Leszek Rycerz

Enthalpies of phase transition in the lanthanide chlorides LaCl3, CeCl3, PrCl3, NdCl3, GdCl3, DyCl3, ErCl3 and TmCl3

Abstract The enthalpies of phase transition in the lanthanide chlorides have been measured with a differential scanning calorimeter and also with a Calvet-type microcalorimeter to within an experimental error of ±2%. The molar enthalpies of fusion of LaCl 3 , CeCl 3 , PrCl 3 , NdCl 3 , GdCl 3 , DyCl 3 , ErCl 3 and TmCl 3 are 55.7, 55.5, 52.1, 48.1, 40.6, 22.8, 31.1 and 35.6 kJ mol −1 respectively. The existence of a solid-solid phase transition was found in DyCl 3 and ErCl 3 at 611 and 1025 K with a molar enthalpy of transition of 1.4 and 5.3 kJ mol −1 respectively. A significant difference (8.5 kJ mol −1 ) between the absolute values of molar enthalpies of crystallization and fusion was found for LaCl 3 at 1124±3 K; this unusual behaviour was explained by an “after-fusion” effect. The entropies of fusion are discussed in terms of structural types of the lanthanide chlorides.

Heat capacity of LaCl3, CeCl3, PrCl3, NdCl3, GdCl3, DyCl3

Abstract Heat capacities of LaCl3, CeCl3, PrCl3, NdCl3, GdCl3, DyCl3 have been measured by differential scanning calorimetry in the temperature range from 300 K to melting temperatures of the compounds. Heat capacities of liquid PrCl3, NdCl3, GdCl3, DyCl3 have also been registered. These results have been compared with literature data and fitted by a linear temperature dependence. The temperature coefficients have been given.

Calorimetric investigation of NdCl3MCl liquid mixtures (where M is Na, K, Rb, Cs)

Abstract The molar enthalpies of mixing ΔmixHm in the liquid NaClNdCl3, KClNdCl3, RbClNdCl3 and CsClNdCl3 binary systems have been measured over the whole composition range at 1124, 1065, 1122 and 1122 K, respectively, with an accuracy of about 6%. The apparatus used was a Calvet-type high-temperature microcalorimeter and mixing of the two liquid components was obtained by the break-off ampoule technique. In the investigated systems, the enthalpies of mixing are negative with minimum values at about −5.7, −16.6, −20.2 and −23.4 kJ mol−1, respectively, at χNdCl3 = 0.4. These values are almost identical, within the experimental accuracy, to those in other previously investigated MClLnCl3 mixtures. The molar enthalpies of formation ΔfHm(K3NdCl6, 1, 1065)/kJ mol−1, ΔfHm(Rb3NdCl6, 1, 1122)/kJ mol−1 and ΔfHm(Cs3NdCl6, 1, 1122)/kJ mol−1, according to the reaction 3 MCI(1) + LnCl3(1)= M3LnCl6(1), are equal to −55.2, −68.8 and −80.8 kJ mol−1, respectively. The least-squares coefficients A, B, C, D, E of the equation for the interaction parameter λ= A + Bχ + Cχ2 + Dχ3 + Eχ4χ4 (in kJ mol−1) are presented.

Practical remarks concerning phase diagrams determination on the basis of differential scanning calorimetry measurements

