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To offer a response to the increasing interest on high-temperature shape memory alloys, mainly driven by the aeronautic and aerospace industries, the design and characterization of Cu-Al-Ni shape memory alloys with high transformation temperatures, between 373 K and 473 K, are approached. The present alloy design is based on the transformation from cubic β3 austenite to the monoclinic β'3 martensite, which offers the advantage of exhibiting not only higher transformation temperatures but also a thermal hysteresis of about 12 K, much smaller than the orthorhombic γ'3 martensite historically considered for this alloy system. The influence of the alloy concentration and the thermal treatments, on the martensitic transformation temperatures is systematically analyzed and a quantitative and predictive equation is proposed. The influence of the stress on the transformation was also studied under two experimental conditions: at constant stress as a function of temperature (shape memory effect) and at constant temperature as a function of the stress (superelastic effect). A double stress-induced transformation from β3 to β'3 and α'3 with an outstanding reversible and reproducible 24% superelastic strain is also reported. The experimental results allows also determine the Clausius-Clapeyron diagrams for this series of alloys, as a fundamental tool for further design of sensing and actuating devices based on these high-temperature shape memory alloys.

Superelasticity is a characteristic thermomechanical property in shape memory alloys (SMA), which is due to a reversible stress-induced martensitic transformation. Nano-compression experiments made possible the study of this property in Cu-Al-Ni SMA micropillars, showing an outstanding ultra-high mechanical damping capacity reproducible for thousands of cycles and reliable over the years. This scenario motivated the present work, where a comparative study of the damping capacity on four copper-based SMA: Cu-Al-Ni, Cu-Al-Be, Cu-Al-Ni-Be and Cu-Al-Ni-Ga is approached. For this purpose, [001] oriented single crystal micropillars of comparable dimensions (around 1 mu m in diameter) were milled by focused ion beam technique. All micropillars were cycled up to two hundred superelastic cycles, exhibiting a remarkable reproducibility. The damping capacity was evaluated through the dimensionless loss factor eta, calculated for each superelastic cycle, representing the dissipated energy per cycle and unit of volume. The calculated loss factor was averaged between three micro-pillars of each alloy, obtaining the following results: Cu-Al-Ni eta = 0.20 +/- 0.01; Cu-Al-Be eta = 0.100 +/- 0.006; Cu-Al-Ni-Be eta = 0.072 +/- 0.004 and Cu-Al-Ni-Ga eta = 0.042 +/- 0.002. These four alloys exhibit an intrinsic superelastic damping capacity and offer a wide loss factor band, which constitutes a reference for engineering, since this kind of micro/nano structures can potentially be integrated not only as sensors and actuators but also as dampers in the design of MEMS to improve their reliability. In addition, the study of the dependence of the superelastic loss factor on the diameter of the pillar was approached in the Cu-Al-Ni-Ga alloy, and here we demonstrate that there is a size effect on damping at the nanoscale. This research was supported by the Spanish Ministry of Economy and Competitiveness, MINECO, projects MAT2017-84069P and CONSOLIDER-INGENIO 2010 CSD2009-00013, as well as by the ELKARTEK-CEMAP project from the Industry Department of the Basque Government, and GIU-17/071 from the University of the Basque Country UPV/EHU, Spain. This work made use of the FIB and ICP facilities of the SGIKER from the UPV/EHU. The author V.F. acknowledges the Post-Doctoral Mobility Grant from the CONICET of Argentina, and J.F.G.-C. also acknowledges the Post-Doctoral Grant (ESPDOC18/37) from the UPV/EHU.

