1H-ENDOR investigations on gamma-irradiated betaine calcium chloride dihydrate at room temperature

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.     

1H ENDOR investigations on gamma-irradiated betaine calcium chloride dihydrate in the incommensurate phase

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°.     

Elastic neutron scattering and dielectric behaviour of 4% brominated betaine calcium chloride dihydrate (BCCD)

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     

Antiferroelectric films of deuterated betaine phosphate

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 T_c^afe = 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 >T_c^afe, which is transformed into an antiferroelectric nonlinearity at T <T_c^afe. 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.     

Coexistence of glass and ferroelectric order in deuterated betaine phosphate0.05 betaine phosphite0.95 crystals

The dielectric spectra of ferroelectric hydrogen bonded betaine phosphate_0.05 betaine phosphite_0.95 (DBP_0.05DBPI_0.95) 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.     

Dynamical studies of highly deuterated betaine phosphate near the antiferroelectric phase transition

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.     

Critical dielectric relaxation of highly deuterated betaine arsenate at microwave frequencies

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.     

Critical behavior of betaine arsenate

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.     

Small- and strong-signal dielectric response in a single-crystal film of partially deuterated betaine phosphite

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.     

Raman spectroscopic study of tris-sarcosine calcium chloride in the antiferroelectric phase

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 $\text{P2}{}_1\text{a (C}{}^5{}_{\mathrm{2h}}\text{)}$ for TSCC in this phase is compatible with our experimental results. The pressure-dependence of the Raman-active soft mode is discussed qualitatively.     

Determination of the deuterium content of deuterated betaine arsenate by electron spin echo envelope modulation

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.     

Dielectric properties of thin films of partially deuterated betaine phosphite with large- and small-block structures

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.