Scripta METALLURGICA Vol. 27, pp. 195-199, 1992 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved

ONSET OF STRUCTURAL TRANSFORMATION IN THORIUM METAL AT HIGH PRESSURES

Kouros Ghandehari and Yogesh K. Vohra Department of Materials Science & Engineering

Cornell University, Ithaca, NY 14853, U.S.A.

(Received March 6, 1992) (Revised May 15, 1992)

Introduction

Novel phases of metals are formed under high pressures as a result of the rearrangement of electrons in different energy bands. Diamond anvil cells can produce calibrated static pressures of several million atmospheres and materials can be studied in-situ with synchrotron x-ray diffraction sources(I-2). Metals, in particular, show a wide variety of phase transformations driven by changes in the electronic structure with pressure. Electron transfer from one band to another is believed to be the cause of several high pressure transformations in rare earth, actinide and transition metals(3-4).

At one atmosphere, the actinide metal thorium (Th), has an unoccupied 5f band approximately 4 eV wide located 1.5 eV above the Fermi energy(5). At high pressures, due to the relative movement of various energy bands, the 5f band should cross the Fermi energy. As a result, the 5f band becomes occupied at high pressures due to electron transfer from the sd-band. This electronic transition is expected to cause structural transformation. Th crystallizes into the face centered cubic (FCC) phase at ambient conditions. X-ray diffraction studies have been carried out on Th up to pressures of 30 GPa(6), 65 GPa(7) and 100 GPa(8). In these earlier studies, the FCC phase was reported to be stable to the highest pressure of 100 GPa(8). Th has recently been investigated at pressures between 100 and 300 GPa where it was found to exist in the body centered tetragonal (BCT) phase(9). However, the mechanism and the onset of the transformation from FCC to BCT were not well understood. It has also been suggested from the similarity of the phase diagrams of cerium (Ce) and Th(10) that an intermediate body centered monoclinic phase (BCM) phase may exist in Th in the pressure range of 70 to 100 GPa.

The purpose of the present research was to examine Th carefully in the range below 100 GPa to determine the precise onset of the FCC to BCT transformation and to investigate the possible existence of an intermediate phase. This study is of great importance in locating the intersection of the 5f band with the Fermi energy and providing stimulus for theoretical calculations of these systems(l 1). In this paper, x- ray diffraction measurements to 76 GPa employing synchrotron radiation reveal the gradual distortion of a face centered cubic lattice into a body centered tetragonal

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196 TRANSFORMATION IN Th Vol. 27, No. 2

lattice beginning at 63+3 GPa at 300 K. This structural distortion is electronically triggered by the lowering of the 5f electronic energy band below the Fermi energy under compression. We provide the first direct structural evidence of the onset of this electronic transition.

Th was studied in a diamond anvil cell by employing the energy dispersive x-ray diffraction (EDXD) technique(I-2) using the A3-Wiggler white beam station at the Cornell High Energy Synchrotron Source (CHESS). Th powder of 99.9% purity was loaded into a 50 ~m sample chamber in the diamond anvil cell. The pressures were measured using the ruby fluorescence technique(12). The ruby lines remained sharp and well resolved to the highest pressure of 76 GPa. Th metal is very soft and is compressed to nearly 50% by a pressure of 100 GPa. Consequently, the pressure conditions in the 50 I~m region are fairly homogenous and excellent quality x-ray data was obtained during both compression and decompression.

Figure 1A shows the EDXD pattern at 60.4 GPa of Th in the FCC phase just before the transformation to the BCT phase. The (111), (200), (220), (311) and (222) diffraction peaks of the FCC phase are shown. Typically, 12 peaks were observed for the FCC phase to energies of 80 keV with the diffraction geometry used. Above 60 GPa, some peaks of the FCC phase first broaden and then gradually split into two peaks. Figure 1B is indexed according to the BCT phase at 69.1 GPa. At higher pressures, the peaks split further apart as can be seen in Figure 1C at 76.5 GPa. This means that the axial ratio of the tetragonal phase is increasing with pressure as can be seen in Figure 2. The axial ratio of the BCT phase is best fitted to a quadratic function of pressure. The resulting fit is (c /a) = ( 0 . 2 6 3 1 ) + (2.797x10-2)P - (1.529x10-4)P 2 where pressure, P, is in GPa. By evaluating the fit at ~/2, which is the ideal value of c/a for the FCC phase, we determine the transition pressure, Pt. At room temperature the pressure Pt is 63+3 GPa. The transition is reversible without significant hysteresis on down- loading of pressure. When the pressure is decreased to 60 GPa, the FCC phase is recovered. No intermediate BCM phase between FCC and BCT phases was detected on increasing or decreasing pressure cycles in Th.

