Lanthanide trifluorides

Ill,A. Another fluorine bomb calorimeter is described (18) and further heats of formation of lanthanide trifluorides measured (1, 11, 16). A revised heat for UF differs appreciably from previous values (7). Xenon difluoride has been used as a thermochemical oxidant (19). 

III, F. More phase diagrams of complex fluorides have been explored using Knudsen cell mass spectrometry (13, 15), and electron impact studies have yielded enthalpies and bond energies (1,3). The heat of formation of MoFmi) has been confirmed (12). Solid state cells have been used with lanthanide trifluorides (24) and NaNiF3 (21). 

Taylor s treatment of lanthanide trifluorides was considerably more cursory than that of the other three trihalides, but nonetheless still provides a starting point (2). More recently, the extraction, preparation, and treatment of the trifluorides have been dealt with in Gmelin (4a). 

The direct measurement of enthalpies of solution of lanthanide trifluorides in water is hardly practicable, in view of their very small solubilities in this solvent. Enthalpies of hydration of lanthanide trifluorides have been measured (217). It should be possible to derive enthalpies of solution, at least for the hemihydrates LnF3-y2H20, by measuring enthalpies of precipitation. 

Solubility Products for Lanthanide Trifluorides in Aqueous Solution at 25°C... 

Solubilities of the later lanthanide trifluorides are similar in liquid hydrogen fluoride and in water, though they are slightly more soluble in the former. Solubilities in liquid hydrogen fluoride (probably 98% HF) decrease steadily from a value of 4 x 10-5 mol EuFs per 100 g HF to 2.1 x 10-5 mol LuF3 per 100 g HF, at 0°C (281). 

Other solubilities relevant to this section include those for LaCl3 in aqueous ammonia (173), lanthanide trifluorides in aqueous hydrofluoric acid (133b, 240b, 281,315), and lanthanide trifluorides in aqueous trifluoroacetic acid (316). Trifluoroacetic acid seems to have a marked solubilizing effect on these trifluorides. 

Fig. 30. Dimorphism in lanthanide trifluorides. The position of YF3 is indicated by a star...

The crystal field energy levels for the lanthanide trifluorides (Dieke et al., 1968 Morrison and Leavitt, 1982)... 

Fig. 6. The variation of Cp (298.15 K) in the lanthanide trifluoride series (o). The broken line connects the values for LaFg, GdF3 and LUF3 for which CeXs is zero at 298.15 K. The solid circles show the values of Qat (298.15 K) obtained by subtracting Cexs (298.15 K) from tiie experimental values.

In fig. 8 the experimental entropies for the lanthanide trifluorides are plotted as a function of the atomic number. The figure shows that. S at for LaF3, GdF3 and LUF3 form approximately... 

Fig. 8. The variation of the SeXp (°) and Slat ( ) in the lanthanide trifluorides. The broken line shows Siat derived from the values for LaF3, GdF3 and L11F3, the solid line shows 5exs derived from the experimental data for the other configurations.

Enthalpy increment measurements to derive the high-temperature heat capacity have been reported for all lanthanide trifluorides except PmF3, as summarised in table 7. The results of the various authors are compared using the reduced enthalpy increment function ... 

Summary of the enthalpy increment measurements for the lanthanide trifluorides... 

Recommended high temperature heat capacity functions for the solid lanthanide trifluorides... 

Three different methods have been used to determine the enthalpies of formation of the lanthanide trifluorides fluorine combustion calorimetry, precipitation calorimetry and galvanic... 

The enthalpy increment of the lanthanide trifluorides in the liquid phase has been measured for most compounds (see table 7). The heat capacity has been derived from these results by fitting them to a 2-term equation H°(T) — 7/°(298.15 K) = a(T/K) + b. In... 

The enthalpies of fusion that have been derived from the enthalpy increment equations for the solid and liquid phase are listed in table 13 and the derived entropies of fusion are plotted in fig. 29. It can be seen that the enthalpies and entropies of fusion for ErF3 to LuF3 are significantly lower than those of the other lanthanide trifluorides. Because these four compounds... 

Fig. 34. The vibrational frequencies of the lanthanide trifluorides O, recommended gas-phase frequencies , experimental data for species isolated in neon matrix , in argon matrix , Adamo and Maldivi (1998) at the BP-DS/TZ, TZd level o Joubert et al. (1998) at the MP2/ECP ), VDZd level A Dolg et al. (1991) for the CISD + Q/ECP >/, ECPDd level V Adamo and Maldivi (1998) at the B3P/ECP5, ECP d level.

The vapour pressures of all lanthanide trifluorides have been measured, including P111F3. Various techniques have been used, which can be divided in indirect techniques (effusion and boiling point methods) which provide accurate data for the total vapour pressure, and direct mass spectrometric techniques, which are less precise, particularly in case of quadrupole mass spectrometry, but give information on the vapour composition and the presence of dimeric or polymeric molecules or dissociation products. 

Experimental structural parameters and vibrational frequencies of the lanthanide trifluorides bond distances in pm, bond angles in degrees, frequencies in cm... 

Uranium trifluoride, which may be obtained by reduction of UF4 with H2 or by reaction of UF4 with UN at 95(FC is a high-melting, nonvolatile solid. It resembles the lanthanide trifluorides in being insoluble in water or dilute acids. 

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