Phase transformations in cerium and thorium metals at ultra high pressures

Scripta METALLURGICA et MATERIALIA

Vol. 25, pp. 2787-2789, 1991 Printed in the U.S.A.

Pergamon Press plc All rights reserved

PHASE TRANSFORMATIONS IN CERIUM AND THORIUM METALS AT ULTRA HIGH PRESSURES Yogesh K. Vohra Materials Science and Engineering Cornell University Ithaca, N. Y. 14853

(Received September 9, 1991) (Revised October 4, 1991) Introduction The role of pressure variable in phase transformations has not been fully exploited in metallic elements and their alloys. The static compression of over 50% in volume can readily be obtained in most metals and this tremendous change in inter-atomic distances can lead to the formation of new exotic crystal structures. The pressure-induced electron transfer amongst existing electronic energy bands and the occupation of new bands are the driving forces in a rich variety of phase transformations. The modern high pressure diamond anvil cell techniques can produce calibrated static pressures of over 300 to 400 GPa range and this technology, when interfaced with the synchrotron radiation sources, can yield rapid structural information(I-3). These capabilities have given new impetus for investigation of phase transformations in metallic systems at extreme conditions of temperatures and pressures and in establishing phase boundaries at high pressures and high temperatures. Cerium(Ce) and thorium(Th) metals occupy special positions in the periodic table at the beginning of the 4-f lanthanide and 5-f actinide series, respectively. Ce has one electron in the localized 4-f shell, apart from the three valence electrons. Th metal, on the other hand, has four valence electrons and an unoccupied 5-f band above the Fermi-energy at ambient conditions. In view of the unoccupied 5-f band, Th metal is normally regarded as a tetravalent transition metal like Ti, Zr, and Hf and its bonding and other electronic properties can be explained within the tetravalent transition metal framework. However, the application of ultra-high pressures causes the deiocalization of the 4-f shell in Ce(4) and it is believed that Ce above 0.8 GPa pressure is a 4-f band metal(5-6). The application of high pressure on Th brings the 5-f hand downwards relative to the Fermi-energy, and 5-f electrons give significant contribution to bonding at ultra high pressures(7). The changes in the electron bonding at high pressures are reflected in the structural phase transformations in these metals at high pressures. It is the purpose of this communication to point out some very interesting similarities between the phase diagrams of Th and Ce. PhA~¢ TrAnsformations in Ce and Th Ce metal has been investigated to 46 GPa at room temperature(8) and its high pressurehigh temperature phase diagram has been investigated to 15 GPa(9). Ce crystallizes in a face centered cubic(FCC) structure at ambient conditions. At 0.8 GPa, it undergoes a very dramatic isostructural transition to another FCC phase with 16% volume change. This transition in Ce at 0.8 GPa is purely an electronic transition associated with the 4f-electron. On further increase in pressure beyond 5 GPa, a structural phase transformation to body-centered monoclinic(BCM) phase is observed(8). The BCM phase of Ce is stable to 12 GPa, above which it transforms to a bodycentered tetragonal (BCT) structure(8,10). The BCM and BCT structures contain two atoms per unit