Phase diagrams of binary systems at constant pressure are representations of oneand two-phase regions with their boundaries being functions of temperature and concentration. The most popular techniques used in determination of phase diagrams are thermal analysis (TA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The first of them, based on recording of cooling curves, has no significant meaning nowadays; however, it is still used, especially in didactics. Actually DTA and DSC are widely used in phase diagrams determination. DSC has an advantage over DTA, because in addition to temperature it gives precise value of enthalpy of thermal effect. Two types of DSCs must be distinguished: the heat flux DSC and the power compensation DSC. The characteristic feature of all DSC measuring systems is the twin-type design and the direct in-difference connection of the two measuring systems which are of the same kind. It is the decisive advantage of the differential principle that, in first approximation, disturbances such as temperature variations in the environment of the measuring system and the like, affect the two measuring systems in the same way and are compensated when the difference between the individual signals is formed [1]. The differential signal is the essential characteristic of each DSC. Another characteristic—which distinguishes it from most classic calorimeters—is the dynamic mode of operation. The DSC can be heated or cooled at a preset heating or cooling rate. A characteristic common to both types of DSC is that the measured signal is proportional to a heat flow rate (in opposition to classical calorimeters where heat flow is measured). This fact—directly measured heat flow rates—enables the DSC to solve problems arising in many fields of application [1]. In the heat flux DSC a defined exchange of the heat to be measured takes place via a thermal resistance. The measurement signal is the temperature difference; it describes the intensity of the exchange and is proportional to the heat flow rate. There are two main types of the heat flux DSC: the disc-type measuring system with solid sample support (disc) and the cylinder-type measuring system with integrated sample cavities. Heat flux DSCs with a disctype measuring system are available for temperatures between -190 and 1,500 C [1]. In the heat flux DSC with a cylindertype measuring system, the outer surfaces of each sample container are in contact with a great number of thermocouples connected in a series between the container and furnace cavity. The thermocouples bands or wires are the dominating heat conduction path from the furnace to samples. Both sample containers are thermally decoupled; heat exchange takes place only with parts of the massive furnace. These apparatuses are available for temperature range between -190 and 1,500 C [1]. The power compensation DSC belongs to the class of heat-compensating calorimeters. The heat to be measured is compensated with electric energy, by increasing or decreasing an adjustable Joule’s effect. The measuring temperature range extends from -175 to 725 C [1]. Differential scanning calorimetry is a relative technique. Because of its dynamic temperature characteristics, the measurements are not made in thermal equilibrium. The relative data must be converted to absolute values by a calibration procedure requiring the employment of standards whose property values and their associated uncertainties are known and established following a metrological procedure [2]. Practical remarks concerning phase diagrams determination on the basis of DSC measurements are illustrated by numerous examples of binary lanthanide halide–alkali halide systems.

Entropies of phase transitions in the M3LnCl6 compounds (MK, Rb, Cs; LnLa, Ce, Pr, Nd) and K2LaCl5

Abstract The molar enthalpies of the solid-solid and solid-liquid phase transitions were determined by differential scanning calorimetry for the compounds K2LaCl5, Rb3LaCl6, Cs3LaCl6, K3CeCl6, Rb3CeCl6, Cs3CeCl6, K3PrCl6, Rb3PrCl6, Cs3PrCl6, K3NdCl6, Rb3NdCl6 and Cs3NdCl6. The K3LnCl6 (LnCe, Pr, Nd) and Rb3LaCl6 compounds, which do not exist at room temperature, are formed at high temperature with a considerable loss of enthalpy (48–55 kJ mol−1) compensated by a high gain in entropy (63–65 J mol−1 K−1). The Cs3LnCl6 (LnLa, Ce, Pr, Nd) compounds are stable at ambient temperature; they undergo a transition in the solid state with a corresponding enthalpy and entropy of 7.4–7.8 kJ mol−1 and 10.9–11.2 J mol−1 K−1. The calorimetric measurements show that Rb3CeCl6, Rb3PrCl6 and Rb3NdCl6 are also probably stable at room temperature. The enthalpies and entropies of fusion are reported for all the compounds mentioned above.

Formation enthalpies of the MBrLaBr3 liquid mixtures (M = Li, Na, K, Rb, Cs)

Abstract Molar enthalpies of mixing of the LiBrLaBr 3 , NaBrLaBr 3 , KBrLaBr 3 , RbBrLaBr 3 and CsBrLaBr 3 systems have been measured with a Calvet-type high-temperature microcalorimeter. The enthalpies decrease gradually from −0.67 kJ mol −1 for the lithium system through −4.32, −10.73 and −14.97 kJ mol −1 for the sodium, potassium, and rubidium systems to −16.81 kJ mol −1 for that of cesium at the composition of the 3 MBr·LaBr 3 compound. The results have been discussed in terms of the conformal solution theory of Davis as well as in terms of relative ionic potentials.

Thermal and conductometric studies of NdBr3 and NdBr3-LiBr binary system

The heat capacity of solid NdBr3 was measured by Differential Scanning Calorimetry in the temperature range from 300 K up to the melting temperature. The heat capacity of liquid NdBr3 was also determined. These results were least-squares fitted to a temperature polynome. The melting enthalpy of NdBr3 was measured separately. DSC was used also to study phase equilibrium in the NdBr3-LiBr system. The results obtained provided a basis for constructing the phase diagram of the system under investigation. It represents a typical example of simple eutectic system. The eutectic composition, x(NdBr3)=0.278, was obtained from the Tamman construction. This eutectic mixture melts at 678 K. The electrical conductivity of NdBr3-LiBr liquid mixtures and of pure components was measured down to temperatures below solidification. Reflectance spectra of the pure components and their solid mixtures (after homogenisation in the liquid state) with different composition were recorded in order to confirm the reliability of the constructed phase diagram.

