Both the nature of the electrical transport and magnetism of high-pressure phases of FeS have been elucidated in relation to the spin-state configuration of Fe. This has been achieved by combined 57FeMössbauer spectroscopy and resistance measurements on samples pressurized in miniature gem-anvil pressure cells to ∼12 GPa in the range 300–5 K. Hexagonal FeS in the range 3–7 GPa exhibits magnetic nonmetallic behavior, whereas monoclinic FeS beyond ∼7 GPa is diamagnetic and nonmetallic. This is compelling experimental evidence to show that, along the room-temperature isotherm, hexagonal FeS has thermally activated charge carriers and a high-spin magnetic-electronic configuration, whereas monoclinic FeS adopts a magnetically quenched low-spin state and concomitant filled valence band

Hearne, G. R.

Seda, Takele

Western Washington University

Masthead LogoWestern Washington UniversityWestern CEDARPhysics & Astronomy College of Science and Engineering8-15-1999Electrical Transport, Magnetism, and Spin-stateConfigurations of High-Pressure Phases of FeSTakele SedaWestern Washington University, takele.seda@wwu.eduG. R. HearneFollow this and additional works at: https://cedar.wwu.edu/physicsastronomy_facpubsPart of the Physics CommonsThis Article is brought to you for free and open access by the College of Science and Engineering at Western CEDAR. It has been accepted for inclusionin Physics & Astronomy by an authorized administrator of Western CEDAR. For more information, please contact westerncedar@wwu.edu.Recommended CitationS. Takele and G. R. Hearne: Electrical transport, magnetism, and spin-state configurations of high-pressure phases of FeS, Phys. Rev. B60(1999) 4401.Electrical transport, magnetism, and spin-state configurations of high-pressure phases of FeSS. Takele and G. R. HearneDepartment of Physics, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg-Gauteng, South Africa~Received 16 February 1999!Both the nature of the electrical transport and magnetism of high-pressure phases of FeS have been eluci-dated in relation to the spin-state configuration of Fe. This has been achieved by combined57Fe Mössbauerspectroscopy and resistance measurements on samples pressurized in miniature gem-anvil pressure cells to;12 GPa in the range 300–5 K. Hexagonal FeS in the range 3–7 GPa exhibits magnetic nonmetallic behavior,whereas monoclinic FeS beyond;7 GPa is diamagnetic and nonmetallic. This is compelling experimentalevidence to show that, along the room-temperature isotherm, hexagonal FeS has thermally activated chargecarriers and a high-spin magnetic-electronic configuration, whereas monoclinic FeS adopts a magneticallyquenched low-spin state and concomitant filled valence band.@S0163-1829~99!01633-1#FeS is of interest to both condensed matter physics andplanetary science. It is an antiferromagnetic semiconductor1,2under ambient conditions and is therefore considered to be astrongly correlated electron system,3 permitting an interest-ing comparison to be made of such narrow 3d-band phenom-ena in transition-metal sulphides against those in transition-metal oxides. It is also a likely component of the cores ofterrestrial planets and of meteorites.A variety of crystallographic structures adopted by troiliteFeS spanning ambient to combined high pressure-temperature (P-T) conditions of from 20 GPa to 1000 K hasbeen comprehensively investigated in previous structuralstudies, yet the magnetic and electrical properties of the highP-T phases have been less extensively investigated. Thegoal of this work is to elucidate the magnetic-electronicproperties of the high-pressure phases, and this has beenachieved by separate57Fe Mössbauer and electrical-transportmeasurements on FeS samples pressurized in gem-anvilcells.The P-T structural phase diagram of FeS from the mostrecent x-ray pressure studies by Kusabaet l.4 using a large-volume press is shown in Fig. 1. The compound is seen toexhibit a number of structural transitions under variousP-Tconditions. Under ambient conditions troilite FeS is a hex-agonal superstructure derivative of the NiAs unit cell and hasaxis lengthsa5)cf and c52cf , whereaf and cf are thelattice