An anomaly in the heat capacity of Eu1.2Mo6S8 at ∼120 K (T0) has been observed and identified as a structural transformation. The heat capacity, electrical resistance, and Hall coefficient for the Eu-rich pseudoternaries have been measured under pressure to obtain the pressure dependence of T0. The depression of T0 by pressure is used to explain the pressure dependence of the superconducting transition temperature and the anomalous temperature dependence of the critical field

Hambourger, Paul D.

Ho, J. C.

Huang, C. Y.

Luo, H. L.

English

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Study of Lattice Instabilities in Snx Euy-x Mo6S8 (y=1.0 and 1.2)

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Cleveland State UniversityEngagedScholarship@CSUPhysics Faculty Publications Physics Department8-1-1982Study of Lattice Instabilities in Snx Euy-x Mo6S8(y=1.0 and 1.2)Paul D. HambourgerCleveland State University, p.hambourger@csuohio.eduJ. C. HoC. Y. HuangH. L. LuoFollow this and additional works at: https://engagedscholarship.csuohio.edu/sciphysics_facpubPart of the Physics CommonsHow does access to this work benefit you? Let us know!Publisher's StatementCopyright 1982 American Physical Society. Available on publisher's site at http://link.aps.org/doi/10.1103/PhysRevB.26.1446.Repository CitationHambourger, Paul D.; Ho, J. C.; Huang, C. Y.; and Luo, H. L., "Study of Lattice Instabilities in Snx Euy-x Mo6S8 (y=1.0 and 1.2)" (1982). PhysicsFaculty Publications. 132.https://engagedscholarship.csuohio.edu/sciphysics_facpub/132This Article is brought to you for free and open access by the Physics Department at EngagedScholarship@CSU. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contactlibrary.es@csuohio.edu.Original CitationHambourger, Paul D., J. C. Ho, C. Y. Huang, and H. L. Luo. "Study of Lattice Instabilities in Snx Euy-x Mo6S8 (y=1.0 and 1.2)."Physical Review B 26 (1982): 1446-1449.PHYSICAL REVIEW B VOLUME 26, NUMBER 3 1 AUGUST 1982Study of lattice instabilities in Sn Eu~ Mo6Ss (y -1.0 and 1.2 )P. D. HambourgerCleveland State University, Cleveland, Ohio 44115J. C. Ho8'ichita State University, 8'ichita, Kansas 67208C. Y. HuangLos Alamos National Laboratory, Los Alamos, New Mexico 87545H. L. LuoUniversity of California, San Diego, La Jolla, California 92093(Received 30 March 1982)An anomaly in the heat capacity of Eul 2Mo688 at —120 K (To) has been observed and iden-tified as a structural transformation. The heat capacity, electrical resistance, and Hall coefficientfor the Eu-rich pseudoternaries have been measured under pressure to obtain the pressuredependence of To. The depression of To by pressure is used to explain the pressure dependenceof the superconducting transition temperature and the anomalous temperature dependence ofthe critical field.The unusual transport properties of the ternarycompound Eu„Mo6SS are currently of great interest.At ambient pressure the electrical resistance (R) andcarrier concentration (n) (obtained from Hall-effectdata) show an abrupt change from metalliclikebehavior (dR/dT & 0 and dn/dT —0) for T & 100K to "semiconductorlike" behavior (dR/dT & 0 anddn/dT & 0) for T & 100 K, with no evidence of su-perconductivity. ' The Hall data' indicate that the ab-sence of superconductivity results from low values ofn (—10'~ cm 3 at 1.5 K) in apparent conflict withband calculations, ' based on the rhombohedral crystalstructure determined at room temperature, whichpredict that EuMot;S8 should be a metal and is likelyto be a high-transition-temperature superconductorbecause of a high density of states. However, super-conductivity has been obtained by application of pres-sure (P) greater than —7 kbar, with a peak in super-conducting transition temperature (T,) at —11 K forP —14 kbar. '4Comparison of the transport and superconductingproperties of Eul 2Mo6SS with those of other ternariessuggests that the resistance anomaly at T —100 K isassociated with a lattice transformation. In particular,the abrupt change in dR /dT at —100 K is rather likethat observed' in CuMo3S4, where it is associatedwith a transformation from rhombohedral symmetryat high temperature to a triclinic distortion at lowtemperature. ' 6 Furthermore, a peak in T, (P) hasbeen observed in CuMo3S4, Cu07MO3Se4, andZnMo5S6, all of which have similar lattice instabili-ties, while no such peak is observed in nontransforrn-ing ternaries. 