A. López Echarri

Specific heat behaviour of K2SeO4 in the 48K–800K temperature range a new phase transition

Abstract A calorimetric study performed on the ferroelectric K2SeO4 in the 48K–800K temperature range confirmed the existence of three phase transition and revealed the occurrence of a new one at 56.03±0.05K (TIVc). The critical measurements for the commensurate-incommensurate phase transition seem to show a cross-over behaviour.

Phase transition study of the ferroelectric (N(CH3)4)2CoCl4

Abstract Specific heat measurements confirm the existence of five phase transitions in (N(CH3)4)2CoCl4 in the 60-330 K temperature range. The thermodynamic function values associated to these phase transitions are closely related with the Zn isomorphous compound although some differences in the low temperature transition set show the role played by the metal ion.

Specific heat critical behaviour of trissarcosine calcium chloride

Abstract Studies about the critical behaviour of the TSCC ferroelectric crystal show discrepancies between the values of the critical exponents obtained from different experimental techniques. /1, -4/. A clarification of the critical behaviour of this ferroelectric was able to be realised from careful specific heat measurements by adiabatic calorimetry which avoid external field perturbation on the critical observation.

Experimental and group theoretical study of the structural phase transition in RbMnCl3 crystals

Abstract X-ray diffraction data, calorimetric and birefringence measurements, as well as group-theory arguments are used to characterize the ferro-paraelastic phase transition observed in RbMnCl3 at 272K.

Critical behaviour of the ferroelastic-incommensurate (N(CH3)4)2CuCl4

Abstract (N(CH3)4)2CuCl4 shows the following phase transition sequence: Commensurate-incommensurate-ferroelastic (incommensurate)-paraelastic (commensurate) with transition temperatures at 298.79 K, 292.8 K and 246.82 K respectively.

Phase transition sequence in (C5H11NH3)2ZnCl4 between 55 and 355 K

Abstract Bis-penthylammoniun tetrachloro zincate (C 5 H 11 NH 3 ) 2 ZnCl 4 , was investigated between 55 K and 355 K by means of adiabatic calorimetry, thermal expansion and optical measurements. The following phase transition sequence was found: V-(141.5K)-IV-(148K)-III-(249.6K)-II-(349K)-I, and the associated thermodynamic functions were obtained. The two lower phase transitions are probably connected with order-disorder mechanisms associated with hydrogen bonds. Phase III is pure proper gyrotropic and phase I is characterized by a high confonnational disorder. The various energy contributions in phase I were estimated in order to give a qualitative description of the molecular movements in the II-I phase transition.

Optical studies of the isotropic point in (CnH2n+1NH3)2ZnX4 crystals with 10⩽n⩽14 and X4Cl4,Cl2Br2

Abstract The dispersion of the optic axial angle of the (CnH2n+1NH3)2ZnX4 crystals with 10⩽n⩽14 and X4Cl4,Cl2Br2 has been investegated between 273 K and 363 K in the spectral interval from 470 nm. to 650 nm. The experimental results show, together with a high anomalous optical dispersion, the axistence of an isotropic point. At this point the optical plane rotates 90 o - around the acute bisectrix. A dependence of the isotropic point temperature with n, X, and the wave length is also found.

Free energy phenomenological parameters for the phase transition sequences of incommensurate (N(CH3)4)2BCl4 compounds with B = Co, Zn y Cu

Abstract From specific heat measurements we have determined the free energy behaviour of A2BX4 organic compounds (A=N(CH3)4; B=Co, Zn, Cu and × = Cl) in the 50-330K temperatures range. The complex phase transition sequence observed in these compounds is well described by the use of a phenomenological model for phase transitions1. The good agreement found with the experimental data have allowed to specify the numerical values of the phenomenological coefficients which account not only for the stability of the phases in a wide temperature range, but also for a good description of the wave vector and for the order parameter in the incommensurate phases2. The gradual disappearance of the ferroelectric phase in Co and Zn compounds by the influence of the pressure3 as well as the presence of two different incommensurate phases in the Co compound is also studied by a suitable pressure dependence of the phenomenological parameters. This method suggests a simple general procedure in order to determine the pressure...

Influence of high order terms in the physical magnitudes obtained from free energy expansions in ferroelectric materials

Abstract From a thermodynamic point of view, the Landau-Devonshire free energy expansion for ferroelectrics with a second order phase transition, can be truncated at the second order term. This expansion gives afairly good picture of the behaviour of the Polarization and Dielectric Constant both, below and above the transition temperature but close enough to it, and it is equivalent t othe molecular field prediction on the critical behaviour. However, we know l that for wider temperature intervals third and fourth order terms are necessary in order to fit the experimental results for Ps with the theoretical predictions.