Lanthanide thermochemical data

Electronegativities, Xp. shown in bold text and rounded to one decimal place, are taken from Allred or from Pauling. The values are based on thermochemical data analysed using Pauling s approach for the elements in their normal oxidation state, viz. for Sc, Y and La, M for Cu. Ag and Au, for the other d-block metals, Ln for the lanthanides and An for the actinides. 

What type of compounds will be covered in this overview. Strictly, only compounds which have at least one metal-carbon bond (with the exception of metal carbides) are called organometaiiic. However, this definition was not adopted in this chapter because many families of compounds that are relevant in organometaiiic chemistry would not be included (e.g., alkoxides). By metal we mean any element from groups 1 (except hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except boron), 14 (except carbon and silicon), and 15 (antimony and bismuth only). Most of the available thermochemical data for all these species are freely available in a single on-line database the NIST Chemistry WebBook Unless stated otherwise, all the data included in this chapter were quoted from that reference. 

Johansson (1978) and Brooks etal. (1984) utilized the energy difference (f d transitions) between trivalent (f"ds ) and tetravalent (f" M s ) cerium and other f-element metals to estimate thermochemical data for compounds. A similar approach has been used by Mikheev et al. (1986) and Spitsyn et al. (1985) to include the divalent as well as the tetravalent state. Again, a critical issue has been the behavior of the heavy actinides with respect to f- d transitions, and the need to utilize and to interpret the few experimental measurements on these actinides, which are discussed below (e.g., section 2.4.1.3). Johansson and Munck (1984) defined a function P (M) that removes the intershell multiplet coupling energy from the atomic reference state (A ,.,up,i j is the energy difference between the baricenter and the lowest level of a multiplet). For the lanthanides P (M) is significantly smoother than P(M),-except for an anomaly at Yb that may be due to an error in experimental data. Unfortunately, Johansson and Munck (1984) point out that spectroscopic data on the heavy actinides are inadequate to correct P(M) to P (M) for the actinides. 

Seifert and Yuan (1991) characterized the ABr-LaBr3 (A = Na, K, Rb, Cs) system and provided thermochemical data. The A = Na system is eutectic. The AjLaBrj (A = K, Rb, and Cs) compounds are stable at ambient temperature, but decomposed peritectically for A = Cs. At temperatures above 430°C A3LaBrg (A = Rb, Cs) crystallize in the cubic elpasolite structure common to the heavier lanthanides at ambient temperature the monoclinic K3MoClg-type structure prevailed, with A = Rb metastable. Cs2LaBr7 was stable over the limited 459 551 °C range. 

Also reliable thermochemical data for the pure metals have been accumulated, like for example their heats of vaporization (cohesive energy at zero temperature) (Habermann and Daane 1964). Since these measurements all involve energy differences between different states of the lanthanide atoms (ionsX seems possible to interrelate the various data to each other and from this gain further understanding of the electronic structure of the lanthanides. 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

A number of efforts have been made to calculate ionization-potential sums from thermochemical data and appropriate Born-Haber cycles. When an isostructural set of compounds is used, and covalence/repulsion corrections are made from a systematic lanthanide-actinide comparison, such sums can be quite reliable, as has been repeatedly demonstrated for the trivalent lanthanides [88]. For example, Morss [89] was able to estimate the sum of the first three ionization energies (/i +I2 + I3) for Pu as... 

There are, as of the time of writing, no thermochemical data on complex oxides containing trivalent actinides (e.g. AmA103 or SrAm204). Indeed, such measurements are still lacking for the lanthanides. 

The dichlorides, dibromides, and di-iodides of Am and Cf have been reported. No thermochemical data are available. Since these dihalides parallel lanthanide dihalides of similar ionic radii, it is possible to estimate their enthalpies of formation by a method similar to that used by Morss and Fahey [107]. The data for this estimation, and the resulting predicted enthalpies of formation, are shown in Table 17.8. The enthalpies of the reactions... 

The thermochemical properties of the lanthanide-hydride systems have been well-catalogued by Libowitz and Maeland (1979). Heats, entropies and free energies of formation have been tabulated for both the dihydrides and dideuterides, as well as data for the La through Nd trihydrides and also the hydrogen-deficient hexagonal phases. Methods are described for typical calculations and are not repeated here. Thermodynamic data and thermal functions are presented by Flotow et al. (1984) and by Ward (1985a and b) for the hydrides of Th through Am. Newer data are reviewed here, and the values in table 1 reflect these updates. 

Each of the four kinetic regions observed in fig. 14 corresponds to control of the bulk hydriding rate by a separate process. The decrease in r as P approaches P is clearly thermodynamic in nature because r approaches zero as the thermochemical activity gradient driving the reaction approaches zero. As described by Haschke (1991), the approach to equilibrium gives rise to the rate decreases observed at high temperature in Arrhenius data for the U-l-H system under isobaric conditions. Unlike the U-UH3 system, equilibrium pressures for the metal-hydride systems of the lanthanides and transuranium actinides are not in the 0.1 to 1 bar range frequently employed in hydride... 

The standard enthalpies of sublimation of rare-earth metals have been measured by a number of workers. Hultgren et al. (1973) have discussed the sources of data and error estimates in their tabulation. Later Morss (1976) has also briefly discussed these data in his comprehensive discussion on thermochemical properties of the lanthanides. Recently Bratsch and Lagowski (1985) have listed a set of values of the sublimitation enthalpies which are also listed in table 1. The values of AH°f for rare-earth metals recommended in table 1 have been used in the recalculation of D values. 

The kind of analysis outlined above can yield accurate assessments of the extent to which f electrons participate in the chemical bonding in the lighter actinides, but they are dependent upon accurate experimental data for actinium and the transplutonium elements. Both thermochemical and optical spectroscopic data are also useful for analysis of the factors determining the oxidation states of the actinide atom in compounds (Johansson 1977a, Brooks et al. 1984). For example, the stability of 450 different halides and oxides of the lanthanides were investigated, and the existence or non-existence of di-, tri- and tetravalent compounds was accounted for very well (Johansson 1977a). 

A systematic treatment of data from the lanthanide and actinide oxides has generated estimated thermochemical values for the oxides of californium [126]. 

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