Micro and nano pillars of Copper-based shape memory alloys (SMAs) with feature sizes between about 2 mu m and 250 nm are known to exhibit ultra-high mechanical damping due to the nucleation and motion of stress-induced martensite interfaces during superelastic straining. While this behavior could be extremely useful to protect micro electro-mechanical systems (MEMS) against vibrations in aggressive environments, a fundamental question must yet be answered in order to envisage further applications, namely, whether this damping is reproducible and stable over long times and many cycles, or whether the damping is a signal of accumulating damage that could compromise long-term usage. In the present paper this crucial question is answered; we show that micropillar arrays of Cu-Al-Ni SMAs exhibit a completely recoverable and reproducible superelastic response, with an ultra-high damping loss factor eta > 0.1, or even higher for sub-micrometer pillars, eta > 0.2, even after thousands of cycles (>5000) and after long times spanning more than four years. Furthermore, the first high-frequency tests on such nanoscale SMAs show that their superelastic response is very fast and relevant to ultra-high damping even at frequencies as high as 1000 Hz. This paves the way for the design of micro/nano dampers, based on SMAs, to improve the reliability of MEMS in noisy environments. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). This work was supported by the Spanish Ministry of Economy and Competitiveness, MINECO, projects MAT2012-36421, MAT2017-84069P, CONSOLIDER-INGENIO 2010 CSD2009-00013, as well as by the Consolidated Research Group IT-10-310 and the ELKARTEK-ACTIMAT project from the Education and Industry Departments of the Basque Government, Spain, and GIU-17/071 from the University of the Basque Country, UPV/EHU. Co-funding from H2020 REACT Project Grant No 640241 from European Community and EOARD Grant No FA8655-10-1-3074 (USA) are also acknowledged. This work made use of the FIB facilities of the SGIKER from the UPV/EHU. CAS acknowledges the support of the Institute for Soldier Nanotechnologies, funded by the U.S. Army Research Office at MIT, under contract number W911NF-13-D-0001.

In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. The observed effects and the measured strain relaxations are in agreement with the predictions of the model proposed to explain this behavior in previous calorimetric studies. In addition, the thermal expansion coefficients of both martensite and austenite phases were measured. The neutron experiments have allowed a complete description of the strains during martensitic transformation, and the obtained conclusions can be extrapolated to other SMA systems.In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. The...

Among the different Cu-based shape memory alloys, the Cu–Al–Be family exhibits a particular technological interest for intermediate and low temperature applications. In this work we studied the martensitic transformation behavior of Cu–Al–Be single crystalline shape memory alloys with three different concentrations. The influence of four specific thermal treatments of quenching and aging on the transformation temperatures has been studied by differential scanning calorimetry and compared with similar studies by mechanical spectroscopy. The analysis of the influence of thermal treatments on the martensitic transformation behavior allows optimize a treatment useful for all the alloys transforming between 200 K and 400 K, avoiding both stabilization and precipitation. In addition, an anomalous jerky behavior has been reported for the alloy with low transformation temperatures, which is discussed in terms of the possible mechanisms.

The temperature memory effects (TME) in shape memory alloys (SMA), which appear after incomplete reverse martensitic transformations, strongly modify the kinetics of successive heating runs. In the present work, these TME have been studied in Cu–Al–Ni alloys by adiabatic calorimetry and optical microscopy. The influence of the different martensitic crystallographic structures, the size of the martensite variants, the physical state of the sample, as well as the number of previous partial cycles are discussed. New results in powdered samples are compared with those obtained in single crystals. The analysis of the experimental data within the frame of a simple thermodynamic model suggests the presence of a high number of martensite plates, which permits a continuous distribution of the elastic energy throughout the crystal, as a necessary condition for these thermal effects. Their knowledge is of paramount importance to improve the reliability of SMA devices.

The multistage martensitic phase transformation of a polycrystalline NiTi shape-memory alloy (50.3 at. %Ni-49.7 at. % Ti) has been studied by means of calorimetric measurements. After a conventional thermal treatment followed by successive thermal cycles, the initial two-step forward transformation splits into four-overlapping stages. However, the reverse martensitic transformation maintains the initial two-step sequence, usually assigned to the B19'-> R -> B2 transformation. The correlation between the forward and reverse steps has been established by means of selected thermal cycles together with an estimation of their enthalpy and thermal hysteresis. These results have also provided information about the storage of the elastic strain energy and the frictional works associated with the variants' nucleation. Moreover, the study around the forward transformation temperature range by means of uncompleted thermal cycles undoubtedly shows the presence of temperature memory effects in both stages. This work has been supported by the Spanish Ministry of Economy and Competitiveness, MINECO, CONSOLIDER-INGENIO 2010 CSD2009-00013, as well as by the Consolidated Research Group IT-1090-16 and the ELKARTEK-ACTIMAT project from the Education and Industry Departments of the Basque Government. The University of the Basque Country has also supported this work with the Research Group grant: UPV/EHU GIU17/071. The authors appreciate the cooperation of J. Rodriguez-Aseguinolaza in the thermal treatments of the samples.

