TCTP structure

Albertson and Elding (1977) estimated a value of 1.1 A as the best lit radius of the cavity for a hard-sphere model of the TCTP structure with equal prismatic and equatorial Ln O distances. In this model, lanthanides of smaller radii than Gd + (r = 1.107 A) for coordination number CN = 9 (Shannon 1976) are expected to destabilize the structure. These calculations were based on the assumption that the van der Waals radius of oxygen is 1.50 A. 

The structures of the primary hydration sphere in solution are similar to those reported for the hydrated crystals (Albertson and Elding 1977), i.e. for the lighter cations n = 9 with tricapped trigonal prism (TCTP) structure and D31, symmetry, and for the heavier cations n = 8 with square prismatic structure and D2h symmetry. The geometric parameters (Ln HjO and H2O H2O distances) were taken from the X-ray data on solutions from Habenschuss and Spedding (1979b, c, 1980). 

The large Stokes shift observed between the absorption and emission maxima of [Ce(H2 0)g] doped in La(ethylsulfate)3 9H2O crystal was attributed to a deformation in the TCTP structure in the excited state. In the rapidly fluctuating solution environment, this deformation leads to dissociation of one of the equatorial waters followed by a shift of one of the vertex waters to an equatorial position to form the excited state [Ce(H2 0)g]. The appearance of a new band was the evidence for the formation of this excited state species of lower hydration (Jorgensen and Brinen 1963, Kaizu et al. 1985, Miyakawa et al. 1988). 

Regardless of the atomic number of the metal ion, the calculated bond energies U of different structures favor formation of the TCTP structure. This is not true for the hydration enthalpies, AH, calculated from the relationship... 

In eq. (27), Affg is the Born hydration enthalpy, n is the difference in the total number of moles between the gaseous and solution species and AH, is the enthalpy of vaporization of water. According to these calculations, the hydration stabilizes the TCTP strueture of La(H2 0)g+ by 15 kJ mol relative to the square antiprism structure of La(H20)g . By contrast, [Lu(H2 0)g] is stabilized in the square antiprism structure by 1 kJ mol relative to [Lu(H20)g] in the TCTP structure. The difference between the hydration free energies [A(AGg)] of the octahydrate and... 

Figure 15 shows plots of the equilibrium distances calculated for the TCTP Ln(H2 0)9 species and for the cubic, square antiprism and dodecahedron Ln(H2 0)g species. The results agree well with the X-ray data of Habenschuss and Spedding (1979b, c, 1980) which indicated a transition from the TCTP (n = 9) structure of La-Nd group to the cubic structure of the Gd-Lu group. 

Fig. 15. Comparison of the experimental values of Ln-O distances for LnfHjO) in solution ( ) and crystals ( ). The crystals all have CN = 9. Also shown as lines are the Ln-O calculated distances for TCTP (CN = 9) and the cubic (solid) square antiprism and dodecahedral (CN = 8) structures.

It is worth noting that a partial substitution of the s cation by Ag completely changes the structure. Thus, InAgl2 and TlAgl2 crystallize in the tetragonal TlSe ( = TCTP" Se2) structure with Ag at the tetrahedral sites. 

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