Enthalpies of Phase Transitions and Heat Capacity of TbCl3 and Compounds Formed in TbCl3–MCl Systems (M=K, Rb, Cs)

The molar enthalpies of the solid–solid and solid–liquid phase transitions were determined by differential scanning calorimetry for pure TbCl3 and KTb2Cl7, RbTb2Cl7, CsTb2Cl7, K3TbCl6, Rb3TbCl6 and Cs3TbCl6 compounds. Both types of compounds, i.e. M3TbCl6 and MTb2Cl7 (M=K, Rb, Cs) melt congruently and show additionally a solid–solid phase transition with a corresponding enthalpy ΔtrsH0 of 6.1, 7.6 and 7.0 kJ mol–1 for potassium, rubidium and caesium M3TbCl6 compounds andΔtrsH0 of 17.1 (rubidium) and of 12.1 and 10.9 kJ mol–1 (caesium) for MTb2Cl7 compounds, respectively. The enthalpies of fusion were measured for all the above compounds with the exception of Rb3TbCl6 and Cs3TbCl6. The heat capacities of the solid and liquid compounds have been determined by differential scanning calorimetry (DSC) in the temperature range 300–1100 K. The experimental heat capacity strongly increases in the vicinity of a phase transition, but varies smoothly in the temperature ranges excluding these transformations. Cp data were fitted by an equation, which provided a satisfactory representation up to the temperatures of Cp discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the liquid phase, which had been previously measured. The results obtained agreed satisfactorily with these experimental data.

Enthalpies of mixing in the DyCl3-NaCl, DyCl3-KCl and DyCl3-PrCl3 liquid systems

Abstract The molar enthalpies of mixing ( Δ mix H m ) in the DyCl 3 -NaCl, DyCl 3 -KCl and DyCl 3 -PrCl 3 liquid binary systems were measured at temperature of 1100, 1070 and 1074 K respectively over the whole composition range under argon at atmospheric pressure. The apparatus used was a Calvet-type high temperature microcalorimeter and mixing of the two liquid components was achieved by the ampoule break-off technique; the experimental uncertainties were about 6% (DyCl 3 -NaCl, DyCl 3 -KCl) and 10% (DyCl 3 -PrCl 3 ). For the first two systems, the enthalpies of mixing are negative over the whole composition range, with minima at x DyCl 3 ≈ 0.3 of about −8 and −21 kJ mol −1 respectively. In the DyCl 3 -PrCl 3 system the enthalpy-of-mixing values are positive, with a maximum at x DyCl 3 ≈ 0.4 of about 760 J mol −1 . The least-squares coefficients A , B , C , and D of the equation for the interaction parameter λ (kJ mol −1 ) = A + Bx + Cx 2 + Dx 3 are presented.

Calorimetric investigation of PrCl3NaCl and PrCl3KCl liquid mixtures

Abstract The present work is part of the thermodynamic research performed on the LnCl 3 MCl systems (where Ln is lanthanide and M is alkali metal). The molar enthalpies of mixing Δ mix H m in the NaClPrCl 3 and KClPrCl 3 liquid binary systems were measured at 1122 K over the whole composition range under argon at atmospheric pressure, with an accuracy of about 6%. The apparatus used was a Calvet-type high-temperature microcalorimeter and mixing of the two liquid components was obtained by the break-off ampoule technique. In the systems, the enthalpies of mixing values are negative, with a minimum value of about −7 and −16 kJ mol −1 for NaClPrCl 3 and KClPrCl 3 , respectively, at χ PrCl 3 ≈Fl 0.4. These values are very similar to those relative to the systems MClLnCl 3 previously investigated. The molar enthalpies of formation Δ f H m (K 3 PrCl 6 , 1, 1122 K)/kJ mol −1 , Δ f H m (Rb 3 PrCl 6 , 1, 1122 K)/kJ mol −1 and Δ f H m (Cs 3 PrCl 6 , 1, 1122 K)/kJ mol −1 , according to the reaction 3MCl(1) + PrCl 3 (1) = M 3 PrCl 6 (1), are equal to −55.9, −66.4, and −80.4 kJ mol −1 respectively, and are almost identical (in the limits of experimental uncertainty) to those of the compounds M 3 LnCl 6 (where M is K, Rb, Cs, and Ln is La, Ce, Nd) previously investigated. The least-squares coefficients A, B, C, D and E of the equation for the interaction parameter λ = A + B χ + C χ 2 + D χ 3 + E χ 4 (in kJ mol −1 are presented.