Rare earth distances

In (La,Ce)P04 the Stokes shift of the emission is smaller 60(X) cm ), and the emi.sston of CeP04 is partly concentration quenched. Energy migration over the Ce ions assists in the transfer to iV and less Tb is needed. Nevertheless the UV output is relatively high (see Table 6.1). This is due to the fact that the Ce -Ce transfer is more efficient than the Ce +-Tb + transfer. At the shortest intemuclear rare-earth distance the transfer rates amount to 10"s and 3 x lO s, respectively. This phosphor system has been discussed at length recently [21]. 

Similar discrepancies in interatomic distances in Fe2GeS4 and Mg2GeS4 were recently noted by Vincent and Perrault (1971) and Vincent and Bertaut (1972). Both the interatomic distances and the unit cell dimensions of the Fe compounds are smaller than those of the Mg compounds in contradiction to the ionic radii of Fe2+ (high-spin) (0.78A) and Mg2+ (0.72 A) in oxides. The same behavior had been noted earlier by Patrie and Chevalier (1966) in the compounds ML2S4 where M =Mg, Fe, Cr, and Mn and L = rare earth. 

Unfortunately, no detailed theory of phonon-assisted exchange processes in rare earths appears to have been made, nor has any attempt been made to determine the distance over which the mechanism is operative, or the rates of exchange. Most likely, these gaps will be filled in the very near future. 

In all these cases, hydride formation corresponds to partial occupation of the available holes, reminiscent of multihole polyhedra behavior. Occupation of all available holes would require a limiting stoichiometry MH3, and would correspond to occupation of the unique hole in isolated polyhedra. This situation is known for some rare earth hydrides (see Table III). Significantly, transformation of the metallic dihydride to the trihydride occurs with a decrease in apparent metal-metal distances and with a large increase in resistivity. These observations indicate a salt-like character and the disappearance of metal-metal bonds (27). 

The quantities B (Rq) and are treated as adjustable parameters and Rq is a reference distance that can be chosen arbitrarily. In principle, the number of ligand shells considered for the calculation of intrinsic parameters is not limited, however, it is usually assumed that only the nearest neighbors of the rare-earth ion contribute significantly to the crystal-field potential. Thus, especially long-range interactions like electrostatic interactions are not accounted for explicitly. Because these interactions are most important for k = 2 parameters, in many cases only the k = 4, 6 intrinsic parameters have been considered. 

While ambient pressure studies must rely on discrete changes of crucial parameters, the high pressure method is capable of generating continuous changes of interatomic distances or relative energies of different electronic states. Moreover, at the same time the chemical composition of the rare-earth compound is conserved under pressure, while ambient pressure studies usually have to consider different compounds. In this sense, the application of high pressure can solve physical problems which can not be accessed by any other method. 

Although the high-pressure method itself is rather simple to use, high-pressure physics is still a somewhat exotic tool for investigating rare-earth compounds. This seems rather surprising if the unique possibilities for example for the determination of intrinsic crystal-field parameters are considered. In many cases, theories explicitly predict certain distance dependences, which can be verified straightforward by the application of pressure, which continuously alters the interatomic distances. 

Besides the YbAgPb type (Section 4.3), La4Ni3Pb4 is the only structure type that has so far exclusively been observed for rare earth-transition metal-plumbides. A view of the La4Ni3Pb4 structure is presented in Figure 25. There are two peculiar features (i) Ni3 triangles with an extremely short Ni-Ni distance of 223 pm, and (ii) chains (spirals) of the Pbl atoms at Pbl-Pbl of 339 pm. Both of these distances are shorter than the Ni-Ni (249 pm) and Pb-Pb (350 pm) distances in the elements (Donohue, 1974). One can thus safely assume significant Ni-Ni and Pb-... 

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