Lanthanide monoxides

The dissociation enthalpies of some lanthanide monoxides were redetermined by Murad and Hildenbrand [378], Murad [380], Hildenbrand [381], as well as Balducci et al. [387] by the study of isomolecular exchange reactions. The enthalpies of dissociation obtained are listed in Table 15. The errors of these values are considerably less than those given in the review by Gingerich [21] for these dissociation enthalpies. Various aspects of the bonding of the lanthanide monoxide series are discussed by Murad and Hildenbrand [378]. 

Figure 7-8. Enthalpies of dissociation of gaseous lanthanide monoxides (o) and monoxide ions ( ).

Figure 7-9. The variation of the promotion energies AE to the 4f 5d 6s state for the gaseous lanthanide monoxides (o) and the 4f 5d for the gaseous monoxide ions ( ) the values were calculated by Gibson [89] from the atomic energy levels given by Martin et al. [114].

The superconfiguration (SC) model proposed by Field (1982) for the lanthanide monoxides proved to be very usefiil for the interpretation of the complex spectra of these... 

The ground state configuration of the lanthanide monoxides may be assumed to arise from either of the two superconfigurations (SC) ... 

Binding energies D, bond lengths R, vibrational frequencies co, and dipole moments for the lanthanide monoxides from configuration interaction calculations including the Langhoff-Davidson size-consistency correction in comparison with experimental data"... 

Binding energies D, bond lengths Rc, vibrational liequencies co and dipole moments ju, for the ground states of selected lanthanide monoxides fiom relativistically corrected density-iiinctional studies in comparison with... 

Carette and Hocquet (1988) have recently applied the ligand-field theory to study the lower-energy lines of the lanthanide monoxides. The spin-orbit parameters were also... 

Lattice parameters of NaCl-type lanthanide monoxides. 

There has been no evidence for the existence of a monoxide of Cf, although other compounds containing divalent Cf are known e.g., dihalides (Haire 1986). It is possible that a monoxide can be attained by high-pressure reaction of californium metal and californium oxide, as has been used for the lanthanide monoxides (Leger et al. 1980) but this reaction has not been tried with Cf. An NaCl-type lattice parameter of 0.48-0.50 would be expected for the monoxide. 

For the monoxides in general, which have questionable or limited existence (see sections 1.2.1 and 2.4.1), thermodynamic calculations have shown that their stability is questionable. Even for the lanthanide monoxides, calculations have shown that these are only marginally stable (Morss 1986). In the case of CfO, Morss reports that this monoxide would be subject to disproportionating to metal and oxygen. 

The lanthanide monoxides have been the subject of a number of thermodynamic investigations (Murad and Hildenbrand 1980, Ames et al. 1967, White et al. 1962, Ackermann et al. 1976, Ackermann and Rauh 1975, 1974, 1971). DeKock and Weltner (1971) have reported the spectroscopic results of a few spectra of these monoxides in argon matrix. But most of the thermodynamic results are based on the studies of high temperature vaporization of the solid oxides. 

One of the earliest systematic studies of the lanthanide monoxides was by White et al. (1962). They measured the vaporization rates of the sesquioxides and determined the enthalpies of formation of gaseous monoxides. Later, they extended this study by including a number of isomolecular equilibration studies (Ames et al. 1967) using high temperature mass spectrometry. Ackermann and Rauh (1975, 1974, 1971) have also studied the vaporization of some sesquioxides... 

The dissociation enthalpies and enthalpies of formation AH ) of lanthanide monoxide... 

As is known, the development of the LFT (Field, 1982) made it possible to refine the enthalpies of atomization of lanthanide monoxides and to elucidate their behavior for RO (Dulick et al., 1986). However, when comparing the performance of different methods, including DFT, for describing the chemical bond in RX, some authors (Heiberg et al., 2003 Wang and Li, 2002) arrived at the conclusion that it is currently impossible to adequately describe the role of the 4f shell in chemical bonding. All corollaries from such a description are qualitative. Among RX molecules, accurate calculations of electronic excitation have only been performed for LaX (Fahs et al., 2002, 2004 see Section 10 for details). 

Among the reported lanthanide monoxides, only EuO is well-characterized, impure YbO can be prepared with difficulty, and metallic (trivalent) monoxides of La, Ce, Pr, Nd, and Sm can be synthesized at high temperature and pressure. Earlier reports of lanthanide monoxides as surface phases are believed to be oxynitrides, oxycarbides, or hydrides [40]. Similarly, earlier reports of PuO are now believed to have been an oxide carbide [97]. 

Thermodynamic calculations have shown how marginally stable the few lanthanide monoxides are, even under the exotic conditions of their preparation, and that classical (divalent) CfO should be unstable with respect to disproportionation [40]. Thus the only hope of synthesis of actinide monoxides would appear to be the high-pressure route for AmO and CfO, an extremely demanding synthetic procedure. 

Figure 7 Dissociation energies of the lanthanide monoxides from quasi-relativistic pseudopotential calculations for the 4f"(T (SC I) and 4f (SC II) superconfigurations in comparison to experimental data (exp) ...

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