Ideal Associated Solution Model Research Articles

THERMODYNAMIC PROPERTIES OF MELTS OF THE Eu-Ge AND Al-Eu-Ge SYSTEMS

The thermochemical properties of the melts of the Eu–Ge and Al–Eu–Ge systems at 1200–1400 K were determined for the first time by the method of calorimetry. It was established that the minimum value of the enthalpy of mixing of the melts of the Eu–Ge system is -49.1±4.4 for an alloy with xGe = 0.45, ΔH̅∞Eu = -145.7 ± 22.3; ΔH̅∞Ge = −166.8 ± 19.8 kJ/mole at T = 1400 K. The curve of enthalpy of mixing of the studied melts is almost symmetrical, which correlates with the behavior of alloys of this and similar systems in the solid state. The obtained data make it possible to consider the entire spectrum of Ge–Ln systems (lanthanides) and to explain what determines the thermodynamic properties of melts of the Eu–Ge system, in particular, and Ge–Ln, in general. Using the defined thermochemical properties of melts and the known phase diagram of the Eu–Ge system, the Gibbs energy, enthalpy, and entropy of formation of melts, associates in melts, and compounds of the Eu–Ge system were optimized and calculated according to the ideal associated solution (IAS) model. The calculated activities of the components in the melts of this system show large negative deviations from ideal solutions. Using the obtained thermochemical data, the temperatureconcentration dependences of Gibbs energies, enthalpies and entropies of the formation of melts and intermetallics, and the coordinates of the liquidus curve of the state diagram of the Eu–Ge system were also calculated according to the IAS model. The calculated liquidus curve and the one known from experimental data do not fully agree with each other. Comparison of ΔHmin of melts with enthalpies of formation, temperatures of melting Ln5Ge3 compounds, and differences in molar volumes and electronegativities of components of Ge–Ln systems depending on the serial number of Ln showed that they are correlated with each other, and ΔHmin of melts and Ln5Ge3 compounds differ little from each other. In addition, all dependences, with the exception of Δχ, are symbiotic and monotonic, with the exception of compounds and melts of binary Ge–Eu(Yb) systems. It was established that the integral and partial enthalpies of mixing of melts of radial sections with xEu/xGe = 0.85/0.15 and 0.3/0.7 of the Al–Eu–Ge system at 1400 K are exothermic. When aluminum is added to GexEu1-x melts, the thermal effect of its dissolution in the first section initially decreases, and then it increases, and in the second – it increases. This is due to the breaking of strong Eu–Ge bonds and the formation of lower energy bonds between Al and Eu. The thermodynamic properties of melts of the Al–Eu–Ge system were calculated using the “geometric” models and the Redlich-Kister-Mujianu model based on similar data for limiting binary systems. It is shown that those calculated according to the Redlich-Kister-Mujianu model with the ternary contribution L = -220 kJ/mole are in the best agreement with the experimental data. Therefore, the same model was used to calculate other thermodynamic properties of melts of the Al–Eu–Ge system.

Дослідження і моделювання термодинамічних властивостей розплавів систем Bi—Cu—Eu і Cu—Eu

The partial and integral enthalpies of mixing of the melts of the Bi—Cu—Eu system were determined for the first time by the method of calorimetry on three radial sections with a constant ratio of two components: xCu/xEu = 0,3/0,7, xEu/xBi = 0,23/0,77, xEu/xBi = = 0,8/0,2 at T = 1400 ± 1 K. It is shown that the partial and integral enthalpies of mixing of the studied melts are mainly exothermic. Moreover, when adding bismuth to CuхEu1-х melt, the thermal effect of dissolution of the latter increases. This is due to the formation of strong bonds between Bi and Eu. In the other two sections, the opposite happens. Using the literature enthalpies of mixing of melts of the Cu—Eu system, investigated by the method of calorimetry at 1313—1480 K in the entire range of compositions, Gibbs energies, enthalpies and entropies of the formation of melts and intermetallics, their temperature-concentration dependences, and from them — the liquidus curve of the state diagram were calculated of the studied system according to the model of the ideal associated solution. It is shown that the activities of the components in these melts exhibit small negative deviations from ideal solutions and a small amount of associates, especially Eu5Cu, is formed in them. The maximum mole fraction of the EuCu associate reaches a value of 0,09, and the other two (Eu2Cu, EuCu5) — 0,05 and 0,02. From the critically analyzed thermodynamic properties of melts of the Cu—Eu, Bi—Cu and Bi—Eu systems, their reliable data were derived, from which similar parameters for liquid alloys of the ternary system Bi—Cu—Eu were calculated according to various known models. It is shown that the experimental mixing enthalpies of melts of the Bi—Cu—Eu system best agree with those calculated according to the Redlich—Kister—Mujianu model. It was established that ΔHmin = = −61,5 kJ/mol at xBi = 0,5. According to the same model, G, S of these melts were calculated. It was established that Gmin = –40 kJ/mol, and Smin = –18 J/mol•K, the minima of which also fall on the boundary subsystem Bi—Eu, i.e., the main contribution to the interaction energy between different-named atoms of melts of the Bi—Cu—Eu system contribute to the components of this subsystem. Keywords: calorimetry, melts, intermetallics, thermodynamic properties, systems, Bi—Cu—Eu, Cu—Eu, ideal associated solution model, phase equilibria.

