Metallization of Fe0.94O at elevated pressures and temperatures observed by shock-wave electrical resistivity measurements

Solid State Communications, Vol. 59, No. 7, pp. 513-515, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.

METALLIZATION OF Feo.940 AT ELEVATED PRESSURES AND TEMPERATURES OBSERVED BY SHOCK-WAVE ELECTRICAL RESISTIVITY MEASUREMENTS E. Knittle and R. Jeanloz Dept. of Geology and Geophysics, University of California, Berkeley, CA 94720, USA and A.C. Mitchell and W.J. Nellis University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

(Received 22 November 1985 by H. Suhl) The electrical resistivity of Feo.940 (wiistite) has been measured under shock-wave conditions to pressures between 72 and 155GPa (0.72 to 1.55 Mbar). The resistivity of FeO at these pressures is approximately 1 ( + / - - 0.6) × 10 -6 ohm-m, as compared with 1.7(+/-- 0.3) x 10 -3 ohm-m at ambient conditions. The absolute value of the electrical resistivity and the increase in resistivity with pressure along the Hugoniot (i.e. the shock compression curve) are consistent with FeO becoming metallic at high pressures and temperatures. INTRODUCTION WOSTITE, Fel_xO, is a non-stoichiometric transition metal monoxide which exhibits large changes in electronic properties at high pressure and temperature relative to those at room temperature and zero pressure. Calculations of the band structure of FeO are unable to account for its zero pressure and room temperature electronic properties [1-3] which are clearly nonmetallic: its electrical resistivity is 1.7(+/--0.3) x 10 -a ohm-m. At ambient conditions, FeO is paramagnetic and in the NaC1 (BI) structure. At 18GPa and room temperature, a phase transition to a distorted (hexagonal) NaCI structure is observed which is antiferromagnetic and corresponds to crossing the phase boundary caused by the elevation of the Ne61 temperature at high pressure [4, 5]. However, neither the application of high pressure alone nor the occurence of the change in magnetic properties and the associated structural distortion causes a significant change in the electrical resistivity [6]. In addition, FeO undergoes a phase transition to an undetermined structure which is observed at approximately 70 GPa in shock-wave measurements of the equation of state [7]. One interpretation of this phase transition is that FeO has metallized under shock-wave conditions. To investigate this possibility, we have measured the electrical resistivity of Feo.940 under shock loading to 155 GPa and observed that the low values of the resistivity at these pressures (~ 1 x l 0 -6 ohm-m) suggest that FeO has indeed become metallic. EXPERIMENTAL The Feo.940 wustite used in these experiments was synthesized in the laboratory of Dr. H. Harrison at

Purdue University. The Feo.940 samples were polycrystalline with grain size of 0.1 to 1 cm. The sample stoichiometry was determined by X-ray diffraction, using the experimentally established relation between unit cell parameters and composition [8]. The diffraction pattern also revealed a trace quantity of magnetite (FeaO4) which, upon examination of a thin section of the sample, was found to occur in highly localized clumps in the wtistite boule. Therefore, these regions were removed before the shock-wave samples were prepared. The electrical resistivity of Fe0.940 along the Hugoniot, or shock-compression curve, was measured using the Lawrence Livermore National Laboratory's two-stage light-gas gun described by Mitchell and Nellis [9]. Strong shock waves were generated by impacting planar projectiles accelerated up to 8kmsec -1 by this gun onto target samples. The FeO samples were cut from large unfractured single crystals, and were slabs approximately 8 mm x 4 mm x 0.4 mm in dimension. The density of the samples was 5.373(+/--0.004)Mg m - 3 . The samples were mounted in a sapphire holder which consisted of two disks 3 2 m m in diameter and 3 m m thick. One disk had a central rectangular depression which contained the sample flush with the holder surface. The change in sample resistance as the shock wave passed was measured using a two-probe technique. Only two probes were used because of the small sample dimensions. Two indium leads were bonded to the sample through the sapphire disk which contained the sample, with the In being in contact with a thin Cu coating deposited on each end of the FeO slab. The sample was then covered by the second sapphire disk

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METALLIZATION OF Feo.940 AT ELEVATED PRESSURES

which was impacted by the projectile. Sapphire was used for the sample holder because it maintains a high electrical resistivity to 160GPa, and it matches the impedance of FeO [7]. Thus, the pressure in the FeO sample can be taken to be the same as that of the sapphire. Indium was chosen for the leads for the ease of attaching coaxial cables for the resistance measurements and because In has a shock impedance close to sapphire. The latter is important to minimize shockwave perturbations in the sample. The flyer plate used in the experiment was vanadium (2.5 mm thick and 24 mm in diameter), also chosen for its close impedance match to the sapphire. The circuit used to measure the change in sample resistance is similar to that described elsewhere [10, 11]. The measured resistances of the high-pressure samples were converted to resistivities by measuring the dimensions of the samples at zero pressure and assuming that the thickness was reduced by the shock-wave compression. The equation of state used to determine this volume reduction in FeO at high pressures is from Jeanloz and Ahrens [7].

