Dhcp structure also

Promethium (Pm) is the last element in the lanthanides to assume dhcp structure at ambient conditions. It is also the only radioactive element besides technetium with stable neighboring elements. The work of Haire et al. (1990), where they obtained Pm-147 as a reactor byproduct is summarized here. An extended discussion on the comparison of Pm with that of Sm, Nd, and actinide materials is given in this work. Pm has been compressed to 60 GPa in a diamond anvil cell and EDXD was employed in observing the structural changes in Pm. The lattice parameters of this Pm sample at ambient conditions are given as a = 0.364nm, c= 1.180nm. 

La and Sm have an additional transformation above room temperature. La transforms from dhcp to fee at 310°C on heating. Impurities tend to stabilize the high temperature fee form. La eontaining 0.8 at.% O heat treated at 400 C for 15 minutes and quenehed to room temperature will retain about 95% of the fee form. However, I,a of the purity given in table 2.1 will transform to over 50% dhep even when quenched. The fee to dhcp transition was shown to be a martensitic type transition by Marcinkowski and Hopkins (1968). The complete conversion of fee to dhcp is also difficult. Beaudry and Palmer (1974b) showed that heat treatment of an as-cast structure for 2 weeks at 275°C did not result in complete conversion to dhcp. They showed that a cold worked sample, heat treated for 2 hours at 425°C to give a recrystallized and strain-free sample of fee phase would transform completely to dhcp when cooled 100°C below the transformation (210°C), then reheated to within 25 C of the transformation and maintained at that temperature (285 C) for 10 days. 

The structural sequence dhcp — ccp was also expected to occur in the next element, curium. But the ccp phase was not observed under pressure. In contrast, the simple hexagonal close-packed (hep) and, at still higher pressure, an as yet undetermined structure is formed. If these results are confirmed by further study, curium structures will have to be considered as another intermediate stage between the lighter and the heavier actinides. ... 

The lanthanide metals should also be investigated to higher pressures than previously applied. It is not excluded that their 4 f electrons also participate in bonding as do the 5 f s of Bk and Cf, after the dhcp, ccp and, possibly, distorted fee phases have been reached. An indication of this possibility can be seen in the recent discovery of the a-uranium structure type in praseodymium (Pr IV) . This structure type was previously observed for cerium, but was thought to be restricted to that metal which has an exceptional position among the lanthanide elements. 

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. 

Fig. 3. The room-temperature atomic volumes of the lanthanide elements and their room-temperature crystal structures. For cerium and ytterbium their just below room-temperature structures and atomic volumes ate also given (dhcp and hep, respective ).

Also noteworthy is the fact that magnetic ordering in the dhcp and Sm structures... 

Marden and Koch (1970) showed by resistivity. X-ray diffraction and dila-tometry that Sm transforms from the room temperature rhombohedral form to another close packed form at about 600 C. The high temperature form was shown to be hep. They also observed that the transition temperature was purity dependent. Further studies by DTA in our laboratory on Sm with the purity listed in table 2.5 gave the transition temperature listed in table 2.14 which is considerably higher than the 600°C reported by Mardon and Koch (1970). Kumar and Srivastava (1969) observed an hep structure in thin films of Sm by electron diffraction. The lattice parameters which they determined were considerably larger than those reported by Mardon and Koch (1970). Boulesteix et al. (1970a) observed Sm to be dhcp in a study of thin films by X-ray diffraction. However, the X-ray data of Mardon and Koch (1970) on bulk samples showed that the hep form is the stable form between the low temperature rhombohedral form and the high temperature bcc allotrope. 

Szuszkiewicz (1973) reported positron annihilation results for the entire series from La to Yb. The data appear to have less resolution than those of Williams and Mackintosh in that the fine structure due to the d bands is not evident in any of the metals. Furthermore, the data analysis was done in the most simple-minded manner. It consisted of finding how many sections of parabola were needed to fit the curve, for instance one for Eu and Yb but two for La and Ce. Not surprisingly, the data also indicated a great deal of similarity in electronic structure between the dhcp members of the group. 

A subsequent, more complete treatment of the band-structure problem for the lanthanides by Skriver (1983b, 1985) confirmed the essence of the Duthie-Pettifor (Duthie and Pettifor 1977) theory, at the same time adding accuracy to the computed structural energy differences. By using full LMTO calculations Skriver was also able to obtain the correct partial occupation numbers - the same as those subsequently calculated by Eriksson et al. (1990c), and shown in fig. 15. By calculating the partial occupation numbers at the pressures required to induce the structural transitions in La, Sm and Gd, Skriver (1983b) was able to associate = 2.0, 1.85 and 1.75 with the dhcp->fcc, Sm-type -> dhep and hep -> Sm-type transition, respectively. Then, calculation of the partial d occupation numbers as function of the pressure, led to fig. 30, which predicts the pressure-induced transitions for the lanthanides. 

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