Термодинамічні властивості розплавів систем Cu—Yb і Cu—In—Yb

The partial and integral enthalpies of mixing of the melts the Cu-Yb system in the composition range 0 < xCu <0,7 and for 5 sections of Cu – In – Yb system with a constant ratio of the other two components (xCu/xIn=0,36/0,64; xCu/xIn= 0,62/0,38; xIn/xYb= 0,62/0,38; xCu/xYb =0,64/0,36; xCu/xYb =0,21/0,79) to x3 = 0.3 by method isoperibolical calorimetry in the temperature range 1453-1473K first were determined. Using the model of ideal associated solutions (IAR), all the thermodynamic properties (Gibbs energies of mixing melts, enthalpy and entropy of formation of intermetallic compounds and associates) of the Cu–Yb system were calculated. It turned out that the activity of the components in the melts of this system exhibit small negative deviations from ideal solutions. Calculations using the IAR model also made it possible to establish that with increasing temperature it increases slightly, but more significantly. Also were obtained the coordinates of the liquidus curve of the diagram state of the studied system. The principal contribution to the enthalpies of mixing in the liquid alloysof the Cu–In–Yb system gives the border subsystem In–Yb. Because the minimum of the mixing enthalpies in the ternary sysytem Cu–In–Yb shifts towards the equiatomic alloy concentration of subsystem In–Yb (-36,5±1,0) at Т= 1453 К. Using the experimental partial and integral enthalpies of mixing, the activities of the components in the melts of binary limiteyted system of melts of the Cu–In–Yb ternary system, calculated according to «geometric» and the Redlich-Kister-Mujian models in a wide range of concentrations. It is established that the experimental ∆H, and calculated analogical datas according to the Redlich-Kister-Mujianu model agree between. It is shown that the activities of the components in the melts of this system, calculated according to the Redlich-Kister model, show small negative deviations from ideal solutions at 1453 K. From these of data calculated G, S. It was found that Gmin = –19 kJ/mol, Smin = –15 J/mol*K for the alloy In0,5Yb0,5. This correlates with the determined thermochemical properties of the melts Cu –In– Yb system. Keywords: calorimetry, the melts, intermetallic, thermodynamic properties, Cu, Yb, In, the model of ideal associated solutions, the Redlich—Kister—Mujianu model, phase equilibria.

Термодинамічні властивості і фазові рівноваги в сплавах системи Eu—Pb

Firstthe partial and integral enthalpies of mixing of the melts of the Eu—Pb system were determined at a temperature of 1100—1350 K in everything range the composition by method isoperibolical calorimetry. It was established that of the melts of the Eu—Pb system are formed with the release of a big amount of heat: the minimum H = –51,7  ± 0,8 (at xPb = 0,4). Using the model of ideal associated solutions, all the thermodynamic properties (Gibbs energies, enthalpy and entropy of formation of melts, intermetallic compounds and associates) of the Eu—Pb system were calculated. It turned out that the activity of the components in the melts of this system exhibit moderate negative deviations from ideal solutions. According to the IAR model, the temperature-concentration dependences of the Gibbs energies, enthalpies and entropies formation of melts and intermetallics were calculated, and from them were obtained the coordinates of the liquidus curve of the diagram state of the studied system. As a result, the temperature-concentration dependences of the thermodynamic properties of all phases and the liquidus of the Eu—Pb system are obtained, those a thermodynamic description of this system is made. Keywords: calorimetry, melts, intermetallics, thermodynamic properties, Eu, Pb, model of ideal associated solutions, phase equilibria.

