ИСТИНА

The purpose of this work is to use the "tetrad-effect" phenomenon (It is connected with the 4felectrons of the lanthanide elements (Ln: La-Lu; their atomic numbers are 57-71)) for the analysis, correction and prediction of the thermodynamic data for the lanthanide (Ln). The standard thermodynamic parameters of the binary lanthanide compounds with other elements of the Periodic Table in solid state obey also the tetrad-effect concept [1, 2]. If the tetradeffect is observed to be the structural parameters of isostructural compounds (as unit cell volume or shortest distances between the atoms RE and second element), this phenomenon should also be extended to the other physicochemical properties of solids. If the tetrad-effect of physical and chemical properties is not observed in the isostructural lanthanide compounds, this indicates an experimental inaccuracy in the determination of these properties. Available information on thermodynamic properties of the lanthanide compounds is usually limited due to great experimental difficulties in the study of lanthanide systems, caused by the high reactivity of the lanthanides (especially, light lanthanides) and their instability in the air. Typically, lanthanide alloys decompose into Ln2O3 and free nanoparticles of the second component when coming into contact with atmospheric air. The use of the tetrad-effect concept can facilitate the prediction of the absent thermodynamic data for the lanthanide compounds [1,2]. The most sensitive to tetrad-effect thermodynamic functions of lanthanide compounds are standard entropies and entropies of formation, because they are the most susceptible to the influence of the 4f-electrons of the lanthanides. As an example, we use the tetrad-effect concept for the analysis and prediction of the standard entropies of Ln2X3 (X=O, S, Se, Te), solid phases, but this approach can also be applicable to other classes of the Ln compounds as LnN, LnB2, LnB4, LnB6, LnF3 and other compounds. The tetrad-effect concept gives us the ability to develop a solid state chemistry theory for lanthanide alloys. We have demonstrated that the concept of tetrad effect and symmetric function: a0+a1 |x|+a2 x2 +a4 x4 (ai are the fitting parameters and x=N - NGd, where N is the atomic number of lanthanide) can be used successfully for the analysis and prediction of the standard entropies at 298 K of solid lanthanide compounds. The measured and calculated values of the standard entropies at 298 K for the Ln2O3, Ln2S3, Ln2Se3, Ln2Te3, solid compounds are given.