Electronic Structure and Bonding in Cerium (Nitride)Compounds:Trivalent versus Tetravalent Cerium

摘要

The assignment of the cerium oxidation state in the recently prepared and structurally characterized Ce_2MnN_3 phase is problematic.Based upon E~0 values from aqueous solution, one would suggest the crystal compositon to be (Ce~(3+))_2Mn~(3+)(N~(3-))_3. The experimental data, however, indicate that Ce_2MnN_3 is nonmagnetic;this alludes to (but does not prove)a cerium oxidation state of +4, giving (Ce~(4+))_2Mn~+(N~(3-))_3. Previous theroetical work supports the latter oxidation state assignment. Here we carry out a systematic theroetical examination (using the TB-LMTO-ASA method)of the electronic structures of a number of Ce compounds, in which the oxidation state on Ce is more easily assigned. The Ce~(3+) compounds examined are CeF_3, CeCl_3,CeBr_3,and CeN. Our Ce~(4+) data set includes CeC, CeO_2,Li_2CeP_2. CeN,which is nonmagnetic despite the fact that it contains Ce~(3+),makes it clear that the absence of a magnetic moment in a compound does not necessarily preculde the presence of Ce~(3+). Our calculations indicate that the remaining Ce-centered electron in CeN has substantial d character and is delocalized to form a partial Ce-Ce bond instead of being highly localized in Cecentered f states, like the unpaired electron in CeBr_3. Careful inspection of the band structures of these compounds shows some qualitative trends that may be useful in assigning oxidation states:in Ce~(3+) compounds there is at least one occupied band that is clearly made up mostly of Ce states, while in systems with Ce~(4+), the small Ce contributions are delocalized across many occupied bands. Using this criterion,we conclude that (Ce~(4+))_2Mn~+(N~(3-))_3 is the correct oxidation state assignment for Ce_2MnN_3. We suspect that the high oxidation state on Ce is stabilized by the large number of N~(3-) ions surrounding it in the crystal.