Speciation in Chloroform + Diisopropyl Ether Binaries in the Light of the Ideal Associated Solution Model Using Viscometric Data

Speciation in Chloroform + Diisopropyl Ether Binaries in the Light of the Ideal Associated Solution Model Using Viscometric Data

Glass-forming ability of Ni-Zr-Al alloys: the interplay of thermodynamic and geometric factors

ABSTRACT Despite decades of intensive rehearses, we still lack robust and physically clear methods to discover new metallic alloys with high glass-forming ability (GFA). Different empirical criteria proposed to estimate GFA have limited applicability. Here we address the impact of thermodynamic and geometric factors on GFA of Ni-Zr-Al alloys. Using the ideal associated solution model, we calculated the Gibbs energy and the mixing enthalpy of the system and compared the results with available experimental and theoretical data. The results show that the model used is a good approximation for calculating the thermodynamic properties of liquid alloys if both binary and ternary compounds are explicitly taken into account. However, we conclude that it is hardly possible to address GFA of Ni-Zr-Al alloys using only thermodynamic properties. We test a few different criteria, proposed in the literature, and show that they also fail to explain GFA in the system. Then, we propose new parameters based on Gibbs mixing energy and effective difference in atomic radii, which allows localising the composition domain corresponding to the best GFA in Ni-Zr-Al alloys. Our findings open up the prospects to develop reliable theoretical methods for discovering new metallic glasses.

Prediction and optimization of bulk metallic glass-forming compositions in Zr-Ni-Cu-Al system by thermodynamic modeling

The search for new criteria to predict glass-forming ability (GFA) in multicomponent metallic alloys is an important task from both fundamental and practical points of view. The decades of research have suggested that both thermodynamic and geometric factors contribute essentially to GFA of the alloys. Recently, P HS and P HSS parameters, which combine the enthalpy of chemical mixing, mismatch entropy, and configurational entropy, have been proposed and proved as successful criteria for the prediction of the GFA in several metallic glasses. Here we apply the thermodynamic modeling technique based on the ideal associated solution model to study GFA in (Ni 0.11 Cu 0.19 Zr 0.70 ) 1-x -Al x , (Ni 0.17 Zr 0.55 Cu 0.28 ) 1-x -Al x alloys with high Zr content. Our calculations reveal a clear correlation between P HS , P HSS parameters, and GFA in Ni-Zr-Cu-Al alloys for which bulk metallic glasses were experimentally fabricated. Based on the calculations, we predict that (Ni 0.17 Zr 0.55 Cu 0.28 ) 1-x -Al x alloys at x (Al) ≤ 0.2 possess high GFA.

Thermodynamic Properties and Phase Equilibria in Ba–Sn Alloys

The mixing enthalpies for liquid Ba–Sn alloys were determined by isoperibolic calorimetry at 1300 K over the entire composition range. Large exothermic mixing effects ( ∆Hmin = –54.8 ±± 1.8 kJ/mole at xBa = 0.56) were found. They are indicative of strong interaction between different components and of short ordering in the liquid alloys. The thermochemical properties of the melts and barium stannides and the phase equilibria in the Ba–Sn system were used to calculate the activities of components, molar fractions of associates, and formation enthalpies and entropies for liquid BaxSn1–x alloys with the ideal associated solution (IAS) model. Two associates, BaSn and Ba2Sn, were selected for the calculation. The calculated activities of components in the Ba–Sn melts show very large negative deviations from the ideal solution, which is consistent with their thermochemical properties. The maximum concentration of each associate is approximately 0.65 for respective compositions. The calculated formation enthalpies for BamSnn intermetallics are quite high exothermic values that agree well with the published data for BaSn3 and only qualitatively for Ba2Sn. The formation enthalpy for the associate agrees with that for the respective BaSn intermetallic and is slightly less exothermic for Ba2Sn. This all indicates that the bonding energies between different atoms in the respective intermetallics and melts are close to each other and quite high. The formation entropy for the Ba2Sn associate is lower and that for BaSn is higher in magnitude. This demonstrates different degrees of ordering in BaSn and Ba2Sn associates and intermetallics. This may be due to different changes in the oscillation frequencies of atoms and other factors. The calculated liquidus curve of the Ba–Sn phase diagram agrees with data for the Sn–Sr system.

