Superconductivity and phase stability of selenium at high pressures

Solid State Communications,

Vol. 13, pp. 1413—1416, 1973.

Pergamon Press.

Printed in Great Britain

SUPERCONDUCTIVITY AND PHASE STABIUTY OF SELENIUM AT HIGH PRESSURES A.R. Moodenbaugh* Institute for Pure and Applied Physical Sciences, University of California San Diego, La Jolla, California 92037, U.S.A. and C.T. Wut and R. Viswanathant Department of Applied Physics and Information Science, University of California, San Diego, La Jolla, California 92037, U.S.A. (Received 19 July 1973 byH. Suhi)

Hexagonal Se shows no indication of a transformation to a metallic, superconducting phase up to 160 kbar. Amorphous Sc transforms at about 130 kbar to an unstable metallic, superconducting state which anneals slowly at room temperature toward a non-metallic, non-superconducting phase. Monoclinic Se behaves much like amorphous Se. X-ray diffraction Indicates that all samples are in the hexagonal phase after release of pressure.

SELENIUM exists in several different forms; the thermodynamically stable form has a chain hexagonal structure while the metastable monoclinic1 and amorphousforms transform to the hexagonal structure when heated.2 In a high pressure X-ray investiption McCann and Cartz found that hexagonal and amorphous selenium transform to similar or identical new unidentified structures above about 140 kbar and that the high pressure modification is retained after pressure is released.3 They associated this new phase with the metsllic4~and superconducting5 (transition temperature T~~ 7K) phase occuring when amorphous Se is pressurized and suggested that supercon. ductivity might be observed at atmospheric pressure in this new modification. However Wittig6 reported

that no superconductivity was detected down to 1.5K on an amorphous sample (which had been superconducting at 160 kbar) when pressure was reduced to roughly 60 kbar. In addition he suggested that the metallic and superconducting phase might not even be the stable form at pressures above 130 kbar. We searched for superconductivity at high pressure in hexagonal, amorphous, and monocimc selenium and examined all samples by X-ray diffraction before pressure was applied and after pressure was released. Our measurements of superconductivity and of resistance vs temperature at high pressure indicate that the behavior of selenium is much more complicated than previously suspected. We find that different forms of Sc behave differently when ressurized p

Supported by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under contract AFOSR/F.44620-72.C-0017.

t Supported by the US. Atomic Energy Commission

American Smelting and Refining Co. provided high purity (99.999+%) selenium. The selenium was

under contract AEC-AT.(04-3)-34.

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SUPERCONDUCTIVITY AND PHASE STABILITY OF SELENIUM

melted in quartz tubes filled with one atm of He. Amorphous selenium was produced by quenching the molten material in liquid nitrogen. Hexagonal selenium was prepared by annealing the previously melted Se at 210 C for one day, then cooling slowly to room temperature over 12 h Small monoclinic crystals were grown by evaporating a saturated solution of selenium in CS2 at room temperature.8 Our X-ray results indicate that monoclinic and hexagonal samples were single phase material prior to high pressure measurements. Amorphous Se X.rays revealed no diffraction lines, .‘~

Direct current electrical resistance measurements were performed on powder samples at high pressure and low temperatures using a four probe Bridgman anvil technique described previously? However, here we used a self-contained press1°instead of the clamp used in the previous study66 of superconductivity in amorphous Se. This change eliminated the possibility that pressure was relieved inadvertently in a clamping process. Pressures quoted in this Communication are

Vol. 13, No.9

might indicate a semiconductor to metal transition). At high pressure we found no superconductivity to 1.5 K. In both cases the resistance at high pressure was non-metallic (See Fig. 1). These samples did not transform to a metallic superconducting phase at high pressures. X-ray diffraction patterns indicate that the material was in the hexagonal phase after release of pressure. Amorphous selenium behaves in a more complicated fashion under pressure consistent with previous measurements of resistance and superconductivity in amorphous Se. We observed a discontinuity in resistance4 (a drop of three to five orders of magnitude as pressure was increased) at a press force corresponding to roughly 130 kbar in each of the two pressure cells studied. Above this pressure the resistance was initially metallic46 and the samples were superconducting.5 Figure 2 illustrates this discussion for Cell 2. Then, after anneal at room ternperature the resistance tended toward a non-metallic

estimated from force applied and probably agree within ±10 kbar with the scale published by Wittig.9 __I0

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We pressurized two seperate cells of hexagonal selenium powder to about 160 kbar. We saw no sharp discontinuity in resistance at any pressure (a discontinuity

perature anneal), the superconducting transition broadened and eventually became incomplete. This general description fits the behavior of both cells.