parameters of a fundamental NiAs subcell. The latticeparameters of the troilite unit cell depicted in Fig. 1 are typi-cally abouta55.966 Å andc511.76 Å, and there is a phasetransformation to the NiAs-type structure of unit-cell dimen-sions (af ,cf) at ambient pressure and an onset temperatureof ;413 K.4 Increasing the pressure at room temperature4,5results in a structural transformation at;3.5 GPa to a hex-agonal NiAs-type superstructure having unit-cell dimensions(2af ,cf). Further increase in pressure along the room-temperature isotherm results in another structural transforma-tion at ;7 GPa to a new phase that may be indexed by amonoclinic unit cell in the space groupP21 or P21 /m, al-though details of the new structure~ .g., atomic positions!are perhaps less well established.5,6Previous attempts have been made to elucidate the mag-netism and nature of electrical transport of the high-P phasesoccurring beyond;3.5 and;7 GPa, but there appears to beambiguity and speculation in this regard because both the57Fe Mössbauer-magnetic7,8 and electrical-transport4,9,10characterization have been limited to studies in the vicinityof room temperature.A precipitous drop in the room-temperature resistance at;3 GPa in previous electrical-transport studies4,10 has beenascribed to a semiconductor-metal transition. However, thechange in resistance at;3 GPa is not a conclusive feature ofmetallization and may be a consequence of a transformationto a new structural phase having a much smaller band gap.Reliable conclusions are perhaps best drawn after establish-ing the temperature coefficient of resistance,dR/dT.The pressure-induced disappearance of a magnetic signa-ture in the57Fe Mössbauer spectrum at;7 GPa and roomtemperature7,8 has been attributed to a collapsed magneticmoment, either due to electron delocalization, suggesting ametallic high-P phase,11 or due to spin crossover of the Featomic spin S from the original high spin S52@Fe21(3d):t2g4 eg2# to diamagnetic low spin S50@Fe21(3d):t2g6 eg0#. A drastic change in the magnetic orderingFIG. 1. Structural phase diagram of FeS derived from recentx-ray studies of Kusabaet al. under variousP-T conditions~Refs.4 and 6!. The dashed phase boundaries~6.5–7.5 GPa! demarcate arange where two phases coexist. Straight solid lines in the structuresshow the unit-cell as well as the octahedral coordination of Fe~small shaded circles! by S ~larger open circles! atoms.PHYSICAL REVIEW B 15 AUGUST 1999-IVOLUME 60, NUMBER 7PRB 600163-1829/99/60~7!/4401~3!/$15.00 4401 ©1999 The American Physical Societytemperature to below room temperature also has to be con-sidered as a possibility due to changes in interatomic dis-tances and bond angles associated with a;5% change inunit-cell volume at the hexagonal̃ monoclinic transition.4A temperature-dependent study of both57Fe Mössbauerand electrical-resistance data to practically accessible liquid-helium temperatures may help to eliminate any ambiguityregarding both the magnetic-electronic state of Fe and thenature of the electrical transport of the high-P phases. Wehave therefore executed variable-temperature studies onpolycrystalline powdered FeS samples at pressures up to;12 GPa by means of both57Fe Mössbauer spectroscopy ina diamond-anvil cell~300–5 K! ~Ref. 12! and electrical-resistance measurements in a gem-anvil cell~300–80 K!. Asample of troilite FeS enriched to;25% 57Fe has been syn-thesized in milligram quantities by reacting the elements inan evacuated quartz tube at 700 °C for a few days. Mo¨ss-bauer spectra under ambient conditions yield almost identicalhyperfine interaction parameters to that obtained by Koba-yashi et al.8 for a sample synthesized using natural abun-dance materials and which they deduced to have a chemicalformula of Fe0.996S. X-ray-diffraction studies independentlyconfirm the stoichiometry of our material as being close toFe0.996S. In our resistance-pressure experiments, four-probecurrent-reversing resistance-temperature (R-T) measure-ments have been performed with a dipstick-type arrangementin a storage Dewar using cubic-zirconia gems as pressureanvils, fine gold-wire electrodes, and an insulating mica gas-ket to constitute the confining sample cavity.13 The lumines-cence line shift at room temperature from fine ruby grainsloaded into the sample cavity has been used to determine thepressure.Figures 2~a!