7In this Brief Report, we present data on the pres-sure and temperature dependence of the heat capacity(C), resistance, and Hall coefficient of Eut 2Mo6SSand Eu-rich pseudoternaries Sn„Eu~ „Mo6SS. Thesedata confirm the existence of a lattice transformationand yield the pressure dependence of the transforma-tion temperature (To). Using our high-pressure data,we then explain for the first time the anomalous elec-trical, magnetic, and superconducting properties ofEul 2Mo6SS and resolve the apparent conflict betweenthe observed transport properties and the band calcu-lations. 2The samples used in this work are the same asthose described in Ref. 1. The heat capacity wasmeasured at ambient pressure by the static adiabaticmethod using a precalibrated platinum thermometerand at pressures up to —8 kbar by the steady-state acmethod' using a Constantan heater driven at 1.5 Hzand a copper-Constantan thermocouple whose output(at 3 Hz) was detected by a lock-in amplifier. Tech-niques for generation of high pressures and measure-ment of resistance and Hall coefficient were asdescribed in Ref. 1.Figure 1(a) shows the static heat-capacity databetween 80 and 150 K measured on severalEul 2Mo6S8 chips held together with GE 7031 varnish.Because of the small available mass (- 3 g) of sam-ple in comparison with the copper block in our ap-paratus, the relatively small contribution (—15%)from the sample to the total measured heat capacityresulted in a scatter in the data of —10%. However,it is clear that there is an anomaly in the heat capacity(C) at To —120 K. The dashed line is drawn simply26 1982 The American Physical Society26 BRIEF REPORTS 14472,0—l.6—I(a)'I I ) t~ ~ 00~~ &~ r+~ r ~yec' ~ 4 ~~ y ~ ~P=7 kbarIOO-"5OiEul.2 MO6 Se(a)80 IO0 l20I140'0' 50ToI I t 1 IIOO l50 200 250T (K)T (K) 5 . . . , l, I rI9 kbar—~ — 13 kbarl6 kbarEul p M06S8III I I IOl4I.O—Alaa0.865 75T (K)85FIG. 1. Heat capacity (C) of Eu~ 2Mo6S8 vs temperature.(a) Ambient pressure, static adiabatic method. Dashed lineis dravrn to aid the reader. (b) I' = 5.7 kbar, ac method.IOtDOOthLD~ I5QIO3kbarL7 kbar~t'rPto aid the reader. In this temperature range, C isdominated by the lattice and hence this anomaly islikely to be associated with a lattice transformationfrom the high-temperature rhombohedral structure tothe low-temperature triclinic distortion as inCuMo3S4, CuQ7MO3Se4, and ZnMoqS6. Figure l(b)shows the temperature dependence of C measured ona —10-mg sample of Eu& 2Mo6S8 by the ac method.An anomaly is visible at To- 77 K. The magnitudeof the anomaly diminished rapidly with increasingpressure, and we were unable to observe the transi-tion in C ( T) data above —8 kbar.Figures 2(a) and 2(b) show the electrical resistancedata for Eu~ 2Mo6Ss and Eu~ OMo6S8 for P up to 19kbar. The greatest change in slope for Eu~ 2Mo6S8 atambient pressure, found by the intercept of twotangent lines as shown in Fig. 2(a), takes place atT —115 + 10 K, which coincides with To in Fig. 1(a)within our experimental error and is identified as thestructural transformation temperature. The pressuredependence of the anomalies in R (determined bythe tangent-lines method) and C for the pseudoter-naries with different Sn concentrations are presentedin Fig. 3(a); from these data we infer that To de-creases with increasing P and is suppressed to 0 K atP —12 kbar. Figure 3(b) displays To as a function ofSn concentration (x) for Sn„EuI 3 „Mo6SS at ambientpressure. To decreases with increasing x, although Tofor x =0.12 deviates somewhat, perhaps due toI I I I I I I I I I I I I I I I I I i I I I I I I I I I I0 50 IOO 150 200 250 300T (K)FIG. 2. Temperature dependence of resistance (8) atvarious pressures for (a) Eu&2Mo6S8 and (b) Eu&OMo6S8.departure from stoichiometry in this sample (actualcomposition Sn009EuI, Mo6S76). This x dependenceindicates that Sn stabilizes the high-temperaturerhombohedral phase as does pressure.Figure 4 shows the carrier concentration as definedby the one-band formula, n = (e IRHI) ', where RHis the Hall coefficient, for Eu~ 2Mo6SS for P up to14.2 kbar. For P ~ 5.4 kbar, RH is ) 0 at high tem-perature and ( 0 at low temperature; we can see thatthe temperature at which RH = 0 decreases with in-creasing P. For P ~ 8.5 kbar, R» 0 at all tempera-tures. Comparison of Fig. 4 with ambient-pressureHall data' for Sn„Eu~ 2 „Mo6SS confirms that the sub-stitution of Eu by Sn has an effect similar to applica-tion of pressure. From Fig. 4 we also find that thetemperature at which the slope of RH with respect toT (at a given pressure) changes most rapidly agreesroughly with To shown in Fig. 3(a).Several conclusions can be drawn from our obser-vations:(1) We identify To as a structural transformation.