Thermal Study of Entropies and Crystal Field Splittings in Heavy Rare Earth Trichloride Hexahydrates. Heat Capacities from 5–300°K

Abstract

Heat capacity measurements were made from 5 to 300°K on salts of the type RCl3·6H2O, with R being Gd, Tb, Ho, and Lu. The lattice contributions to the heat capacities of the magnetic salts were obtained by using an interpolation between the heat capacities of the Gd and Lu salts. The crystal field splittings of the lowest J levels of the Tb3+ and Ho3+ ions were studied by comparing the calorimetrically determined magnetic entropies with those calculated on the basis of extant optical spectral data. This comparison indicated that the spectral levels were identified correctly in the case of the Ho salt but incorrectly in the case of the Tb salt. The total entropies of the Gd, Tb, Ho, and Lu salts at 298.15°K are 400.80, 403.84, 403.51, and 374.54 J mole−1·deg−1, respectively. The standard state partial molal entropies of the solutes for the above salts in aqueous solution were determined using the present measurements and previous data from solution calorimetry. The heat capacity of the Lu salt contains an anomalous peak between 279 and 285°K. The enthalpy change associated with the peak is 13.8±1 J. The entropy change is less than 0.3 J mole−1·deg−1. Possible causes of the anomaly are discussed.

The Variation in the Standard Entropies of Aqueous Tripositive Lanthanide Ions

“…Published reviews of the energy levels of complexes, in which the lanthanide ions are bound solely or mainly to water molecules, suggest that, at 298.15 K, the operation of this ligand field splits the ground term, with its characteristic J value, and reduces the internal entropies below R ln (2 J + 1) by about 1.5 ± 1 J K –1 mol –1 . Values for the magnetic entropies of trichloride hexa-hydrates appear in Tables , , and . We give references to the energy levels from which they were derived by means of Boltzmann-type calculations for neodymium, gadolinium, terbium, dysprosium, holmium, erbium, and thulium . At ytterbium where the analysis of the ground state multiplet is incomplete, we have used data from the spectrum of [Yb­(H 2 O) 9 ]­(C 2 H 5 OSO 3 ) 3 , in which the ytterbium ion is surrounded by water molecules .…”

“…Values for the magnetic entropies of trichloride hexa-hydrates appear in Tables , , and . We give references to the energy levels from which they were derived by means of Boltzmann-type calculations for neodymium, gadolinium, terbium, dysprosium, holmium, erbium, and thulium . At ytterbium where the analysis of the ground state multiplet is incomplete, we have used data from the spectrum of [Yb­(H 2 O) 9 ]­(C 2 H 5 OSO 3 ) 3 , in which the ytterbium ion is surrounded by water molecules .…”

“…It was assumed that, in GdCl 3 ·6H 2 O, the eight degenerate levels of the gaseous ion were split into four equally spaced doublets covering just 1 cm –1 . However, initially, account was not taken of this double degeneracy when assessing the entropy below a temperature of 14 K. As a result, R ln 2 must be added to the entropies in Table 4 of reference . This includes the value at 298.15 K, which was quoted as 400.80 J K –1 mol –1 but, as Rard has pointed out, now becomes 406.56 J K –1 mol –1 .…”

“…At lanthanum n appears to be 7 but the values at cerium and praseodymium have not been clearly established. From neodymium onward however n = 6, and we have a series of hydrates with the same monoclinic structure at room temperature, − and the formula LnCl 3 ·6H 2 O. It is with the entropies of this isostructural series of solids that we shall be particularly concerned.…”

“…Their entropies have been determined from the variation of heat capacities with temperature. Experimental data of this kind are available for neodymium, gadolinium, , terbium, dysprosium, holmium, , erbium, and lutetium. , For samarium, europium, thulium, and ytterbium, they are not. Rard, who reviewed previous assessments of the entropies, states that missing values had been obtained by subtracting magnetic entropies {approximately R ln­(2 J + 1)} from the established values to get lattice entropies, S Θ latt .…”