Analysis of thermodynamic properties of Ca – Si – Fe melt

The model of ideal associated solutions was used for the analysis of thermodynamic properties of the Ca – Si – Fe melt. Chemical equilibrium, as per the law of mass conservation between associates and monomers in the assumed model version, was performed without consideration of mole fractions of these particles in solution but with consideration of the absolute number of their moles. It allows taking account the changes in the associated solution mole composition depending on the concentration of its components. The understudied binary sub-system Ca – Si was analyzed most comprehensively. Using the latest data of temperature dependency of heat capacity for five types of intermetallics of this sub-system, types of stable associates in it were defined, i.e. Са2Si, СаSi in the solution range with low contents of silicon in solution and СаSi, СаSi2 in the solution range with high contents of silicon in solution. Thermodynamic properties of the corresponding intermetallics in the databases Terra, Astra and HSC notably differ from the computed properties of the associates. The reason of disagreement of experimental and reference data consists apparently in the inaccurate reference information based on the previous underestimated studies of intermetallics’ heat capacities. Analysis of mixing energy of Ca – Si alloy components has shown that concentration and temperature dependencies of excessive free energy closely follow the so-called pseudosubregular model of binary solutions. Only two types of stable associates were defined for the other sub-system Fe – Si, i.e. Fe3Si and FeSi. On the whole, energies of formation of these associates and respective intermetallics agree well. The third sub-system Ca – Fe was not considered because of the very limited mutual solubility of its components. Thus, only three associates, i.e. CaSi, CaSi2 , FeSi, are valid out of five possible in the triple system Ca – Si – Fe in the range with high concentrations of silicon. A calculation under this condition of thermodynamic properties of calcium silicon melts for CK10 – CK30 grades has shown that activity of silicon in them at temperature 1873 K constituted 0.6 – 0.7, whereas activities of other components do not exceed 0.01.

Modelling thermodynamic properties of binary Cu–Eu and ternary Al–Cu–Eu melts

Model calculations of the whole set of thermodynamic properties of liquid alloys for the binary Cu–Eu and ternary Al–Cu–Eu systems have been performed. Authors used the ideal associated solution model (IAS model) for calculation of the entropies and excess Gibbs energies of mixing for these systems. The binaries were given as the Redlich-Kister polynomials. The thermodynamic properties for the ternary system are described using the Redlich-Kister-Muggianu formalism. A comparison of the surfaces of excess Gibbs energy and entropy of mixing for liquid Al–Cu–Eu alloys at 1350 K demonstrates that the ordering related to the formation of rather strong associates in the Al–Eu system significantly affects the concentration dependence of the excess Gibbs energy of mixing in the liquid phase at this temperature.

Thermodynamic Properties and Phase Equilibria of Nd–Ni Alloys

Isoperibolic calorimetry has been used for the first time to determine the mixing enthalpies of binary Nd–Ni liquid alloys in the range 0 < xNi < 0.5 at 1733 K and range 0.55< xNi < 1 at 1773 K. The binary Nd–Ni melts are characterized by significant negative mixing enthalpies with a minimum of –33.9 ± 0.8 kJ/mole at xNi = 0.63 and 1750 K. In the temperature range studied (1733–1773 K), the mixing enthalpies of the melts are described by polynomial ΔH = xNi (1 – xNi )× × (–75.32 – 103.41 xNi + 273.45$$ {x}_{Ni}^2 $$ – 817.02$$ {x}_{Ni}^3 $$ +548.58$$ {x}_{Ni}^4 $$). The ideal associated solution (IAS) model was used to calculate the activities of components, molar fractions of associates, Gibbs energies, and mixing entropies using our ΔH and $$ \varDelta \overline{H}i $$ and literature data on the formation enthalpy of nickelides and the Nd–Ni phase diagram. All intermediate phases were considered stoichiometric, with zero excess heat capacity. Five associates were selected for the calculation: Nd2Ni, NdNi, NdNi2, NdNi3, and NdNi5. Most of them agree in composition with the intermetallides in this system. Only one of them does not exist in solid state. This is associate Nd2Ni, whose composition is close to that of the Nd7Ni3 intermetallide. The activities of the Nd–Ni melt components show high negative deviations from ideal solutions. Associates of simplest composition, NdNi and NdNi2, are predominant in the melts. The mixing entropies of Nd–Ni liquid alloys are negative (minimum value is close to –6.4 J/mole · K). All our thermodynamic properties indicate that there is strong energy of interaction between Ni and Nd. The liquidus curves of the Nd–Ni system were calculated using the IAS model parameters.