(2) The data of Fig. 3(a) show that for P & 12kbar the rhombohedral "high-temperature" phase is1448 BRIEF REPORTS 26l60)lf40-l20-l00-(cm 3)2R—10Eui 2 Mo6 Se 0 kbar+ 54 k bar8,5 k borx I 2,2 kbor/t l4. 2 kbar80- 060 j Eu Mo S (Q)40 you Mo s (p)l2 6 8Ol2 I08 6 820 - aF-u, o~o8S8(R)I I s t0 2 4 6 8P (kbar)l0 l2 P4 $0I ph(„nj( z(g os ++~XX+ pti f+p p+ +++xol. hx f~ ~~x"e4I20-ll(b)I9IO 0~It00 50080 — S"xEul 2-x SloeFIG. 4. Temperature dependence of the carrier concen-tration, n =—(e~RnI) at various pressures. (RH is the Hallcoefficient. )00—00IG. II I02 03XI0.4 0.5retained down to T —0 K. The transport data onthis phase are characterized by relatively large n, pos-itive dR/dT, and high-T, superconductivity and thusare consistent wih a metallic band structure and highdensity of states. The fact that the variation of T,with pressure in Eu-rich samples above 12 kbar isnearly the same as that of metallic Sn& 2Mo6SS, aspointed out in Ref. 1, is further evidence thatEui 2Mo6SS remains metallic down to low tempera-tures for P & 12 kbar. Our results therefore explainthe abrupt appearance of high-T, superconductivityupon application of pressure and support the bandFIG. 3. (a) Pressure dependence of structura1 transfor-mation temperature (To) for various pseudoternariesSn„Eu~ „Mo6S8 (y =1.0 and 1.2). (R) and (C) refer to datataken from anomalies in resistance and heat capacity, respec-tively. (b) Variation of To with Sn concentration (x). (A11data from anomalies in resistance. )calculations of Jarlborg and Freeman, which ofcourse apply only to the "high-temperature" rhom-bohedral phase.(3) Our data on the "low-temperature" phase arecharacterized by rather small n, positive dn/dT, andnegative dR/dT, suggesting the opening of a gap atthe Fermi surface upon cooling below To. The factthat n and 8 level off as T 0 suggests that the gap-ping is partial rather than complete. Our finding thatdn/dP is positive at low temperature (Fig. 4), sug-gesting that the gap diminishes with increasing pres-sure, is qualitatively consistent with the depression ofTo under pressure. Such gapping might be due tocharge-density waves. The appearance of gapping atT ( To suggests that the low-temperature phase mayhave symmetry lower than that of the high-tempera-ture phase and could therefore be triclinic as inCuMo3S4.(4) The depression of Ta to 0 K for P ) 12 kbar isconsistent with the anomalous temperature and pres-sure dependence of the critical field observed in Eu-rich samples above 12 kbar. Furthermore, the factthat dn/dP is positive explains why the negative'conduction-electron spin polarization at the supercon-ducting Mo site originating from the Eu'+, as ob-tained from the critical-field data, increases withincreasing P since high spin polarization requireslarge n.High-pressure x-ray work is currently in progress to26 BRIEF REPORTS 1449determine the pressure dependence of To more pre-cisely.Since the completion of our work we have been in-formed that Baillif, Dunand, Mueller, and Yvon'0have observed the lattice transformation by x-ray dif-fraction at T —110 K and have found the low-temperature phase to have the triclinic crystal struc-ture (distortion).ACKNOWLEDGMENTSWe wish to thank C. W. Chu, A. J. Freeman, D.W. Harrison, M. B. Maple, and F. Welty for discus-sions and assistance. This work was supported inpart by the U. S. Department of Energy and by theNational Science Foundation under Grant No. DMR81-08421.D. W. Harrison, K. C. Lim, J. D. Thompson, C. Y. Huang,P. D. Hambourger, and H. L. Luo, Phys. Rev. Lett. 46,280 (1981).T. Jarlborg and A. J. Freeman, Phys. Rev. Lett. 44, 178(1980).M. Marezio, P. D. Dernier, J. P. Remeika, E. Corenzwit,and B. T. Matthias, Mater. Res. Bull. 8, 657 (1973).4C. W. Chu, S. Z. Huang, C. H. Lin, R. L. Meng, M. K.Wu, and P. H. Schmidt, Phys. Rev. Lett. 46, 276 (1981).R. N. Shelton, Ph.D. thesis (University of California, SanDiego, 1975) (unpublished).6A. C. Lawson, Mater. Res. Bull. 7, 773 (1972).7R. N. Shelton, A. C. Lawson, and D. C. Johnston, Mater.Res, Bull. 10, 297 (1975)~C. W. Chu and L. R. Testardi, Phys. Rev. Lett. 32, 766(1974).9C. Y. Huang, D. W. Harrison, S. A. Wolf, W. W. Fuller,and H. L. Luo, Physica (Utrecht) (in press).R. Baillif, A. Dunand, J, Mueller, and K. Yvon, Phys.Rev. Lett. 47, 672 (1981).

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Study of Lattice Instabilities in Snx Euy-x Mo6S8 (y=1.0 and 1.2)

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