A single crystal with a selected composition of a Cu–Al–Ni shape memory alloy, undergoing two martensitic phase transformations when cooling from the common parent phase, β 3 → β 3 ′ + γ 3 ′ , has been studied by adiabatic calorimetry. As the coexistence of two different martensites has a strong influence on the transformation kinetics, both the forward and the reverse phase martensitic transformation have been carefully studied by means of very low thermal rate dynamic thermograms. Temperature memory effects have been observed in the β 3 ′ transformation but not in the γ 3 ′ one. Finally, radiation-cooling procedures were used to study the forward transformation and showed the interaction between the two types of martensite variants on the nucleation processes. This last technique was found useful to evaluate the latent heat released at each individual nucleation event, throughout the transformation progress. The consequent sample self-heating alters this process, which dramatically affects the β 3 ′ / γ 3 ′ ratio and the final martensitic state.

The martensitic transformation kinetics during partial cycling and the so-called “hammer” effect has been carefully characterized in Cu–Al–Ni shape memory alloys. These temperature memory effects were measured by adiabatic calorimetry and analyzed within the frame of a new thermodynamic model. A straightforward study of the nucleation processes explains on quantitative grounds the shift of the reverse transformation to higher temperatures and the presence of a secondary Cp peak associated to these phenomena. The optical observations support the calorimetric results. Finally, the release of the elastic energy in the martensitic state due to the different thermal cycles has been determined.

A thermodynamic study, based on high-sensitivity adiabatic calorimetry, of the martensitic transformation undergone by Cu–Al–Ni shape memory alloys is presented. From the specific heat data, the thermodynamic function values, and in particular the crystal free energy, as functions of temperature, have been obtained. These results have permitted a careful estimation of the phase transformation temperatures of each β3′ martensite plate as a function of its stored elastic energy. Within this frame, the distribution density of the elastic energy states in the martensitic phase is directly derived from the specific heat data. It also permits a simple analysis of the nucleation processes and gives a convincing explanation of the temperature memory effects.

The temperature memory effect exhibited by Cu–Al–Ni shape memory alloys was studied by means of adiabatic calorimetry and microscopic observations. The harmonic, anharmonic and electronic contributions to the lattice specific heat were estimated by using the experimental data of the metallic components. The obtained results provide an accurate baseline for the quantitative study of the martensitic phase transformations as a function of the thermal history in these alloys. The specific heat of a Cu–Al–Ni sample was measured from 140 to 350 K throughout the phase transition region, and the temperature memory effect was carefully studied. These results are in good agreement with the optical observations as a function of temperature. The global behaviour of the martensitic transformation as regards the temperature memory effect is discussed and interpreted in terms of the microscopic mechanisms of nucleation and motion of the martensite plates.

We present an accurate study by AC calorimetry on the thermoelastic martensitic transformation in Cu–Al–Ni shape memory alloys, both under quasi-isothermal conditions and by using a dynamic method. The AC calorimetry results suggest that the martensitic transformation exhibits some kinetic effects strongly dependent on the temperature rate, in agreement with internal friction results. Calorimetric results obtained by DSC do not exhibit this behaviour, but dynamic measurements obtained under oscillating fields show a net dependence on the temperature rate.

Adiabatic calorimetry measurements have been performed on Cs2CdBr4 from 90 to 270 K. Phase transitions at 252.03, 237.02, and 154.88 K were found. The thermal expansion coefficients and the elastic constant values were used to separate the harmonic and anharmonic contributions to the specific heat, which permitted us to establish a baseline for the precise determination of the thermodynamic functions associated with the phase transitions. Anharmonic quantities, such as the Gruneisen parameter and the isothermal compressibility, were also calculated. Finally, no experimental evidence of the suggested transitions at 208 and 130 K was found.

The specific heat of urea/n-heptadecane was measured by adiabatic and AC calorimetry from 80 K to 330 K. The experimental curves show that the phase transition undergone by this crystal is characterised by a double peak anomaly centred around T = 158.9 K. The experimental resolution around this temperature has been improved by numerous heating thermograms with rates varying from 0.1 °C/h to 1.9 °C/h. Below 1 °C/h the same double peak is found but, as the heating rate is lowered, the profile of the specific heat curves progressively changes and exhibits two additional small anomalies at T = 158.35 K and T = 158.85 K. Moreover, a small thermal hysteresis (

An AC calorimeter ranging from 4.2 to 400 K has been built and optimized in our laboratory for the study of solid–solid phase transitions. Only a small amount of sample is required (with typical dimensions of (2–6) mm ×(2–4) mm ×(0.2–0.5) mm and mass around 1–25 mg) and different types of solids such as insulator crystals or metal samples can be studied. Small-diameter thermocouples (25 μm) are used as temperature sensors and also as sample support. This system can be operated discontinuously with temperature jumps as small as 0.025 K or by continuous heating or cooling with scanning rates from 0.06 up to 6 K/min. Both procedures are governed by an automatic control program and data acquisition which provides very low dispersion of the C p points (0.1%). The whole experimental set up was tested by new measurements of the triglycine sulphate (TGS) specific heat. Finally, the ferroelectric phase transition in the TGS is studied within the frame of the Landau theory. With this model, the specific heat is correctly described down to 15°C below the transition temperature leading to accurate calculations of the phase transition thermodynamic functions.

