Crystal-field excitations

The electronic spectrum (36) of the pol5uner is dominated by a very broad ultraviolet band, with shoulders at 280 and 470 m/t, which tails into the visible region and is responsible for the deep brown color of the polymer. Very weak crystal field excitations are found at 640 and 880 m. From the latter transition one can estimate that for high-spin Fe +, Dq = 1100 cm i. This value is typical of Fe3+ in octahedral coordination with oxygen ligands, but the X-ray evidence (see below) indicates that the coordination is tetrahedral, so that Dq seems anomalously high. However, the coordination symmetry is actually lower than tetrahedral, since both hydroxide and oxide ligands are involved. 

Crystal-field excitations were observed by neutron scattering (Murray and Buyers 1980). It is argued that the crystal field is responsible for the non-magnetic character of the ground state. Consistently no magnetic ordering was found by the neutron diffraction (Shaked 1978). 

Loong CK, Soderholm AL, Abraham MM, Boatner LA, Edelstein NM (1993a) Crystal-field excitations and magnetic properties of TmP04. J Chem Phys 98 4214-4222 Loong CK, Soderholm AL, Hammonds JP, Abraham MM, Boatner LA (1993b) Neutron study of crystal-field transitions in ErP04. J Appl Phys 73 6069-6071... 

Two well-defined crystal-field excitations at 12.2 and 18.1 meV and a quasi-elastic excitation centered at zero energy were derived from inelastic neutron scattering experiments of ferromagnetic YbNiSn. A crystal-field potential based on a superposition model was presented in that work (Adroja et al. 1998). 

Whilst there have been several theoretical investigations of the effect of hybridisation on the crystal-field excitations within the ground multiplet (Maekawa et al. 1985, Lopes and Coqblin 1986), there have been relatively few in which the spin-orbit level is explicitly included. Cox et al. (1986) have shown, in the context of the Anderson impurity model, that when is comparable to the spin-orbit splitting, the inelastic peak is broadened and shifted to lower energies. Given that the cross-section is weak, at about half the intensity of the praseodymium spin-orbit cross-section, they concluded that the transition was unlikely to be seen except in heavy-fermion compounds with low values of This appears to be confirmed by the failure to observe such a transition in CePdj in recent measurements on HET (Osborn, unpublished). On the other hand, the... 

On the other hand, good agreement with the de Haas-van Alphen measurements on UPdj (Ubachs et al. 1986) is only obtained by treating the f electrons as core states (Norman et al. 1987), whilst photo-emission results show that there is no f-electron density at the Fermi level in this compound (Baer et al. 1980). It is significant that UPdj is the only actinide metal in which well defined crystal-field excitations have been observed by neutron spectroscopy (Shamir et al. 1978, Murray and Buyers 1980, Buyers and Holden 1985). All these results indicate that the uranium ions in UPdj have a localised f configuration and behave more like stable lanthanide ions. 

In the case of cubic crystal symmetry the T, and F symmetry components are Raman allowed. In fig. 15 the well known vibrations of the octahedra have been included to demonstrate the corresponding symmetry. The peak near 95 cm appears only in F symmetry with the Tj and F components being zero. The symmetry analysis is consistent with the identification of the 95 cm line as due to a crystal-field excitation, but does not allow a separation of the two transitions. 

The excitations in systems composed of non-Kramers ions were studied by Fert and Campbell (1978) and Bieri et al. (1982). Here the ground state doublet is split to various degrees, corresponding to a wide distribution of crystal field excitations down to zero energy. This leads to a specific heat contribution at low temperatures which is nearly independent of temperature. 

Pr + ( H4), and Sm + ( H5/2) ions. These studies have revealed both pure crystal-field excitations in R2CUO4 systems, as well as crystal-field excitations in RBa2Cuj06 systems that derive their Raman intensities from a strong magneto-elastic coupling with phonons. For a more detailed discussion of these studies, the reader is referred to the recent review article by Cardona (1999). 

By time resolved spectroscopy we found the broad band peaking at approximately 785 nm with relatively short decay time of 5 ps (Fig. 4.52a) evidently connected to Cr " center in weak crystal field. Excitation by CW laser with 532 and 780 nm revealed many luminescence bands and lines (Fig. 4.52b-f), which may be ascribed to Mn ", Fe, Nd " and the broad stmctured band peaking at approximately 700 nm or to Cr in average crystal field or to... 

Certainly the clearest conclusion from the examples of this chapter is the total absence of sharp features in the inelastic response function of anomalous lanthanide and metallic actinide materials. This contrasts strongly with the sharp dispersionless crystal-field excitations observed in most lanthanide compounds, in which the exchange interactions are weak (fig, 2), and with the sharp spin-wave excitations found in systems with strong exchange interactions. In many of the early studies with neutron inelastic scattering, for example of the heavy lanthanides or transition metals and their compounds, the width of the excitations was never an issue. It was almost always limited by the instrumental resolution, although it should be stressed that this resolution is relatively poor compared to that obtained by optical techniques. However, the situation is completely different in the materials discussed in this chapter. Now the dominant factor is often the width indeed in some materials the width of the over-damped response function is almost the only remaining parameter with which to characterize the response. 

In sect. 5 we discuss a large number of paramagnetic systems that exhibit no sharp crystal-field excitations. For the most part the materials are of intermediate valence (IV). Most of the experiments have been performed on polycrystalline materials. As discussed above, for the most part the interactions that lead to intermediate behavior are stochastic in nature so that little dispersion is anticipated - see also below. A good test for this is whether there is agreement between the neutron magnetic intensities (local susceptibility) and the bulk susceptibility. There is now considerable evidence that the magnetic response of these systems is inelastic at low temperatures. The energy of the peak position is... 

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