Trihalides lanthanide

Rowley, A. T. et al., Inorg. Chem. Acta, 1993, 211(1), 77 Preparation of metal oxides by fusing metal halides with lithium oxide in a sealed tube leads to explosions if halide hydrates are employed, particularly lanthanide trihalide hydrates. The preparation succeeds with anhydrous halides. This will be purely a question of vapour pressure above an exothermic reaction the question is whether the vapour is water, or metal halide, and the reaction oxide formation, or hydration of lithium oxide. Like other alkali metal oxides, hydration is extremely energetic. 

Undoubtedly, the best method for the production of pure anhydrous lanthanide trihalides involves direct reaction of the elements. However, suitable reaction vessels, of molybdenum, tungsten, or tantalum, have to be employed silica containers result in oxohalides (27). Trichlorides have been produced by reacting metal with chlorine (28), methyl chloride (28), or hydrogen chloride (28-31). Of the tribromides, only that of scandium has been prepared by direct reaction with bromine (32). The triiodides have been prepared by reacting the metal with iodine (27, 29, 31, 33-41) or with ammonium iodide (42). 

Fig. 1. Enthalpies of solution of lanthanide trihalides in aqueous media ( ) anhydrous trichlorides (183) and trichloride hexahydrates (189) in water (A) trichloride hex-ahydrates in dilute hydrochloric acid (190) ( ) trichloride hexahydrates in aqueous magnesium chloride solution (191) ( ) anhydrous triiodides in water (192). Values for the trichlorides refer to 25°C, for the triiodides to 20°C. Filled symbols represent experimental determinations, open symbols represent estimates.

In hexamethylphosphoramide, sodium reduction of lanthanide trihalides gives solutions of Eu2+ (pale yellow), Yb2+ (yellow), Sm2+ (red-violet) and Tm2+ (red-brown). The latter has a half-life at 4.5 hours. The solution of SmCI2 tends to be more stable (half-life, 60 hours) than those of SmBr2 (30 hours) or Sml2 (25 hours).660,661 In liquid ammonia, Eu2+ or Yb2+ may be... 

Preparation of metal oxides by fusing metal halides with lithium oxide in a sealed tube leads to explosions if halide hydrates are employed, particularly lanthanide trihalide... 

Heat capacity of the liquid trihalides 175 the lanthanide trihalides 234... 

The lanthanide trihalides have been subject of studies for many decades. The main incentive has been the scientific interest in the physical and chemical properties of compounds of the trivalent lanthanide ions, which are unique in the period system as they regularly vary along... 

Lanthanide bromides and iodides have found important applications in a completely different field. They are added as additives in high-pressure discharge lamps in the lighting industry to improve the arc stability and the colour quality. The latter is due to the contribution of the multiline spectrum of the doped rare earths which are added to the salt mixture. Lanthanide trihalides of dysprosium, holmium, thullium, gadolinium and lutetium are used frequently for this purpose (Hilpert and Niemann, 1997). 

These high temperature processes can be modelled adequately by equilibrium thermodynamics. For such calculations reliable thermodynamic data are a priority. Although numerous studies of the thermodynamic properties of the lanthanide trihalides have been published in the past, the available information is still not complete. But because the properties change regularly within the lanthanide series, estimates can help to obtain the data that are lacking. 

The selected transition and melting temperatures for the lanthanide trihalides, in K... 

The heat capacity of the trivalent lanthanide trihalides consists of a lattice component, arising mainly from the vibrations of the ions in the crystal, and an excess component (Westrum Jr. and Grpnvold, 1962 Westrum Jr., 1970 Flotow and Tetenbaum, 1981 Westrum Jr., 1983) ... 

Selected enthalpies of formation of the solid lanthanide trihalides, in kJ mol-1... 

For the liquid lanthanide trihalides data from enthalpy-increment measurements and heat capacity (DSC) measurements are available, as summarised in table C. 1 of Appendix C, the recommended values are given in table 14. The majority of the results have been reported by two... 

The thermodynamic functions of the gaseous lanthanide trihalides have been calculated using standard statistical thermodynamic methods which relate the functions Cp, S, and H to the molecular partition function Q (Lewis et al., 1961) ... 

Fig. 32. The bond length of the lanthanide trihalides , experimental values from ED ) o, experimental values from ED (rg) , Adamo and Maldivi (1998) for the BP-DS/TZ, TZd level 0 Joubert et al. (1998) at the MP2/ECP/J, VDZrf level A Dolg et al. (1991) at the CISD + Q/ECP/j/, ECP/jrf level V Adamo and Maldivi (1998) at the B3P/ECP51, ECP d level. For details of the computations see original papers.

The structure and vibrational spectra of the lanthanide trihalides have been studied extensively in the past decade. Due to the recent developments in the experimental and theoretical... 

As was already discussed in the previous sections, dimeric molecules contribute significantly (up to 10%) to the total vapour pressure of the lanthanide trihalides. However, experimental information is only available for a few systems, which is often highly uncertain. As a result it is difficult to predict trends in the lanthanide series or estimate unknown values. 

In the present chapter we have presented a careful evaluation of the thermodynamic and related properties of the lanthanide trihalides. It is shown that the properties of these compounds vary regularly within the four series (F to I) and in most cases clear trends are observed. 

The variation in the heat capacity and entropy of the solid lanthanide trihalides can be described by a lattice contribution that linearly varies with atomic number within each crystallographic class of compounds, and an excess contribution that depends on the electronic configuration (crystal field) of the lanthanide ions. A distinct difference is observed between... 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

Although the literature on the thermodynamic and related properties of the lanthanide trihalides on which our evaluation and conclusions are based is extensive, it is far from complete. As we have demonstrated in many instances the gaps in the experimental information can be filled with estimates (e.g., the standard entropies and the enthalpies of formation) based on the observed systematics. However, this is not always possible for the following reasons ... 

Appendix A. The transition and melting points of the lanthanide trihalides... 

Appendix B. The enthalpies of formation of the solid lanthanide trihalides Table B.l The enthalpy of formation of LaA3(cr) at 298.15 K Aand Aare the enthalpies of solution of La(cr) and LaX3(cr) in HCl(aq), respectively ... 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

G. Lanza, Z. Varga, M. Kolonits, M. Hargittai, On the Effect of 4f Electrons on the Structural Characteristics of Lanthanide Trihalides. Computational and Electron Diffraction Study of Dysprosium Trichloride. J. Chem. Phys. 2008, 128, 074301-1-14. 

The lanthanide trihalides demonstrate very clearly the effect of varying the cation and anion radii upon the structure type adopted (Table 3.1). 

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