Dhcp structure

The polymorphism of the lighter actinides reflects the existence of numerous bonding (including 5f) electron states of almost identical energies. The observation of dhcp structures for the transplutonium metals indicates only a slight participation of the predominantly localized 5f electrons in the bonding. 

The heavier actinides with localized 5f-electrons adopt the dhcp structure (similar to some lanthanides) at ambient pressure and temperature. At high pressures they undergo structural transitions to lower symmetry structures, marking the pressures at which 5f-electron delocalization takes place 23 GPa for Am (Benedict et al. 1985a), 43 GPa for Cm (Benedict et al. 1985b), 32 GPa for Bk (Benedict et al. 1984) and 41 GPa for Cf (Benedict et al. 1984). 

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. 

It is convenient to discuss the results of the calculations by separating the rare earth metals into groups. We will discuss the metals Y and Sc together with the trivalent heavy lanthanides Gd, Tb, Dy, Ho, Er, Tm and Lu because of their close similarity in band structure. The trivalent light lanthanides La, Pr and Nd and Pm form another group because they have the dhcp structure, see ch. 2, section 6. The Ce is in a class of its own. The complicated structure of Sm makes a band calculation prohibitively difficult. 

The magnetic structure of Nd and polycrystalline Pr may be explained on the basis of nearly parallel (nesting) pieces of Fermi surface. The authors suggested a number of such pieces which may nest in the a direction of the crystal as shown in fig. 3.12. More detailed discussion will be found in section 4.2. Recent neutron scattering results in /3-Ce (Stassis, 1975) seem to indicate that it has a similar magnetic structure to Nd. Pm has the dhcp structure (Koehler et al., 1973), and is situated between Nd and Sm on the periodic table. We may conjecture that the two metals /3-Ce and Pm in the dhcp phase have the same band structure as is discussed here. 

According to the model of McHargue and Yakel (1960), see fig. 4.5, /3-Ce transforms to y-Ce by glide on the 0001 planes in the (lOTO) directions in the dhcp structure. The glides take place on two adjacent planes while the neighboring pairs of two adjacent planes remain stationary. 

In the light lanthanides, the first element. La, is a superconductor with a dhcp structure. The second element, Ce, exhibits a number of phases with intriguing and different behavior from the other lanthanides, which is due to the presence of both localized and delocalized aspects of the 4f-electrons. As a result, Ce has been the focus of much research. The next materials, Pr and Nd, are normal trivalent lanthanides, while Pm is a radioactive element, which prevents a detailed study of this material. These three elements have the dhcp structure. Sm has a typical and complicated Sm-type structure and a trivalent configuration. The last element, divalent Eu, has a bcc structure. 

Anderson et al. (1958) measured the effect of lutetium additions on the superconducting transition temperature of lanthanum and reported that alloys containing 55 and 80 at% La each had the lanthanum dhcp structure. These results are consistent with those reported above by Lundin. 

Jayaraman et al. (1966) examined several binary alloys of a light rare earth metal with a heavy rare earth metal and found that a Ce-70 at% Gd specimen transformed from its normal Sm-type structure to dhcp structure during a 5 hr treatment at 4.0 GPa and 450°C. Since this alloy did not decompose to the elements during the heating-pressure cycle, Jayaraman et al. concluded that cerium remains in its trivalent state under the above conditions. 

Effect of pressure on phase relationships Jayaraman et al. (1966) in a study of the effects of high pressure and temperature on binary cerium-rare earth alloys, tested three cerium-yttrium alloys that, under normal conditions, had the hep the Sm-type and the dhcp structures. Pressure-induced phase transformations in the sequence hep - Sm type dhcp fee were observed in several other intra rare earth alloy systems tested but in the Ce-Y... 

Phase relationships as a function of pressure Jayaraman et al. (1966) studied pressure-induced transformation in several rare earth alloys, including a Nd-Tm alloy, which they stated was considerably less rich in Tm than Nd q. jTmo. 5 due to loss of Tm as vapor in the preparation of their alloy. They stated that the Sm-type phase is centered at Ndo.63Tmo.37. Their alloy had the Sm-type structure under normal conditions (a = 3.50A, c = 26.00A) but transformed to the dhcp structure (a = 3.60 A, c = 11.50 A) during treatment for 5 hr at 4.0 GPa and 450°C in a piston-cylinder apparatus. 

In his investigation of the nature of the formation of the Sm-type structure, Lundin (1966) included some alloys in the neodymium-lutetium system that combine the heaviest of the heavy lanthanides with the heaviest of the light lanthanides that still have the dhcp structure. His neodymium metal was 99.9 -I- wt% pure... 

Curium metal exists in two modifications, a double hexagonal close-packed (dhcp) structure (a-lanthanum type) and a high-temperature cubic close-packed (fee) structure. Using Cm, the dhcp form was found to have lattice constants a = 3.496(3)andc = 11.331(5) A, giving a calculated density of 13.5 gem and a metallic radius of 1.74 A [27,97]. In a recent pressure study, slightly less accurate lattice constants were obtained (yielding a calculated density of 13.8 g cm ) but did establish that the dhcp phase was stable at least to 6.5 GPa [162]. 

Nd has a double hexagonal crystal (dhcp) structure, with two inequivalent lattice sites of approximately cubic and hexagonal symmetries. In addition, the fact that the magnetic order in Nd is multi-q at low temperatures means that the magnetic structure of Nd is probably the most complex of any of the lanthanides, and explains why a complete understanding of its magnetic structure has emerged only in recent times. 

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