Thermodynamic Properties of Ni–Sb Alloys

The thermochemical properties of Ni-Sb alloys at 1241, 1363, and 1457 K in the range 0 <= x(Ni) <= 0.5 were studied by calorimetry. The ideal associated solution (IAS) model was employed to calculate the thermodynamic properties of Ni-Sb intermetallics and melts. The minimum mixing enthalpy is - 26.9 kJ/mole at x(Ni) = 0.58 and agrees with the values for Sb-3d-metal alloys. Our and literature data were used to predict the formation enthalpies of melts and intermetallics in the binary Sb-3dtransition metal systems.

Chemical interaction, thermodynamics and glass-forming ability of Cu-Zr-Al melts

Using the ideal associated solution model, we analyze the impact of chemical interaction on thermodynamic properties and glass –forming ability of Cu-Zr-Al alloys. For two sections Cu0.33Zr0.67-Al and Cu0.5Zr0.5-Al, we calculate activities and thermodynamic functions of mixing in liquid state taking into account both binary and ternary chemical complexes. Our calculations relate thermodynamic properties of the melts with their chemical local order characteristics. The analysis of the results reveals that composition range corresponding to high glass-forming ability of Cu0.5Zr0.5-Al alloys correlates strongly with the range where CuZr associate dominates in the melt.

МОДЕЛЮВАННЯ ТЕРМОДИНАМІЧНИХ ВЛАСТИВОСТЕЙ РОЗПЛАВІВ ПОТРІЙНОЇ СИСТЕМИ Ge-Mn-Gd

In the present work, the Gibbs energies of mixing of liquid alloys of the Ge-Mn-Gd ternary system were determined, which was made on the basis of an analysis of published data on the thermodynamic properties of liquid alloys of boundary binary systems that form the ternary Ge-Mn-Gd, as well as on the basis of the model calculations in these binary systems. To determine the activities of the components, the Gibbs energies of mixing, and the enthalpies of mixing of liquid alloys of the Ge-Mn(Gd) systems, for which alloying process is accompanied by significant heat release, an ideal associated solution model was applied. For the melts of the Mn-Gd system, which are characterized by rather insignificant exothermic effects, a model of regular solutions was used. The surface of the Gibbs energy of mixing for the alloys of the Ge-Mn-Gd ternary system has been determined on the basis of the concentration dependences of the Gibbs energies of mixing obtained for constituent binary systems under the assumption of additivity of pair interactions using the Redlich-Kister-Muggianu method. The obtained topology of the Gibbs energy isolines projections is compared with the thermochemical properties of liquid alloys of this system that we have determined earlier. A comparative analysis of the topology of these surfaces in the Ge-Mn-Gd system led to the conclusion that the surfaces of ΔG and ΔmH monotonically decrease from the manganese-rich angle of the diagram towards the Ge-Gd side of the concentration triangle. The minimum value of the thermodynamic characteristics of mixing of the ternary liquid alloys corresponds to the composition, which coincides with the composition of the most stable intermetallic compound in the Ge-Gd system. From the course of isolines of free energies and integral enthalpies of mixing, one can also conclude about the influence of a short-range order, existed in the Ge-Mn system near the composition with a mole fraction of mangan greater than 0.7, on the properties of ternary alloys in the vicinity of this composition. Thus, the topology of isolines and the large exothermic values of the obtained thermodynamic properties allow us to make a reasonable conclusion that the strong interaction between unlike components inherent in the Ge-Gd system in the solid state is also maintained for liquid alloys of the Ge-Mn-Gd system.

The Thermodynamic Properties and Phase Equilibria in Ce–Sn Alloys

The thermodynamic properties of Ce–Sn alloys have been determined by calorimetry at 1500–1650 K in the composition range 0 ≤ xS ≤ 0.2 and calculated using the ideal associated solution model. The minimum mixing enthalpy is found to be –67.2 ± 0.5 kJ/mol at xS = 0.45, and the activities of components exhibit very large negative deviations from the ideal solution.

