C, which is not accompanied by a change in the dielectric loss tangent tanδ. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures T >Tcafe, which is transformed into an antiferroelectric nonlinearity at T <Tcafe. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling ξP2η2 between the polar order parameter P and the nonpolar order parameter η with a positive coefficient ξ. The E–T phase diagram has been constructed. 相似文献
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The dielectric spectra of ferroelectric hydrogen bonded betaine phosphate0.05 betaine phosphite0.95 (DBP0.05DBPI0.95) was investigated in the very wide temperature (300–20 K) and frequency (20–35 GHz) regions. The dielectric dispersion was analyzed in terms of distribution of relaxation times, using Tichonov regularization method. Strongly asymmetric and broad distribution of relaxation times below ferroelectric phase transition temperature T c?≈?253 K clearly differs from the one that is usually observed in ferroelectrics. The observed disorder in deuterons system close to ferroelectric phase transition temperature is an embryo of coexistence ferroelectric order and dipolar glass disorder observed at low temperatures. 相似文献
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Dynamical studies of highly deuterated betaine phosphate near the antiferroelectric phase transition
H. J. Brückner H. -G. Unruh G. Fischer L. Genzel 《Zeitschrift für Physik B Condensed Matter》1988,71(2):225-229
The dielectric response and the Raman spectra of single crystals of deuterated betaine phosphate are studied around the antiferroelectric phase transition. The dielectric data between 10 MHz and 11 GHz can be explained on the basis of a simple Debye-relaxation with a critical slowing-down of the relaxation rate on approachingT
C
. Using the Cole-Davidson form of the dielectric function we succeeded in fitting the data in the whole frequency range from 10 MHz to 11 GHz and from 64–400 GHz over a temperature range from 145–280 K. Raman spectra clearly indicate that the doubling of the unit cell does not take place at the antiferroelectric transition temperature, but some degrees below. 相似文献
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H. J. Brückner 《Zeitschrift für Physik B Condensed Matter》1989,75(2):259-264
The dielectric behavior of highly deuterated betaine arsenate in the frequency range up to 11 GHz is described by a Cole-Davidson relaxation. The characteristic dynamics show a critical slowing down near the para-antiferroelectric phase transition. The temperature dependence of the dielectric constant is analyzed in terms of a pseudo-one-dimensional Ising model. Evidence is given for a dependence of antiferroelectric coupling on deuteration which can explain the change from a ferro- to an antiferroelectric ordering on deuteration. Measurements of the dielectric relaxation under a dc bias field parallel to the critical axis elucidate the unusual phase transition behavior at low temperatures. A multicritical point appears in theE-T-phase diagram. Frustration effects due to competing ferro- and antiferroelectric interactions are not seen. 相似文献
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The critical exponents , and of undeuterated and partially deuterated ferroelectric betaine arsenate are estimated. Small but remarkable deviations from the tricritical values are observed. The obtained values are tested via scaling of the isothermals.At some distance aboveT
c a cross-over of the effective critical exponent to a value typically for short-range forces is observed. A suitably defined cross-over temperature range decreases with increasing deuteration and seems to vanish near the antiferroelectric phase boundary. 相似文献
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Poly- and single-crystal films of betaine phosphite deuterated to ∼20% have been grown by evaporation on NdGaO3 (001) substrates with a preliminarily deposited planar interdigital structure of electrodes. The small-signal dielectric
response in the 0.1–100.0-kHz frequency range has revealed a strong anomaly in capacitance upon the transition of the films
to the ferroelectric state. Application of a bias field brings about suppression and a slight shift of the dielectric anomaly
toward higher temperatures. The strong-signal dielectric response has been studied by the Sawyer-Tower method over the frequency
range 0.06–3.00 kHz both in the para- and ferroelectric phases. In contrast to the case of a plane-parallel capacitor, in
the planar structure studied, the dielectric hysteresis loops exhibit a very small coercivity at low frequencies, which grows
with increasing frequency. This difference should be assigned to different domain structures formed in a planeparallel capacitor
and in a planar structure in a saturating field. The growth of hysteresis with increasing frequency in a planar structure
is considered to be associated with the domain wall motion. 相似文献
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The polarized Raman spectra of (CH3NHCH2COOH)3CaCl2 (TSCC) have been obtained applying hydrostatic pressure in the paraelectric, ferroelectric and in the pressure-induced antiferroelectric phase. The phase transition between the paraelectric or the ferroelectric and the antiferroelectric phase appears to be of first order. No cell doubling could be observed in the antiferroelectric phase. The space group for TSCC in this phase is compatible with our experimental results. The pressure-dependence of the Raman-active soft mode is discussed qualitatively. 相似文献
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An alternative method for the determination of the deuterium content in the hydrogen bonds of ferroelectric/antiferroelectric deuterated betaine arsenate is presented. Carbon radicals formed by gamma irradiation of the betaine arsenate have been used as paramagnetic probes. The deuterium content in the hydrogen bonds has been determined by analyzing the modulation in the electron spin echo spectra of the carbon radical (CH3)3N+CHCOO? which arises because of the dipolar interactions between the paramagnetic radical and the deuterium nuclei. 相似文献
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E. V. Balashova B. B. Krichevtsov E. I. Yurko G. A. Pankova 《Physics of the Solid State》2014,56(10):1997-2004
Ferroelectric thin films of partially deuterated betaine phosphite (DBPI) have been grown by evaporation on NdGaO3 substrates with a preliminarily deposited structure of interdigitated electrodes. The block structure of the films is a texture in which the polar axis b is oriented in the plane of the film and the a* axis is perpendicular to this plane. Typical dimensions of the single-crystal blocks in DBPI films substantially exceeds the distance between the interdigitated electrodes (d = 50 μm). However, DBPI films in which the block structure has characteristic dimensions of the order of d have also been grown. Investigations of the dielectric properties of the films have demonstrated that the dimensions of the block structure have little effect on the behavior of a small-signal dielectric response, which, in the phase transition region, is characterized by a strong anomaly of the capacitance of the structure at T = T c and by a glass-like behavior of the capacitance C and dielectric loss tangent tanδ in the temperature range of 120–200 K. By contrast, the low-frequency strong-signal dielectric response (dielectric hysteresis loops) in the structures with small blocks differs significantly from that observed for large-block structures. The difference in the frequency behavior of the hysteresis loops in the large-block and small-block structures is associated with the limitation of motion of domain walls in the case of small blocks. 相似文献