I. Ruiz-Larrea

The phase transition sequence in tetramethylammonium cadmium chloride (TMCC)

The structural phase transitions in were investigated by means of adiabatic calorimetry and thermal expansion measurements. Two phase transitions have been studied on cooling and on heating at about 104 K and 118 K. The shapes of the observed specific heat anomalies, as well as the existence of thermal hysteresis, confirm the first-order character previously assigned to these phase transitions. From the experimental data and the harmonic specific heat obtained from the known frequencies of the vibrational modes, a calculation of the Gruneisen parameter as a function of the temperature is also given. Finally, the suitability of different Landau potentials for the two phase transitions is briefly discussed.

Thermodynamic study of the temperature memory effects in Cu–Al–Ni shape memory alloys

This work presents careful adiabatic calorimetry measurements of the temperature memory effects on a Cu–Al–Ni shape memory alloy single crystal. These effects, which appear after partial martensitic transformations, have been systematically studied. The subsequent delay of the transformation as a function of the thermal history is described by means of a straightforward model, which makes full use of the phase transformation thermodynamic functions, carefully determined from the specific heat results. This model predicts on quantitative grounds, the decrease in the transformed mass in the successive thermal cycles. It also provides information about the distribution of the elastic energy among the martensitic plates in the low temperature phase, which is a characteristic feature of these alloys. The knowledge of the elastic states is not only required to analyze the properties of the martensitic phase but also to describe the kinetics of any physical quantity in the reverse transformation.

A new (C2H5NH3)2ZnCl4 crystal with a pure Pnma-P212121 ferroelastic phase transition

Optical, calorimetric, and X-ray measurements performed on (C2H5NH3)2ZnCl4 (C2ZnCl) together with a group theoretical analysis have revealed that this crystal shows a pure first-order ferroelastoelectric phase transition at 243.3 K. In the low-temperature phase the crystal shows two explicit forms of ferrocity: gyrotropy and piezoelectricity, which is a common fact in this class of secondary ferroics. The experimental data show that the corresponding phase transition is similar to that observed in (C5H11NH3)2ZnCl4 (C5ZnCl) at 249 K. The thermodynamic function values for this phase transition in both compounds and the X-ray results confirm that the conformational changes must be excluded from the physical mechanisms involved in the onset of the ferroelastoelectric properties. The microscopic origin of this phase transition should be related to the freezing of the dynamical disorder of the N-H...Cl hydrogen bonding and with the corresponding tilting of the hydrocarbon chains. The experimental data seem to be in agreement with the Landau phenomenological theory except near Tc, where the experimental specific heat is well described by an empirical power law. Finally, as found in some other crystals of this family, C2ZnCl crystals also exhibit an isotropic point around 25 K. The isotropic point temperature shows a very strong dependence on n (chain length) for n or=12.

The lattice contribution to the specific heat in tetramethylammonium tetrachlorometallates

The lattice specific heat in tetramethylammonium tetrachlorometallates was calculated from accurate Cp measurements as well as from the available spectroscopic data. The experimental information on various members of this family and some related compounds was also used to analyse the various contributions to the lattice specific heat. The anharmonic correction was performed using the Nernst-Lindemann law, together with the elastic constants and with the thermal expansion coefficients. The frequencies of the Raman and infrared inactive modes were estimated from the active associated motions in related organic molecules. Einstein and Debye functions were used to calculate the harmonic specific heat from the phonon spectrum. The final result shows an excellent fit to the experimental data and provides a useful method to obtain a baseline for the calculation of the phase transition thermodynamic functions. This method has permitted the authors to study the specific heat contribution of the tetramethylammonium groups, which is also compared with the values obtained by other procedures. Finally, the main sources of errors are discussed.

The specific heat of [N(CH3)4]2ZnBr4 around the ferro-paraelastic phase transition by adiabatic calorimetry

Abstract A calorimetric study of [N(CH3)4]2ZnBr4 is presented. This compound exhibits a second-order phase transition at 287.20 ± 0.12 K from a high-temperature orthorhombic phase (space group Pnma) to a monoclinic ferroelastic phase (space group P21 /a). Specific heat measurements were carried out by automatic adiabatic calorimetry from 170 to 370 K, and the value obtained for the phase transition entropy was ΔS = 1.95 ± 0.1R (where R is the ideal gas constant). A simple phenomenological model is found to describe the specific heat below Tc. The experimental set-up and procedure are also described.