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cell at (0,0,0) and (1/2,1/2,1/2). There is a wide range of stability for the BCT phase in Ce, and this phase is stable to the highest pressure of 46 GPa to which Ce has been investigated(8). Th metal has been recently investigated to 300 GPa at 300 K in a diamond anvil cell(7). Th crystallizes in the FCC phase at ambient conditions. Above 100 GPa, it adopts the BCT structure like Ce, eventhough transformation pressure is an order of magnitude higher. The BCT phase of Th is stable in the range of 100-300 GPa and has an extensive stability range, similar to Ce. The existence of the BCM phase has not yet been established in Th. Table 1 gives the summary of phase transformations along with the transition pressures in Ce and Th at 300 K. The other known phase transformations for Ce and Th at elevated or low temperatures under ambient pressure are not listed in Table 1. Figure 1 shows the crystal structure data on Th to 300 GPa and Ce to 46 GPa(8). The data is plotted as an axial ratio ( e/a ) as a function of pressure. The FCC phase is represented by the body centered tetragonal cell with the ideal c/a of 1.414 in Figure 1. The BCM phase of Ce has two of its lattice parameters nearly equal(8) and is also plotted in Figure 1. It is clear from Figure 1 that there is a remarkable similarity in the sequence of phase transformations in Ce and Th, and the axial ratio variation is very similar. It should be added that there are no measurable volume discontinuities at the FCC-BCM and BCM-BCT transformations in Ce and Th within the experimental uncertainties of 1%. This is reflected in the measured pressure-volume data for Ce(8) and Th(7), which can be fitted by a single curve through these phase transformations. The BCM and BCT forms of Th and Ce can be viewed as the continuous distortion of the FCC lattice as the pressure is increased above the transformation pressure, and there is a general trend of increasing c/a with increasing pressure. The Ce c/a ratio tends to flatten out at 20 GPa, and in Th similar behavior is observed above 200 GPa. The change in the diffraction patterns at the BCM-BCT transition is very subtle, and the existence of the BCM phase in Th is not established. The pressure range of 70-100 GPa in Th needs to be investigated further. Conclusions The comparison of the structural data at high pressures for Ce and Th metals shows remarkable similarities. This result is of great interest in metal physics because the starting electronic structure of Ce and Th are radically different. Ce has one electron in a localized 4f-shell while the 5f-band in Th is unoccupied at ambient conditions. However, the application of high pressure broadens the 4f-band in Ce and lowers the 5f-band in Th in such a way that their electronic structure and bonding properties at high pressures are indeed very similar. This results in a similar phase diagram at ultra-high pressures. Acknowledgements This work is supported by the Metallurgy Program, National Science Foundation under Grant No. DMR - 9017194. Yogesh Vohra acknowledges useful discussions with Dr. Jagan Akella and Dr. Ulrich Benedict. References (1) H. K. Man, Y. Wu, R. J. Hemley, L. C. Chen, J. F. Shu, and L. W. Finger, Science 246, 649 (1989). (2) Y. K. Vohra and A. L. Ruoff, Phys. Rev. B42, 8651 (1990). (3) A. L. Ruoff, H. Xia, H. Luo, and Y. K. Vohra, Rev. Sci. Instrum. 61, 3830 (1991). (4) B. Johansson, Philos. Mag. 30, 469 (1974). (5) H. L. Skriver and J. P. Jan, Phys. Rev. B21, 1489 (1980). (6) A. K. McMahan, J. Less. Comm. Metals 149, 1 (1989). (7) Y. K. Vohra and J. Akella, Submitted for publication. (8) J. S. Olsen, L. Greward, U. Benedict, and J. P. Itie, Physic& 133B, 129 (1985). (9) L. G. Khvostantsev and N. A. Nikolaev, Phys. Status Solidi A77, 161 (1983). (10) S. Endo, N. Fujioka, and H. Sasaki, J. Phys. Soc. Japan 42, 882 (1977).

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Vol. 25, No. 12

TRANSFORMATION IN Ce AND Th

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Crystal Phases and Transformation Pressures for Ce and Th at 300 K FCC -- Face Centered Cubic, BCM -- Body Centered Monoclinic, and BCT -- Body Centered Tetragonal Transformation Pressure ( GPa ) ....................................................... Ce Th

Phase Change

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FCC to FL"C

0.8 GPa

FCC to BCIVl

5 OPa

70-80 OPa ?

BCM to BCT

12 GPa

100 GPa

BCT Phase Stable to 46 GPa

BCT Phase Stable to 300 GPa

I

I

!

Ce

I

BCT

1.7

(~ 000

0

Oooo O O O

@

0

~1.6 U

BCM & 1.5

FCC DO

1.4 1.7

I 10

I 20

I 30

Th

BCT 0

0

~1.6 U

0

~

0000

I 40 0000000

6) BCM ? A

1.5

FCC 0

1.4

0

0

0 iO

70

I

I

140 210 PRESSURE ( GPo )

I

280

350

FIG. 1 The axial ratio ( c/a ) for various observed crystal phases of Ce to 46 GPa and Th to 300 GPa at 300 K. The FCC structure is indicated by the ideal c/a of 1.414. Ce data is from Ref. 8 and Th is from present experiments.