Lanthanides structure sequence

The next two elements, berkelium and cahfornium, were recently found to have identical structural sequences under pressure (Fig. 2 b, c). The first high pressure transition for both Bk and Cf is dhcp ccp as in the lanthanides. Thus the lanthanide character of heavy actinides again seems confirmed. But a second transition to the low symmetry a-uranium type structure follows in both metals. This transition reflects the start of 5 f participation in bonding. The transition pressures increase monotonically on going from Am to Bk and Cf 5, 7 and 17 GPa for the dhcp ccp transition, 10, 25, 30 GPa for the ccp An III (low symmetry phase) transition. The second transition in Cm occurs at 18 GPa this transition pressure fits well into the sequence of delocalization pressures. But the dhcp hep transition in Cm occurs at 12 GPa and thus does not fit into the increasing Z sequence with respect to both structure type formed and transition pressure. ... 

Erbium has a total of 11 electrons in its 4f shell. The crystal structure sequence hcp Sm-type dhcp dfcc seen in lanthanide metals with decreasing atomic number and increasing pressure is also observed in erbium. 

The structural phase transitions in thulium have been studied to 195 GPa (Montgomery et al., 2011). The lanthanide crystal structure sequence, hep Sm-type dhep dfee, is observed below 70 GPa. It is to be noted that the pure/cc phase is not seen in this study. The hexagonal hR24 phase (Montgomery et al., 2011) and orthorhombic Cmmm (Pravica et al., 2006) are used... 

It is clear ffran the data presented in this chapter that the trivalent lanthanide metals show the following structural sequence hep Sm-type dhep - fcc —> effee (hR24 phase—hexagonal with 24 atoms celt) with increasing pressure. The long standing debate oti the structure of the dfcc phase now appears to be settled with... 

It was not until about 25 years later, however, that the final details became available concerning the room-temperature crystal structure sequence in the lanthanides. 

The crystal structure sequence as one proceeds along the lanthanide series from La to Lu goes from fee to hep with two intermediate structures which are composed of a mixture of both fee and hep close-packed layers, one being 50% of each and the other being f hep and 3 fee. The relationship of the close-packed layer sequences in these four structures is shown in fig. 2.5. A layer is classified as an h (hexagonal) layer if the atoms just above and below the layer are aligned... 

This correlation between the stable structures and d-occupation number can be related to the crystal structure sequence as observed in the trivalent lanthanides metal series, as we will discuss in the next section. 

The relation between the observed structural sequence in the lanthanides and the d-band occupancy can be examined qualitatively using the above values of the number of occupied d-electrons. The study of the pressure-induced structural transition of Lu above (Min et al. 1986c) shows that the dependence of the d-band oecupaney on the structure is small at the given lattice constant the differences obtained between struetures are only 0.02 or 0.03 electrons. Hence it is possible to relate, within a small uneertainty, the observed structures in the lanthanide metals with the absolute values of the d-band occupancy obtained in the fee structure. From fig. 6, the resulting ranges of d-band occupancy (JV ) for the stable structures are < 1.7 for hep and 1.75 [Pg.178]

Work on correlating the crystal structure sequence in the pure metals and intra rare earth alloys started in about the mid-1960s, shortly after the discovery by Spedding et al. (1962) of the formation of the Sm-type structure in alloys between a light lanthanide metal and a heavy lanthanide (or yttrium) metal. The next major impetus was provided by the high pressure work of Bell Telephone Laboratory s research group (Jayaraman and Sherwood, 1964a, b McWhan and Bond, 1964) who found that under pressure hep Gd transformed to the Sm-type structure Sm (rhombohedral, Sm-type structure) transformed to the dhep structure and dhep La transformed to the fee structure. Thus it became evident that the crystal structure sequence in the lanthanide series at 1 atm pressure varied from fee dhep Sm-type(or 8)- hep, while pressure caused the reverse structure sequence. 

X-ray diffraction studies on Y under pressures up to 50 GPa at room temperature (Vohra et al. 1982, Grosshans 1987) gave crucial experimental evidence for the hypothesis that the structural sequence of the regular lanthanides can be attributed purely to the progression of the s to d transfer under pressure, without any (significant) contributions from f electrons. In fact, the typical structural sequence of the regular... 

In any case, it can be noticed that Sc under pressure follows a difierent structural sequence than Y and the regular lanthanides, which could be explained on the ground that the 3d electrons of Sc in their open 3d shell tend to be localized more strongly than the 4d electrons in Y or the 5d electrons in the regular lanthanides, due to the absence of an inner, closed d-electron shell in Sc, which in the other cases leads to an extra orthogonaUty condition and, therefore, to extra core repulsion. This difference in the 3d transition metals with respect to the 4d and 5d transition metals also seems to account for the various differences in Ti, Mn, Fe, Co, and Ni with respect to their 4d and 5d counterparts. 

First, the radius ratio r, = R s/R p was introduced as a useful parameter to scale the structural transitions to critical values of this parameter, leading to the observation that all the regular lanthanides, and, somewhat less rigorously, also Y and the heavier actinides from Am on, follow the same structural sequence, with respect to this scaling. 

In the sequence of structures from the large to the small rare-earth elements, the lanthanide contraction is manifested as shown in Figs. 19a and 19b. Within a structure, the cell volume diminishes linearly with the atomic number. If a certain, limiting value is reached, there is... 

Bis(ethylacetoacetonate)-lanthanide(III) alkoxides, represented by structure (314), also initiate the well-controlled ROP of CL.895 Mn increases linearly with conversion (with Mw/Mn<1.10 throughout), and increasing [M]0/[I]o- Kinetic analysis implies a first order dependence on the lanthanide initiator, consistent with a non-aggregated active site. Block copolymers with moderately narrow polydispersities (1.25-1.45) have also been prepared using these initiators. NMR spectroscopy confirms well-controlled block sequences suggesting that these initiators are less susceptible to transesteriflcation than other lanthanide alkoxides. Initiation occurs exclusively at the alkoxide bond, and the tris(ethylacetoacetonate) analogs are inactive under the same conditions. 

An-An alloys. A summary ofthe phase diagrams for adjacent actinide metals is shown in the connected binary phase diagrams of Fig. 5.11. The structure of this diagram resembles that reported in Fig. 5.10 for the lanthanides notice, however, that such a sequence of interconnected diagrams could be used as a generalized diagram in a more limited way only, possibly for the heavier actinides from americium onward. 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

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