Vol. 13, No.9

SUPERCONDUCTIVITY AND PHASE STABIUTY OF SELENIUM

However, Cell 1 showed sharper transitions (0.25 K

smallest width vs smallest of 1.0 K for Cell 2). Cell 2 tended toward a non-metallic state faster than Cell I by a factor of twenty. In both cells the resistance at room temperature rose with time. X-rays showed that these samples were in the hexagonal phase after pressure release, _____________________________

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in resistance near 130 kbar as pressure was raised, as in amorphous selenium. However, the resistance at high pressure was non-metallic (resistance ratios R2~I

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metallic material which are unstable and in time transform to the high pressure non-metallic state seen in the amorphous and hexagonal samples. Our X-ray patterns taken after release of pressure show that

FIG. 3. We present a plausible interpretation of the above

observations, generally agreeing with a130 suggestion by 6 Just after squeezing to above kbar, amorWittig. phous Se is almost completely in the metallic, superconducting state. This form is not stable and sluggishly transforms to a non-superconducting, non-metallic phase. This phase is probably not the lower pressure amorphousphase since it is not likely that the pressure inside the cell went down with time. Pressure reduction has never been observed using this self-contained press when a manometer was included in the cell. Also, the metal to non-metal transition occured with the cell at 160 kbar (Cell 1) as well as at 130 kbar (Cell 2). Similarity of X-ray patterns after pressure release of —.

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hexagonal and amorphous samples leads us to believe that the high pressure non-metallic amorphous phase is identical to high pressure hexagonal Se. The initial sharpening of superconducting transitions may indicate that the remaining high pressure metallic material had been strain annealed in the time that some of that same phase had transformed to the non-metallic form. Our results for monoclinic to the amorphous selenium results. Weare sawrelated a discontinuity

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monoclinic Se also had transformed to the hexagonal structure. In these samples there were a few weak lines corresponding to the original monoclinic phase. After pressure release two or three faint unidentifled lines, in addition to hexagonal lines, appear in

X-ray patterns of (originally) hexagonal, amorphous, and monoclinic samples. However, they do not correspond to the lines of the unidentified high pressure phase retained after pressure release observed by McCann and Cartz.3 We appreciate the encouragement of B.T. Matthias, H.L. Luo, and J. Wittig. Acknowledgements

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REFERENCES 1. BURBANK RD.,Acza Oysr. 4, 140 (1951); BURBANK R.D.,Acta Oystallogr. 5, 236(1952); MARCH R.E., PAULING L. and MCCULLOUGH JD.,Acra Crystallogr. 6,71(1953). 2. ABDULLAYEV GB., ASADOV Y.G. and MAMEDOV K.P., In The Physics of Se and Te (Edited by COOPER W.C., p. 179. Pergarnon Press, London (1969). 3 MCCANN D.R. and CARTZ L.,J. Chem. Phj’s. 56,2522 (1969). 4. BALCHAN AS. and DRICKAMER H.G.,J. Qiem. Phj’s. 34, 1948 (1961); RIGGLEMAN B.M. and DRJCKAMER H.G.,J. Oiem. Phys. 37,446(1962). 5. WITTIG J.,Phys. Rev. Let:. 15, 159 (1965).

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SUPERCONDUCTIVITY AND PHASE STABILITY OF SELENIUM

Vol. 13, No.9

6. WI1TIG 3.,!. (item. Phys. 58,2220(1973). 7. GRIFFIThS C.H. and FFI’TON B., In The Physics ofSe and Te (Edited by COOPER W.C.,) p. 163. Pergamon Press, London (1969). 8. IIZIMA S. and NICOLET M.A,,JetPropolsion Laboratory SpacePrograms Summary 37—48, Vol. HI p. 70. (1967). 9. WITTIGJ.,Z.Thys. 195,215(1968). 10. EJCHLER A. and WI1TlG J.,Z. Angew. Phys. 25,319 (1968).

Hexagonales Selen zeigt keine Andeutung einer Transformation zum metallischen, supraleitenden Zustand bis hinauf zu 160 kbar. Amorphes Selen geht bei etwa 130 kbar zu einem instabilen, metallischen, supraleitenden Zustand über von welchem es jedoch bei Raumtemperatur langsam zu einer nicht metallischen, nicht supraleitenden Phase transformiert. Monoklines Selen verhâlt sich etwa wie amorphes Selen. Rontgenstrahlenanalyse zeigt nach Wegnahme des Drucks alie Proben in der hexagonalen Phase.