Discussion

From the gradual increase of c/a above ~/2 in Figure 2, it is obvious that this transition is continuous. Furthermore, it is thermodynamically a second order transition since there is no measurable change in the volume at the transition. In Figure 3, the plot of volume versus pressure shows a smoothly varying curve across the transition. In fact the data can be fitted with a single curve. The electronic transition from one band to another also has a profound effect on the measured pressure-volume relationship. Further studies are necessary to develop an equation of state with thermodynamically relevant parameters for Th. It is of interest to note that the high pressure behavior of Ce and Th differ in the way the FCC phase

Vol. 27, No. 2 TRANSFORMATION IN Th 197

transforms to the BCT phase. In the actinide element, Th, we have shown that it happens continuously. However, in the lanthanide element, Ce, there is an intermediate phase. The difference in the high pressure behavior of Ce and Th is related to the details of the pressure induced electronic rearrangement in these metals.

This continuous second order transformation is unique amongst known phase transformations in metals at high pressures. In most transformations under high pressures, the crystal structure changes discontinuously across the transition and the structural parameters such as the axial ratio (c/a) of the high pressure phase are relatively insensitive to pressure. However, the Th transition described in this paper has continuous distortion. The c/a changes from the ideal FCC value of 1.414 at 63 GPa to 1.65 at 200 GPa(9)which represents a total change of 17%. This is a direct result of the continuous electron transfer from the sd band to the 5f band starting at 63 GPa and continuing to pressures well over 200 GPa. Our experimental results represent the first direct structural evidence of the onset of this novel electronic transition.

Acknowledgments

This work is supported by the Metallurgy Program, National Science Foundation, under Grant No. DMR-9017194. The authors acknowledge Dr. Jagan Akella and Dr. Ulrich Benedict for many stimulating discussions.

References

(1) B. Buras and L. Gerward, Prog. Crystal Growth Charact. 18, 93 (1969). (2) H.K. Mao, Y. Wu, R.J. Hemley, L.C. Chen J.F. Shu and L.W. Finger, Science 246,649

(1989). (3) H.L. Skriver, Phys. Rev. 1331, 1909 (1985). (4) A.K. McMahan, J. Less-Common Met. 149, 1 (1989). (5) H.L. Skriver and J.P. Jan, Phys. Rev. B 21, 1489 (1980). (6) T.M. Benjamin, G. Zou, H.K. Mao and P.M. Bell in Carnegie Institution Geophysical

Laboratory Year Book, 80, 280 (1981). (7) G. Bellussi, U. Benedict and W.B. Holzapfel, J. Less-Common Met. 78, 147 (1981). (8) J. Akella, Q. Johnson, G.S. Smith and L.C. Ming, High Pressure Research 1, 91

(1988). (9) Y.K. Vohra and J. Akeila, Phys. Rev. Lett. 67, 3563 (1991). (10) Y.K. Vohra, Scripta Metallurgica 25, 2787 (1991). (11) J.M. Wills, O. Eriksson and A.M. Boring, Phys.Rev. Lett. 67 2215 (1991). (12) H.K. Mao, P.M. Bell, J.W. Shaner and D.J. Steinberg, J. Appl. Phys. 49, 3276 (1978).

198 TRANSFORMATION IN Th Vol. 27, No. 2

m

0 0

C 76.5 GP: ,~,~ BCT ..-.. ~',

i l l l i

B 69.1 GPo BCT ¢,q

I N I I

A 60.4 GPa PCC

o A

i H i i

21 27 34 40 46 ENERGY (keV)

Fig. 1 Energy dispersive x-ray diffraction patterns of thorium at various pressures in a diamond anvil cell. The product of energy and interplanar spacing is Ed - 5.4470 + 0.0014 keV nm. The pressure was measured by the ruby fluorescence technique. (A) FCC phase at 60.4 GPa before transformation. (B) BCT phase at 69.1 GPa. (C) The highest pressure spectrum taken in the BCT phase at 76.5 GPa.

Vol. 27, No. 2 TRANSFORMATION IN Th 199

In I ~ I i i

T oFcc • o BCT

V

0

0 0 0 O 0 0 0 0 0 0 0 " B ~ B - ~ Q "

0 20 40 60 80 PRESSURE (GPQ)

Fig. 2 The axial ratio (c/a) as a function of pressure in the range of 0 to 76 GPa. The FCC phase below 60 GPa is indicated by the ideal value of ~/2. The tetragonal data is fitted by a quadratic function shown by the solid curve. Pt denotes the transition pressure.

0 0

> 0 P~

d

ffl Q

| ~ I i i

" q _

I ! I

0 20 40 60 80 PRESSURE ( GPa )

Fig. 3 The measured pressure-volume relation of Th between 0 and 76 GPa. The solid curve is a cubic fit to the data and no volume discontinuity at the FCC to BCT transition pressure Pt is detected.