Entropies lanthanide metals

Data on Metals. A compilation of the available entropy values for lanthanide metals is given in Table V. It will be noted that these values also include significant estimated magnetic contributions to the entropy and the precision indicated may be deceptive. Experimental values are... 

Another contribution involved the measurement of the high-temperature heat contents of the rare earth metals. An analysis of the data revealed that the entropy of transformation, AS, for the close packed to bcc transformation in the rare earth metals depended upon the number of valence electrons (Dennison et al. 1966c), i.e. AS,r 0.2 e.u. per valence electron. Extension of these observations to the remainder of the periodic table indicated that the entropy of transformation of metals which posses two or more of the common metallic structures and the entropy of fusion depend upon both the crystalline structure of the phases involved and the number of valence electrons (Gschneidner 1975). These results were then used to predict the entropies of fusion for 16 metals, including two lanthanide metals - promethium and lutetium - and the entropies of transformation of 5 metals including promethium, for which no reliable experimental values existed. 

From the latter equation the calculated boiling point of Cm is 3110°C. The derived heat of fusion, entropy of fusion, and average second-law entropy are 13.85 kJ mol , 9.16 JK mol , and 106.7 3.0 J K mol , respectively. Low-temperature condensed-phase thermodynamic parameters await the availability of long-lived isotopes. For excellent discussions of thermodynamic, electronic, and magnetic effects in curium and other actinide and lanthanide metals, the reader is referred to recent articles by Ward and Hill [34]. 

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

Metal complexes also can show contradictive behavior with benzo-15-crown-5 the complexation of lanthanide perchlorates is entropy-driven in acetonitrile, while the complexation of lanthanide nitrates with corresponding disubstituted derivatives is primarily enthalpy-driven, with small entropic differences.44... 

EuRu4Sbn is metallic and becomes ferromagnetic for temperatures below 3.3 K (Takeda and Ishikawa, 2000b). The low temperature saturation moment is about 6.2/xb, 89% of the Eu+2 value. Low temperature heat capacity measurements indicate that the magnetic entropy removed due to magnetic order is also only about 90% of its expected value (Rln8). It is likely that the lanthanide site is not completely filled in this compound although mixed valence behavior can not be ruled out with the available data. 

The lattice contribution in the lanthanide fluorides is only known with sufficient accuracy when the f shell of the metal ions is empty (4f°) or completely filled (4f14). In these cases Sexs is zero and the experimental entropy corresponds to Siat. Also in case the f-shell of the metal... 

The thermodynamic parameters (i.e.) the enthalpy and entropy values showed the formation of inner-sphere chloro complexes in the case of all the lanthanides. The enthalpies for the formation of monobromo complexes of lanthanides are also positive but smaller in magnitude than the corresponding chloro complexes. The complex formation enthalpies follow the sequence A//°(C1) > A//°(Br) > A//°(I) which is unusual for hard metal(III) ions. 

Metals. Kruglikh, et al. (104) measured saturated vapor pressures of erbium, samarium, and ytterbium by the Knudsen effusion method, and standard (average) sublimation entropies of 18.4, 20.7, and 25.6 cal./(gram atom °K.) were derived. Nesmeyanov, et al. (146) studied the vapor pressure of yttrium by an integral variant of the effusion technique. Similar studies at higher temperatures by Herrick (70) on samarium metal have been interpreted in good accord by both first and second law methods. Ideal gas thermodynamic functions have been derived from 100 °K. to 6000°K. at 100° intervals for both actinide and lanthanide elements by Feber and Herrick (45). 

Data on the interesting Schottky transformations in the lanthanide hydrated ethyl sulfates by Meyer and his collaborators (89,138, 140) are suggestive and of considerable interest, but they do not cover a sufficiently extended range to permit entropy calculations. Vapor pressure data of metallically bonded lanthanide-magnesium alloys have been used also to deduce enthalpies of formation of CsCl-type compounds. Since data were unavailable, ACp values were assumed to be zero (147). 

Even more striking is the anomalous position of Th in the entropy-radius relationship of Fig. 4. In following the IVA elements, the shift to smaller radius at Hf corresponds to the gross effect of the lanthanide contraction in the previous row. Note that Th is far over into the trivalent metal area, corresponding to a very large radius Ci.e., lower valence for a supposedly tetravalent metal). 

The heat capacity of Th02 was determined in an adiabatic calorimeter from 10.2 to 305.4 K. The heat capacity and entropy at 298.15 K were calculated to be (61.76 + 0.06) and (65.24 + 0.08) JK mol respectively when corrected for the modem atomic weight of thorium, these become 61.74 and 65.23 J K mol. The sample of thoria used was ground from electrically-fused material, and was analysed to have a Th content of (87.54-87.93) mass% (theoretical 87.88). Chemical analysis showed the total content of lanthanide elements to be < 150 ppm, similar to the total content of other metals, measured spectroscopically. 53 heat capacity measurements were made, extending from 10.2 to 305.4 K. For the entropy calculations, the heat capacity was extrapolated to 0 K using a Debye function. The heat capacity and entropy from this excellent study were adopted by the review. 

For the two common lanthanides lanthanum and neodymium that there are no quality heat capacity measurements available between 20 and 298 K and the selected entropy values at 298 K are therefore little more than educated guesses. Further measurements are also required on alpha cerium between 20 and 96 K in order to obtain a precision value for the entropy at 298 K. What is woiryingly being shown up by these reviews is that for many of these elements, selected values in a particular region are generally based on only one set of measurements since other measurements differ so significantly that they cannot be considered. It is possible that measurements on the pure metals will become fewer and fewer as there is a concentration oti commercially exploiting the many extraordinary properties of lanthanide compounds. 

The breakdown of the resultant pentacoordinated intermediate is rate limiting, since the first step is an intramolecular reaction facilitated by a favorable activation entropy term. In this step, the metal-bound water functions as a general acid catalyst. The water bound to the lanthanide(III) ions has a pA a in the range 8-9, which should be further decreased in the bimetallic clusters since the second trivalent ion should further withdraw electrons from the... 

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