–2~c! illustrate selected57Fe Mössbauer spec-tra measured at room temperature in each of the three phasesoccurring along the room-temperature isotherm as proposedin the recent structural study of Kusabaet al.4 This is inagreement with the most recent Mo¨ssbauer pressure data ofKobayashiet al.,8 showing a collapse of the magnetic sextetat room temperature to yield only a quadrupole-interactiondoublet. Our Mo¨ssbauer pressure studies to cryogenic tem-peratures@e.g., Fig. 2~d!# show that the nonmagnetic quad-rupole doublet persists to the lowest attainable temperatureof 5 K, strongly suggestive of diamagnetic behavior. Thepossibility that the monoclinic phase has a spin-orderingtemperature below 300 K at pressures beyond;7 GPa maytherefore be discounted.Normalized resistance-temperature measurements at se-lected pressures spanning the different structural phases uponrising pressure have been depicted in Fig. 2~e!, demonstrat-ing that a negative temperature coefficient of resistance,dR/dT,0, obtains in the hexagonal and monoclinic struc-tures of Fig. 1. This is indicative of behavior typical of asmall-band-gap semiconductor, in contradiction to the specu-lation of all previous work restricted to measurements at orclose to room temperature that claim intermediate-P ~3.5–7GPa! and high-P ~.7 GPa! phases to be metallic.4,8,9Arrhenius-type plots and analysis of the data in Fig. 2~e!show~semiconducting! carrier transport in the high-pressurephases near room temperature to involve activation energiescomparable to that of troilite FeS at ambient pressure,namely, 10–20 meV.2The magnetic and electrical properties of transition-metalcompounds may be primarily influenced by the electronicconfiguration of the cation. In octahedral coordination thisdepends on the relative values of interatomic~Fe-S! andintra-atomic (Fe:3d spin pairing! exchange interaction ener-gies Dcf and Dsp, respectively.14 The crystal-field splittingDcf of ;1–2 eV represents, within a ligand-field picture, theenergy difference between threefold-degenerate 3d:t2g anddoubly degenerate 3d:eg orbitals of the cation in the FeS6octahedron; see Fig. 3~a!. Alternatively,Dcf is a measure ofthe difference in strength ofp-bond ands-bond cation co-valent mixing with ligand orbitals. Typical intra-atomic-exchange stabilization energiesDsp;2 – 3 eV favor maximalunpaired electron spins and associated orbital occupation tominimize interelectronic repulsion~Hund’s rule!. Upon in-creasing pressure and a corresponding reduction of the unit-cell volume, Dcf may become comparable to or exceedDsp,Dcf>Dsp. It is then energetically favorable for majority-spineg(↑)-derived electrons to occupyp-bonding orbitals ast2g(↓)-derived electrons@see evolution of Fig. 3~b! to Fig.3~c!# representing the high-spin to low-spin spin-pairingtransition that leads to a significant alteration of the elec-tronic configuration.The magnetic nonmetallic behavior of hexagonal FeS~3–7 GPa! suggests a high-spin 3d:t2g4 eg2 cation electronicconfiguration@see Fig. 3~b!# and thermally activated chargeFIG. 2. 57Fe Mössbauer spectra of FeS in the various structuralphases at elevated pressures~a!–~c!. Magnetic spectra at pressuresin ~a! and~b! have been recorded at room temperature, whereas thespectra at higher pressure in~c! and ~d! indicate nonmagnetic be-havior at both room temperature and 5 K, respectively. Solid linesare theoretical fits to the data. The bottom panel~e! has resistance-temperature data at selected pressures normalized to the resistanceat room temperature.4402 PRB 60BRIEF REPORTScarriers. The high-spin state has a partially filled minority-spin t2g(↓)-derived subband contribution to the 3d valenceband, anticipated to render metallic behavior to FeS in con-ventional band theory@see Fig. 3~a!