Jaime Del Cerro

Antiferroelectric phase transition in titanite: Excess entropy and short range order

Abstract The antiferroelectric A2/a↔P21/a phase transition in titanite may be described using a thermodynamic model where the principal contribution to the excess entropy is assumed to be configurational rather than vibrational; G = A/2 Q2 + B/4 Q4 + λT [(1 + Q) ln (1 + Q) + (1 - Q) ln (1 - Q)]. Such a model is likely to be a valid description of a phase transition where strain effects are large enough to maintain mean field behavior, but not so large that vibrational effects dominate in the excess entropy. Best-fit parameters for A/λ and B/λ are determined from X-ray measurements of the order parameter. The magnitudes of the three parameters are then determined using calorimetric data, with the results A = -337.6 J/mol, B = -112.5 J/mol, λ = 0.34 J/mol-K. The model is compared with measurements of dielectric susceptibility, and is found to give good, but not perfect, agreement with experiment. The excess entropy associated with the transition is far smaller than expected for a simple order-disorder model. This is interpreted as evidence for significant short-range order above TC.

Phase transitions in lawsonite a calorimetric study

The specific heat of lawsonite, CaAl 2 Si 2 O 7 (OH) 2 .H 2 O, has been measured in the temperature range 125 K - 325 K. An anomaly is seen at 273 K, which is related to the Cmcm — Pmcn phase transition. The magnitude of the total excess entropy associated with this transition is not reproducible, varying in the range 5.93 — 6.24 J K −1 mol −1 . On heating, the specific heat anomaly is consistent with a tricritical phase transition. However, on cooling, significant hysteresis is observed, and the form of the C P anomaly is quite different. In all measurements extensive pre-transitional effects are observed above T C . Analysis of existing specific heat data in the temperature range 75 K — 175 K shows an anomaly associated with the Pmcn — P2 1 cn phase transition. The excess entropy associated with this transition is 6 (1) J K −1 mol −1 . These data are interpreted as showing that both transitions are caused by the interaction of proton ordering and displacive changes in the aluminosilicate framework. The standard entropy of lawsonite at 298 K is recalculated, incorporating the effects of the two transitions. Two methods are used for this recalculation, giving values of S 0 298 = 233.27 and 234.96 JK −1 mol −1 respectively.

Domain structure of latgs single crystals

Abstract Single crystals of TGS are grown with different proportions of L-alanine. The crystal habit is related with the sense of the polarization and with the polarization of the seed. A static study of the domain structure by the etching technique shows the differences between the surfaces (010) and (010).

The influence of an electric field on the latent heat of the ferroelectric phase transition in KDP

The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystal have been measured simultaneously for various electric fields with a conduction calorimeter. The specific heat presents a strong anomaly, but these simultaneous measurements allow us to evaluate the latent heat accurately. The latent heat decreases with field, and the value of the critical electric field—that where the latent heat disappears—is estimated to be (0.44 ± 0.03) kV cm−1. Incidentally, we have measured simultaneously the dielectric permittivity, which suggests that latent heat is developed as domains are growing.

Temperature Cycling Influence on Pyroelectric Figure of Merit of Triglycine Sulphate Doped with Alanine and Deuterated Triglycine Sulphate Doped with Alanine and Bivalent Platinum Crystals

While heating and cooling in the range 20-60° C, the pyroelectric coefficient, the dielectric permittivity and the specific heat of three samples of triglycine sulphate (TGS) family have been measured. These samples were TGS doped with alanine (ATGS), deuterated TGS doped with alanine (ADTGS), and deuterated TGS doped with alanine and bivalent platinum (ADTGS:Pt). Using these data, the thermal hysteresis of the pyroelectric figure of merit F v of these three crystals has been studied in this temperature range. The highest value of F v was obtained for the ADTGS:Pt crystal while the widest temperature range in which F v can be considered constant was obtained for the ADTGS crystal.

Influence of the rate of temperature change on the dielectric tail of single crystal Rb2CoCl4

Abstract The influence of very small rates of temperature change on the dielectric permittivity of incommensurate crystal Rb2CoCl4 was studied. It is shown that in the commensurate phase the permittivity excess depends on the cooling rate and the tail is present even for rates as low as 0.002 K min−1. Relaxation processes of the permittivity were studied in both the temperature range where C and IC phases coexist and at temperatures where the discommensurations are not stable. Results are compared with data of Rb2ZnCl4 and K2ZnCl4. Data suggest that the dielectric tail is more stable in Rb2CoCl4 than in K2ZnCl4 and is a consequence of revival discommensurations.

Pyroelectric coefficient of α-LiIO3 below room temperature

Abstract The pyroelectric coefficient of TGS single crystal is measured by means an experimental system which is described. Below room temperature the pvroelectric coefficients of two α-LiIO3 single crystal grown from different pH solution are measured. There is evidence of the influence of crystal growth on pvroelectric properties of α-LiIO3.

Long Time Permittivity Decay in Commensurate Phase of Rb2ZnCl4 Single Crystal.

The decay of the permittivity of Rb 2 ZnCl 4 is studied over ten days in the commensurate phase. No equilibrium state is actually found after an experience of twelve days. Assuming that the heating branch may represent the equilibrium state a three-component relaxation law is obtained with three different relaxation time. These relaxation times are connected to three different mechanisms of permittivity decay: six domain wall merging, pair annihilation and pinning to defects.

Electric field dependence of pyroelectric figure of merit and other thermodynamic parameters in TGS

Abstract The adiabatic electric susceptibility of TGS is measured near the critical temperature and at several applied electric fields (160,300,500 and 800 V/cm). Using the values of specific heat and pyroelectric coefficient measured previously on the same sample, the variation of isothermal susceptibility, specific heat at constant polarization and figure of merit with values of electric field are obtained.

Influence of Electric Field in Commensurate-Incommensurete Phase Transition of Rb 2 ZnCl 4

The commensurate-incommensurate phase transition in Rb 2 ZnCl 4 single crystal is studied under three external applied field. We measured simultaneously the dielectric permittivity and the heat flux exchanged by the sample, We study the evolution of the transition enthalpy with the applied field. We also observe the formation of a polar incommensurate phase during the cooling process with external field.