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH3)4]2MnBr4 and [N(CH3)4]2ZnBr4 are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.

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 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.

Some examples of calorimetric measurements in several crystals containing at least one tetramethylammonium unit in compounds of (N(CH3)4)2BX4 and (N(CH3)4)BX3 families are presented. They show the possibilities of using adiabatic calorimetry in the precise characterization of complex phase transition sequences, as well as accurate calculations of the phase transition thermodynamic functions and the specific heat behaviour around the critical temperatures. In addition, complementary experimental information such as spectroscopic, elastic and thermal expansion data establish the different contributions to the specific heat in a crystal. In particular, anharmonic quantities, such as the isothermal compressibility and the Gruneisen parameter, can be calculated. Finally, the suitability of phenomenological models for a good description of the various phase transitions will be briefly discussed.

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.

Calorimetric measurements of Cs2ZnI4 showed four distinct phase transitions at 117, 108, 104 and 94 K. The shape of the observed specific heat peaks indicates that those at 108 and 94 K are first order in nature. This sequence is in agreement with previous results using different experimental techniques, and supports the hypothesis that the phase transition sequence of Cs2ZnI4 is the same as that of Cs2HgBr4 and can be written as Pnma (Z=4) 2nd order to T1=117 K incommensurate 1st order to T2=108 K ferroelastic P21/n (Z=8) 2nd order to T3=104 K ferroelastic P1 1st order to T4=94 K ferroelastic P1.

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 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 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.

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.

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.

Raman scattering and calorimetric measurements are used to study the ferro-paraelastic phase transition undergone by [N(CH3)4]2ZnBr4 at 287.19 K. The knowledge of its vibrational spectrum, together with the available elastic and thermal expansion data, allow to obtain the specific heat of the phase transition and the associated values for the thermodynamic functions. The transition entropy (ΔS = 1.95 R ± 0.1) confirms the order-disorder character assigned to this phase transition. Finally, the temperature dependence of the thermodynamic functions is discussed within the frame of the Landau theory. Ramanstreuung und kalorimetrische Messungen werden benutzt, um den ferro-paraelastischen Phasenubergang von [N(CH3)4]2ZnBr4 bei 287,19 K zu untersuchen. Die Kenntnis seines Schwingungsspektrums zusammen mit den zur Verfugung stehenden elastischen Daten und Werten der thermischen Ausdehnung, erlaubt, die spezifische Warme des Phasenubergangs und die verknupften Werte fur die thermodynamischen Funktionen zu erhalten. Die Ubergangsentropie (ΔS = 1,95 R ± 0,1) bestatigt den Ordnungs-Fehlordnungscharakter, der mit diesem Phasenubergang verknupft ist. Schlieslich wird im Rahmen der Landau-Theorie die Temperaturabhangigkeit der thermodynamischen Funktionen diskutiert.

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.

An experimental study of the ferroelastic phase transition in urea/n-heptadecane CO(NH 2 ) 2 /C 17 H 40 composite around the structural phase transition undergone by this crystal at 159 K is presented. The temperature dependence of the macroscopic spontaneous strain and the optical birefringence around this temperature has been determined. A phenomenological model limited to the hexagonal-orthorhombic change of the urea sublattice leads to a linear relation between these quantities and the phase transition entropy. The experimental data agree with this description exception made of the near vicinity of the phase transition, where the influence of the alkane chain ordering processes cannot be excluded.

The phase-transition sequence undergone by [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{ZnI}}_{4}$ has been studied by adiabatic calorimetry. The specific-heat results show the P${112}_{1}$/a-P${2}_{1}$ca phase transformation around 215 K to take place in two independent steps. The corresponding specific-heat anomalies differ by only \ensuremath{\approxeq}0.4 \ifmmode^\circ\else\textdegree\fi{}C on heating, and \ensuremath{\approxeq}3 \ifmmode^\circ\else\textdegree\fi{}C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appearance of the two specific-heat peaks. This fact has been unambiguously confirmed by means of an exhaustive experimental specific-heat study and by the use of the thermodynamic-functions values associated with this anomalous phase transformation.