#. Therefore the nonme-tallic behavior must be attributable to a gapd in the t2g(↓)subband; see Fig. 3~b!. The gap may originate either or bothfrom strong electron correlations~on-site Coulomb repul-sion! represented by the Hubbard splittingU of the 3d bandat narrow enough bandwidths15,16 or from pronounced Fe-Feinteractions considered possible across shared faces of FeS6octahedra along thec axis17 in the structure of the hexagonalphase illustrated in Fig. 1.The nonmetallic and diamagnetic state of the monoclinicphase atP.7 GPa is consistent with afully occupiedt2g-derived valence-bandstructure as a result of spin pairingto yield a low-spin 3d:t2g6 eg0 electronic configuration havingquenched atomic spinS50, depicted in Fig. 3~c!. Kusabaet al.4 have shown that from ambient pressure up to;7 GPa,preceding the structural transition to the monoclinic phase,there is a;10% reduction in unit-cell volumeV and anadditional ;5% reduction at the transition. SinceDcf}V25/3, derived from the inverse fifth power dependence ofDcf on average Fe-S interatomic distanceR,14 we anticipatethat a 10% reduction in unit-cell volume alone for FeS wouldincreaseDcf by ;20% from an ambient estimate of;1.6 eV~Ref. 14, p. 276! up to;1.9 eV at;7 GPa. At this pressure,Dsp may be anticipated to decrease from its ambient estimateof 1.7–2.0 eV~Ref. 14, p. 29! due to increased cation-ligandorbital mixing. Thus the reduction in unit-cell volume up to‘‘modest’’ pressures of;7 GPa may quite plausibly lead tothe spin-crossover conditionDcf>Dsp, invoking the elec-tronic change depicted in the sequence Fig. 3~b!˜Fig. 3~c!.Note added in proof.A detailed XRD pressure study hasrecently been performed to elucidate the phase occurring atP.7 GPa at room temperature. The study confirms that thestructure is monoclinic~see Ref. 18!.Financial support from the NRF-ORP~Pretoria! is ac-knowledged with gratitude. L. M. Madzwara assisted in theinitial stages of this project.1J. L. Horwood, M. G. Townsend, and A. H. Webster, J. SolidState Chem.17, 35 ~1976!.2J. R. Gosselin, M. G. Townsend, and R. J. Tremblay, Solid StateCommun.19, 799 ~1976!.3K. Shimada, T. Mizokawa, K. Mamiya, T. Saitoh, A. Fujimori, K.Ono, A. Kakizaki, T. Ishii, M. Shirai, and T. Kamimura, Phys.Rev. B57, 8845~1998!.4K. Kusaba, Y. Syono, T. Kikegawa, and O. Shimomura, J. Phys.Chem. Solids59, 945 ~1998!.5Y. Fei, C. T. Prewitt, H.-K. Mao, and C. Bertka, Science268,1892 ~1995!.6K. Kusaba, Y. Syono, T. Kikegawa, and O. Shimomura, J. Phys.Chem. Solids58, 241 ~1997!.7H. King, D. Virgo, and H. K. Mao, Carnegie Inst. Wash. Publ.77,830 ~1978!.8H. Kobayashi, M. Sato, T. Kamimura, M. Sakai, H. Onodera, N.Kuoda, and Y. Yamaguchi, J. Phys.: Condens. Matter9, 515~1997!.9S. Minomura and H. G. Drickamer, J. Appl. Phys.34, 3043~1963!.10C. Karumakaran, V. Vijayakumar, S. Vaidya, S. N. Kunte, and S.Suryanarayana, Mater. Res. Bull.15, 201 ~1980!.11G. K. Rozenberg, M. P. Pasternak, G. R. Hearne, and C. C. Mc-Cammon, Phys. Chem. Miner.24, 569 ~1997!.12G. R. Hearne, M. P. Pasternak, and R. D. Taylor, Rev. Sci. In-strum.65, 3787~1994!.13R. L. Reichlin, Rev. Sci. Instrum.54, 1674~1983!.14R. G. Burns,Mineralogical Applications of Crystal Field Theory~Cambridge University Press, Cambridge, England, 1993!.15J. B. Goodenough, J. Solid State Chem.5, 144 ~1972!.16S. Satpathy, Z. S. Popovic, and F. R. Vukajlovic, Phys. Rev. Lett.76, 960 ~1996!.17J. B. Goodenough, Mater. Res. Bull.13, 1305~1978!.18R. J. Nelmes, M. I. McMahon, S. A. Belmonte, and J. B. Parise,Phys. Rev. B59, 9048~1999!.FIG. 3. Schematic of the possible energy band structure of FeSfor various electronic configurations. The highest occupied Fe:3dand S:3p bands are shown as solid rectangles, and symbols aredefined in the text: ~a! metallic high-spin state,~b! high-spin statewith semiconducting gapd, and~c! semiconducting low-spin state.In the case of~b! and ~c!, the effect of on-site Coulomb repulsion,the HubbardU, is implicitly included, possibly giving rise to thegap d. Shaded 3d electronic states then represent lower Hubbardsubbands.PRB 60 4403BRIEF REPORTS

Electrical Transport, Magnetism, and Spin-state Configurations of High-Pressure Phases of FeS

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Electrical Transport, Magnetism, and Spin-state Configurations of High-Pressure Phases of FeS

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