M. L. Santos

Critical behaviour in (BPI)x(BP)1−x

Abstract Measurements of dielectric constant, dielectric loss and pyroelectric coefficient as a function of temperature have been performed in Betaine Phosphite (BPI), Betaine Phosphate (BP), and mixed crystals of (BPI)x(BP)1−x for x=0.05; 0.15; 0.30 and 0.60. The behaviour observed for the mixed compounds has a good resemblance to that of structural glasses.

Dielectric and pyroelectric behaviour in (BPI)x (BP)1−x

Abstract Measurements of dielectric constant, dielectric loss and pyroelectric coefficient as a function of temperature have been performed in mixed crystals of Betaine Phosphite (BPI) and Betaine Phosphate (BP) ((BPI)x (BP)1−x), for x = 0.80, 0.90, 0.94 and 0.97. The obtained results are compared with the ones previously reported for x = 0.00, 0.05, 0.15, 0.30, 0.60 and 1.00. Three regions are apparent: one where the ferroelectric properties are dominant (x > 0.90), another which can be related to a glassy phase (0.30< x <0.90) and a third one mainly associated with an antiferroelectric behaviour of BP (x < 0.30).

Study of lattice dynamics and phase transitions in betaine phosphate by comparison with betaine phosphite via infrared reflectivity spectroscopy

The temperature dependence of infrared reflection spectra of betaine phosphate single crystal is reported for polarizations parallel to the a- and b-axes. The spectra were fitted with the factorized form of the dielectric function. A mode assignment is proposed on the basis of a comparison with the spectra of betaine phosphite and deuterated betaine phosphate. Some very asymmetric phonon lines are observed, and the importance of mode couplings is emphasized. A picture of a tendency towards transverse optical mode softening of the low-frequency spectrum upon approaching either side of the phase transition temperature emerges for the polarization parallel to the b-axis, consistent with the peak observed in the temperature dependence of the dielectric constant. On the other hand, the temperature dependence of the ionic effective charges does not display any appreciable discontinuity at the phase transition, contrary to what is observed at the temperature of the transition to the ferroelectric phase of the parent compound betaine phosphite. This is consistent with the very low level of the spontaneous polarization below in betaine phosphate. The similarities of the behaviours with those of the compounds of the KDP family are discussed.

Detailed Structural X-Ray Study of (Betaine Phosphate)1—x(Betaine Phosphite)x Compounds

We have determined the structure of a series of mixed compounds BP 1-x BPI x of betaine phosphate (BP) and betaine phosphite (BPI) at room temperature through single crystal X-ray diffraction measurements. In agreement with previous studies the observed symmetry for all studied concentrations was P2 1 /c. The evolution of the structure as a function of BPI content and the behaviour of the unit cell parameters a and b as a function of x are analyzed. A calculation of the electric dipolar moment of the H 3 PO 4 tetrahedron was effectuated as a function of x and its relevancy is discussed. A comparison between the BPI concentration in the solution and in the crystal was also made along the series.

Lattice dynamics and phase transitions in betaine arsenate

Abstract In this work we present an experimental study of infrared reflectivity spectroscopy in betaine arsenate (BA). The results obtained corroborate the order-disorder character of the phase transition in BA at 120 K, observed by Raman spectroscopy. They also provide a clear evidence of the proton ordering in the hydrogen bonds, associating different ordering temperatures to different bonds orientations. Precursory effects are observed well above 120 K.

Crystal Structure of the Ferroelectric Phase of (BP)0.08(BPI)0.92

We have determined the structure of (BP) 0.08 (BPI) 0.92 both in the ferrodistortive (at 294 K) and in the ferroelectric (at 146 K) phases, by single crystal X-ray diffraction measurements. A detailed comparison between the structures revealed small but significant structural distortions, consisting almost exclusively of rotations which evidence the order-disorder nature of protons in the bonds linking neighbouring molecules in the chains and confirm the increase of the hybridization of the orbitals of the O-H valence electrons in these bonds upon cooling. These observations suggest that the proton ordering may be, if not the only contribution to the macroscopic polarization, at least the mechanism triggering the para-ferroelectric phase transition in (BP) 0.08 (BPI) 0.92 .

Ferro-antiferroelectric compétitions in the (BP)x(BA)1- x mixed system

We have attempted to understand the low temperature microscopic structure and dipolar ordering of the (BP)x(BA)1 −x mixed system in a previous work, by performing detailed pyroelectric, dielectric and polarization reversal measurements, complemented with Raman and infrared spectroscopic investigations. Whenever the same experimental techniques are used, our experimental results agree extremely well with those reported in the literature. However, our use of slightly different methods disclosed new aspects of the phase sequence, which allowed us to trace an alternative phase diagram for the (BP)x(BA)1−x mixed system. The present paper is aimed at emphasising the most relevant of these previous results. Particular attention will be paid to the role of ferro-antiferroelectric competitions in the mixed system (BP)x(BA)1 −x.

Ferroelectric Phase in Betaine Phosphite Studied by Raman Scattering

In this work we present a detailed study of the temperature dependence (35 K – 290 K) of the Raman spectra in a single crystal of betaine phosphite. A softening of a relaxational mode is observed above and below the para-ferroelectric phase transition at Tc2∼ 213 K, which appears to have its primary origin in the different internal dynamic deformations and reorientations of molecular units. Our results suggest the existence of a certain degree of local disorder between 110 K and 170 K.

Phase Transition in Betaine Cadmium Chloride Monohydrate

Betaine cadmium chloride monohydrate undergoes a phase transition at 241 K. This transition was disclosed by using thermal differential analysis and Raman spectroscopy. This phase transition is clearly related to a structural change where the water molecules lose their degrees of freedom, becoming more tightly bonded, on cooling.

Crystal structure of trimethylglycine 2-hydroxy-1,2,3-propanetricarboxylic acid (1:1) adduct, C6O7H8 · C5NO2H11

02^ υ Abstract _ Ci 1H19NO9, triclinic, PI (No. 2), a = 7.175(3) k,b= 10.065(4) A, c = 11.099(3) A, a = 103.85(4)°, β = 97.50(3)0, γ = 105.32(4)°, V= 734.3 A3, Z=2, Rgf(F) = 0.045, wR^F2) = 0.126, T= 293 K.