The specific heat of the ferroelectric phase transition in N(CH3)4CdBr3

The specific heat of N(CH3)4CdBr3 from 50 to 300 K has been measured by adiabatic calorimetry, using both static and dynamic methods. The obtained results have permitted a careful study of the ferro-paraelectric phase transition the crystal shows at 160 K. The available spectroscopic data have been used to generate a reliable baseline which accounts for the normal lattice contribution to the specific heat. These results allow for an accurate estimation of the phase transition thermodynamic functions: ΔH=2620 J·mol−1 and ΔS=18.04 J·(mol°C)−1. These high values are in agreement with the predictions of the 6 well potential Frenkel model.ZusammenfassungUnter Einsatz sowohl dynamischer als auch statischer Methoden wurde mittels adiabatischer Kalorimetrie die spezifische Wärme von N(CH3)4CdBr3 zwischen 50 und 300 K gemessen. Die erhaltenen Resultate erlaubten eine behutsame Untersuchung der ferroparaelektrischen Phasenumwandlung, die das Kristall bei 160 K aufweist. Die zur Verfügung stehenden spektroskopischen Angaben wurden benutzt, um eine zuverlässige Basislinie zu erhalten, welche den normalen Gitterbeitrag zur spezifischen Wärme berücksichtigt. Diese Resultate erlauben eine genaue Schätzung der thermodynamischen Funktionen der Phasenumwandlung: ΔH=2620 J·mol−1 und ΔS=18.04 J·(mol°C)−1. Diese hohen Werte stimmen mit den Vorhersagen der 6 besten Frenkelschen Modelle überein.

The phase transition sequence in thiourea (SC(NH2)2). A heat capacity study

Abstract A careful adiabatic calorimetric study of the phase transitions sequence in thiourea has been made. Phase transitions at 169 K, 171 K and 200 K have been studied. The associated enthalpies are ΔH 169K/R = 3.167 K, ΔH 171K/R = 0.103 K, ΔH 200K/R = 13.547 K. Up to the experimental resolution of the system, there is no evidence for the existence of calorimetric anomalies at 161 K or 177 K, as suggesed by dielectric techniques carried out at non-zero electric fields. Making use of the published temperature dependent Raman spectroscopic data, the different contributions to the heat capacity have been determined. This method permits one to assign very accurate values for the thermodynamic functions of the various phase transitions undergone by the crystal.

The phase transition sequence in betaine calcium chloride dihydrate by adiabatic calorimetry

Abstract The specific heat of (C3)3NCH2COO.CaCl2.2H2O (BCCD) has been measured by adiabatic calorimetry from 20 K to 330 K. This experimental study refers to the rich variety of phase transitions showed by this crystal and has permitted a precise characterization of its calorimetric response. Most of the previously reported phase transitions show a clear specific heat anomaly. In some cases, however, these measurements do not permit one to confirm the occurrence of phase transitions when they are related to very small jumps of the modulation wave vector. The results presented in this work confirm a noticeable relationship between the wave-vector behaviour and the associated phase transition entropy. In fact, this correlation has permitted an unambiguous identification of some of the crystal stable phases.

The specific heat of tetramethylammonium salts

New measurements of the (N(CH3 )4 )2 MnBr4 specific heat by adiabatic calorimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are compared with previous calorimetric studies on similar tetramethylammonium bromide compounds. The thermodynamic behaviour of the tribromides and tetrabromides derivatives together with the influence on the phase transition parameters of the cation and halogen molecular substitutions are examined. The thermal relaxation experiments permit to study the behaviour of the crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions is used to describe the thermodynamic behaviour of some of these crystals.

The specific heat of N(CH3)4MnBr3 by adiabatic calorimetry

The specific heat of N(CH3)4MnBr3 has been measured by adiabatic calorimetry, using both static and dynamic methods. The obtained results have permitted the calorimetric characterization of the phase transition which the crystal shows at 143 K. The comparison with other compounds of the family has been used to generate an adequate baseline for the normal lattice contribution to the specific heat. These results allow for an accurate estimation of the phase transition thermodynamic functions: ΔH = 218 R K and ΔS = 1.58 R. The calorimetric data are very close to those found for the isomorphous N(CH3)4CdBr3 and suggest a similar frequency spectrum for the lattice vibrational modes and the anharmonic contributions to the specific heat.