The gamma to alpha phase transformation in cerium
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INTRODUCTION
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CERIUM is one of the most fascinating elements of the periodic table. At ambient pressure, it can exist in one of four allotropic phases, 1 and in a fifth phase under pressure. 2 It is unique among the elements in that it undergoes a polymorphic transformation directly from one face-centered cubic structure to another face-centered cubic structure of different density. For example, the 3'-Ce (295 K ambient pressure phase) transforms under pressure to the a-Ce high density phase, and the reverse transformation occurs on decompression. On heating under pressure, the cell size of the a-Ce increases more rapidly than that of the 3'-Ce until they are identical at 600 K and 20 kbars pressure, the critical point (CP) in the pressure-temperature phase diagram of cerium by Koskenmaki and Gschneidner, 3 shown in Figure 1. At 295 K, the transformation occurs at about 8 kbars, where the 3' ~ a volume change is an enormous 18 pct. This large volume change suggests a modification of the electronic structure and bonding in cerium. Many of the theoretical models for this behavior, as well as other properties of cerium, are reviewed by Koskenmaki and Gschneidner. 3 Smith and K m e t k o 4 suggest that this behavior observed in cerium is part of a more general phenomenon seen for elements found in other parts of the periodic table, most notably among the actinides. Microstructural studies of the phase transformations in cerium that take place below 295 K have been performed by McHargue and Yakels and Rashid and Altstetter; 6 Koch and McHargue 7 have looked at the effects of phase transformations that occur as a result of plastic deformation. These results and many others are summarized in Reference 3. There are a number of inconsistencies in the results reported by different investigators, and many of these are believed to be attributable to impurities in the cerium. In many ways, the transformation behavior of Ce is similar to that for the face-centered cubic delta phase of the Pu-2 at. pct A1 alloy stabilized to 295 K. 8 This alloy transforms to the t~' phase (a primitive monoclinic crystal structure with a ~20 pet density increase) on cooling to low temperatures, or under hydrostatic compression. This dramatic change in density is attributed, at least in part, to the bonding contribution of the 5f electrons. During cooling the
E.G. ZUKAS and J.O. WILLIS, Staff Members, and R . A . PEREYRA, Senior Technician, are with Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545. Manuscript submitted October 24, 1985. METALLURGICALTRANSACTIONS A
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8 12 16 PRESSURE (kbars)
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Fig. 1--Pressure-temperature phase diagram for cerium. 3
phase appears to transform directly to the a ' phase, but it is essentially impossible to achieve complete transformation. Ce transforms from the fcc 3, phase to either the double close-packed hexagonal/3 phase or to the high density fcc a phase. Under hydros
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