A calorimetric study of urea/n-nonadecane, CO(NH(2))(2)/C(19)H(40), and the deuterated derivatives, CO(ND(2))(2)/C(19)D(40) and CO(NH(2))(2)/C(19)D(40), around the structural phase transition temperature is presented. For this purpose differential scanning (DSC), temperature-modulated (AC) and adiabatic calorimetry have been used and the obtained results are compared. Leaving apart the noticeable peak associated with the main phase transition at 158.5, 149.4 and 154 K respectively, small anomalies of the specific heat are found at lower temperatures and their corresponding entropic and enthalpic changes are reported. Heating and cooling experiments show the influence of the temperature rate and the thermal history on the detailed profile of the specific heat traces. The presence of thermal hysteresis and latent heat as a way to characterize the order of the phase transitions is discussed. Finally, a tentative approach to the urea and the alkyl chain contributions to the specific heat and their influence on the phase transition mechanisms is presented.

Specific heat measurements performed on the ferroelectric (N(CH3)4)2ZnCl4 reveal that the phase transition occurring in this compound can be classified into two groups. The commensurate to incommensurate to ferroelectric to paraelectric sequence belongs to the first group, whereas in the second group, for which a new anomaly was detected, the authors have three phase transitions associated with the organic group movements. The critical behaviour for the commensurate-incommensurate phase transition is discussed.

The prototype-incommensurate-ferroelectric-ferroelastic phase transition sequence in (N(CH3)4)2CuBr4 is studied by adiabatic calorimetry in the temperature range 60 to 343 K. The thermodynamic function values assigned to the successive phase transformations permit a comparative study with other tetramethylammonium tetrahalogenmetallates. The enthalpies involved in their phase transition sequences can be related to the packing of the crystal unit cell and to the position of each compound in a common pressure-temperature phase diagram. In the case of (N(CH3)4)2‥CuBr4, this diagram is correctly described by a simple phenomenological model. A brief discussion on the critical exponents associated to the specific heat (α = α′ = 0.01 for ‖T − T0‖ < 0.2 K) and to the order parameter, for the prototype-incommensurate phase transition, is also included. La sequence des transitions de phase prototype-incommensurable-ferroelectrique-ferroelastique dans le cristal (N(CH3)4)2CuBr4 a ete etudie par calorimetrie adiabatique dans l'intervalle de temperature 60 a 343 K. Les valeurs des fonctions thermodynamiques associees aux successives transformations de phase ont permis un etude comparatif avec des autres tetramethylammonium tetrahalogenmetalates. Les enthalpies associees aux transitions de phase sont relationees avec la densite de la maille unite aussi bien qu'avec la position de chaque compose dans le diagramme des phases pressure-temperature commun. Dans le cas de (N(CH3)4)2CuBr4, ce diagramme est decrit convenablement par une modele phenomenologique tres simple. Une bref discussion sur les exposants critiques associes a la chaleur specifique (α = α′ = 0,01) et au parametre d'ordre dans la transition de phase prototype-incommensurable est aussi presente.

A complex phase transition sequence was found in (C3H7NH3)2CuCl4 crystals. The specific heat Cp, thermal expansion Delta d/d0 and linear birefringence Delta b have been used to study this sequence. By comparison with the isomorphous manganese compound, two unusual features can be identified: a re-entrant phase ( gamma -phase) and a lock-in phase transition ( epsilon - zeta ) with a flipping modulation vector. The experimental results are discussed within the framework of the Landau theory.

We present the specific heat measurements performed on (N(CH3)4)2FeCl4 from 60K to 350K by Adiabatic Calorimetry. Phase transition temperatures as well as the corresponding values of the thermodynamic functions have also been determined. In addition, experimental results on the relaxation times as a function of temperature reveal a new and simple technique to study the evolution of the sample thermal conductivity around solid-solid phase transitions.

Accurate specific heat measurements on tetramethylammonium tetrachloroferrate (III) have been performed by means of Adiabatic Calorimetry in the temperature range 60–350K. Continuous heating and cooling methods as well as the conventional pulse technique have been used. Complementary DCS measurements were also carried out. We have found calorimetric evidence of phase transitions at 236.1K, 291K, 307K, 347K and 381K (on heating). The associated thermodynamic functions have been calculated from the specific heat curve by numerical integration. Finally, results on the phase transition hystheresis as well as on the sample thermal relaxation are also presented.