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Table 1. Electrical resistivity o f Feo.9~O ., ,. ,. ,. ~. .,. . .~,,~,~ .... loading Sample m

Impact Velocity (km/sec)

Pressure (GPa)

V/V~

FeO6 FeO2 FeO4 FeO5 FeOl FeO3

3.471 4.509 4.670 5.361 5.860 6.216

72 100 111 126 147 155

0.758 0.713 0.708 0.686 0.675 0.667

Resistivity (X 10 -~ ohm-m) 2.20 ± 1.07 ± 0.85 ± 1.03 ± 1.35 ± 1.60 +

0.44 0.54 0.17 0.21 0.34 0.32

Temperature c (K) 1250 1800 1950 2630 3330 3600

alnitial densities were Po = 5.373 (0.004)Mg m-3. b Calculated from Hugoniot measured by Jeanloz and Ahrens [7]. eCalculated Hugoniot temperatures from Jeanloz and Ahrens [7].

E

in Fig. 1 by comparison with the shock-wave electrical resistivity data of iron [13] and two iron-silicon alloys [10]. The FeO shock-wave point at 72GPa is interpreted as only a partial conversion to the high-pressure metallic phase in accord with previous Hugoniot measurements of the equation of state [7]. In addition, the electrical resistivity of FeO increases with increasing pressure and temperature. This behavior is similar to that of the other metals under shock-wave conditions (e.g., Fig. 1). The large change in electrical resistivity of FeO at high pressures and temperatures cannot be due to a temperature effect alone on the room-temperature resistivity of semiconducting FeO (see Table 1 for the calculated Hugoniot temperatures of the shock-wave resistivity data). The effect of temperature on the electrical resistivity of semiconducting FeO can be calculated from:

tO

P = Po exp ( - - E a k T ) ,

RESULTS The results of the shock-wave electrical resistivity data for Feo.940 are given in Fig. 1 and Table 1. At zero pressure the value for the resistivity is 1.7 (+/--0.3) x 10 -3 ohm-m, in good agreement with

io~

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I

I

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Feo.940

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I

I

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x

I

I

I

, ~ / I

l

I

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o ~

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Shock-Wave Data I

I

I

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150

Pressure (GPa)

Fig. 1. Measurements of the electrical resistivity of Fee.94) under shock loading are compared with previous data for iron [13] and two iron-silicon alloys [10]. values measured by Tannhauser on FeO of different stoichiometry [12]. At high pressure (above 72GPa), the resistivity drops three orders of magnitude to approximately 10 -6 ohm-m. The absolute magnitude of the resistivity is consistent with that of a metal as indicated

where p is the electrical resistivity, Po is a pre-exponential factor, Ea is the activation energy for conduction, k is Boltzmann's constant and T is the temperature. Using E a = 0 . 0 7 e V and p o = 3 . 3 x 10-Sohm-m from the data of Bowen et al. for FeO [14], this equation predicts less than an order of magnitude decrease in the electrical resistivity of FeO to 1200K, the calculated Hugoniot temperature at 72 GPa. Thus, the absolute value of the resistivity in our experiments and the increase of resistivity observed with increasing Hugoniot pressure and temperature, strongly support the interpretation that FeO becomes metallic at shock-induced pressures exceeding 72GPa. The zero-pressure electronic properties of wiistite are well described by a localized-electron model [3, 15], in which the conduction mechanism is by small polaron hopping [ 15, 16]. In contrast, we propose that the highpressure metallization is associated with overlapping of

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METALLIZATION OF Feo.940 AT ELEVATED PRESSURES

the Fe 3d and hybrid O 2p-Fe 4s bands under high compression as predicted from pure band theory. Acknowledgements - This work was supported by NASA and the U.S. Department of Energy under contract No. W-7405-Eng-48. The w~istite sample material was supplied by Dr. H. Harrison of the Materials Sciences Council at Purdue University. We thank J. Donovan and P.C. McCandless for fabricating and assembling the sample holders, J.I. Miller for fabricating the projectiles and C.D. Wozynski and K.C. Pederson for firing the two-stage gun. We are grateful to Q. Williams and L. Falicov and the reviewer for helpful comments on the manuscript.

6. 7. 8. 9. 10. 11. 12. 13.

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