Chromous capacities of ferrochromium and matte smelting slags

Purpose: of this paper is to extend the slag capacity concept to sparingly soluble cationic species encountered for example in matte smelting and ferrochromium processing in terms of chromous capacities. Design/methodology/approach: In this context in order to develop the chromous capacity the experimental data on slag-metal equilibrium for ferrochromium smelting and on chromium distribution between slag and matte phases encountered in sulphide smelting of PGM (Platinum Group Metals) and chromium containing copper nickel concentrates were used. Findings: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. These values indicate the very low and moderate chromous ion dissolving ability of ferrochromium smelting slags and matte smelting slags respectively. Research limitations/implications: Due to highly reducing conditions in ferrochromium smelting and the imposed low oxygen partial pressures in sulphide smelting of chromium and PGM containing concentrates chromium in the slag was assumed to be in its divalent state. The slag-metal/matte reaction for dissolution of chromium into the slag as a cation under reducing conditions -in contrast to reactions for dissolution of species such as sulphur and phosphorus into the slag as anions under oxidizing conditions – required the release of electrons. Thus to maintain charge neutrality the dissolution of chromium from the metal or the matte phases into the slag as Cr2+ ions was accompanied by consumption of these electrons by reduction of Fe2+ ions in the slag into metallic Fe which dissolved in the metal/ matte phase. This was supported by the experimental equilibrium data. Hence the slagmetal/ matte reaction defining the chromous capacity involved both O2- and Fe2+ ions of the slag phase. Moreover for matte smelting the thermodynamic activities of species in the matte phase were assumed to conform to ideal associated solution model due to lack of data. For future work it is advisable to determine such activities. Practical implications: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. The results clearly indicate that in matte smelting lower basicity slags would be preferable to render chromium more soluble in the slag in contrast to ferrochromium smelting where higher basicity slags would be suitable to minimize chromium losses to the slag. Originality/value: This research and hence the paper is probably the first on dissolution of chromium as chromous ion in slags, thus it is novel and original.

Analysis of the Glass-Forming Ability of Fe–Er Alloys, Based on Thermodynamic Modeling

The Fe–Er phase diagram and thermodynamic properties of all its phases are assessed by means of self-consistent analysis. To refine the data on phase equilibria in the Fe–Er system, an investigation is performed in the 10–40 at % range of Er concentrations. The temperature–concentration dependences of the thermodynamic properties of a melt are presented using the model of ideal associated solutions. Thermodynamic parameters of each phase are obtained, and the calculated results are in agreement with available experimental data. The correlation between the thermodynamic properties of liquid Fe–Er alloys and their tendency toward amorphization are studied. It is shown that compositions of amorphous alloys prepared by melt quenching coincide with the ranges of concentration with the predominance of Fe3Er and FeEr2 associative groups that have large negative entropies of formation.

Thermodynamic Properties of Alloys of the Sn–Yb System

Thermochemical properties of liquid alloys of the Sn–Yb system are studied via calorimetry at 1110–1310 K over the ranges of concentrations 0 < x < 0.47 and 0.82 < x < 1. Considerable negative heat effects of the mixing of these alloys are identified. Activities of the components, the Gibbs energies and entropies of mixing, and the liquidus curve of the phase diagram of this system are calculated using the ideal associated solution (IAS) model and agree with data from the literature.

Thermochemical Properties of Binary Ba–In Alloys

The thermochemical properties of binary Ba–In melts were studied using calorimetry at 1070–1320 K over a wide composition range. It is shown that these melts are characterized by significant exothermic heat effects of mixing. The ideal associated solution model was used to calculate the activities of components, Gibbs energies, and mixing entropies of the alloys and to construct the liquidus curve in the phase diagram. These parameters agree with the literature data.

Thermodynamic characteristics and phase equilibria in the alloys of the Ge–La system

Mixing enthalpies of melts of the Ge–La system have been measured using isoperibolic calorimetry within two concentration ranges. For the first range (0 < x < 0.16 at 1520 K and 0.16 < x < 0.29 at 1570 K), agreement with the known literature data is observed within the experimental error. The second range (0.78 < x < 1 at 1470 K and 0.7 < x < 0.78 at 1580 K) has been studied for the first time. The melts are characterized by very strong exothermal effects of mixing, which have almost symmetrical concentration dependence: ΔH = ΔH = −245 kJ/mol at 1470 K. A thermodynamic optimization of the activities of the components and the phase diagram of the system have been conducted based on the obtained experimental data, using an ideal associated solution (IAS) model.