By applying a hydrostatic pressure, we have successfully suppressed the valence change transition in EuPdAs. The studied compound EuPdAs crystallizes in a P63/mmc space group. Through resistivity and magnetic susceptibility measurements, we find that EuPdAs shows a phase transition at 180 K and another transition below 10 K at ambient pressure, as was reported before. The overall transport and magnetic behavior is to some extent similar to that of the parent phase of iron based superconductors. With application of a hydrostatic pressure, the transition at 180 K is sensitively suppressed with a pressure as low as 0.48 GPa. However, superconductivity has not been induced with pressure up to 1.90 GPa

Baoxuan Li

Hai-Hu Wen

Jianzhong Liu

Xiyu Zhu

Yufeng Li

Advances in Condensed Matter Physics

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Pressure Induced Suppression to the Valence Change Transition in EuPdAs

By applying a hydrostatic pressure, we have successfully suppressed the valence change transition in EuPdAs. The studied compound EuPdAs crystallizes in a P63/mmc space group. Through resistivity and magnetic susceptibility measurements, we find that EuPdAs shows a phase transition at 180 K and another transition below 10 K at ambient pressure, as was reported before. The overall transport and magnetic behavior is to some extent similar to that of the parent phase of iron based superconductors. With application of a hydrostatic pressure, the transition at 180 K is sensitively suppressed with a pressure as low as 0.48 GPa. However, superconductivity has not been induced with pressure up to 1.90 GPa.</jats:p

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Research ArticlePressure Induced Suppression to the Valence ChangeTransition in EuPdAsBaoxuan Li, Jianzhong Liu, Yufeng Li, Xiyu Zhu, and Hai-Hu WenCenter for Superconducting Physics and Materials, National Laboratory of Solid State Microstructures and Department of Physics,National Center of Microstructures and Quantum Manipulation, Nanjing University, Nanjing 210093, ChinaCorrespondence should be addressed to Hai-HuWen; hhwen@nju.edu.cnReceived 6 December 2014; Accepted 8 February 2015Academic Editor: Viorel SanduCopyright © 2015 Baoxuan Li et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.By applying a hydrostatic pressure, we have successfully suppressed the valence change transition in EuPdAs.The studied compoundEuPdAs crystallizes in a P63/mmc space group.Through resistivity andmagnetic susceptibility measurements, we find that EuPdAsshows a phase transition at 180K and another transition below 10K at ambient pressure, aswas reported before.Theoverall transportand magnetic behavior is to some extent similar to that of the parent phase of iron based superconductors. With application of ahydrostatic pressure, the transition at 180K is sensitively suppressed with a pressure as low as 0.48GPa. However, superconductivityhas not been induced with pressure up to 1.90GPa.1. IntroductionTheEuPdAs alloy adopts the hexagonal CaIn2-type structure.The transport and magnetic measurements of this materialreveal a similarity to the parent phase of iron based super-conductors [1, 2]. Particularly, there is a clear transition ataround 180K, which is of first order and accompanied witha large volume collapse caused by a strong contraction ofthe 𝑐-axis [3–5]. The transition at 180K was reported to beinduced by the fluctuation of Eu valence from 2.15 to 2.40between 300K and 4K as revealed by experiments of Moess-bauer measurement and 𝜇SR method [3, 6, 7]. However,whether this transition is related to some kind of magnetictransition is still unclear. And as it is known in the ironbased superconductors, there is a close relationship betweenthe high temperature phase transition and the emergenceof superconductivity [8, 9]. Inspired by this similarity, weintend to check whether the transition at 180K can besuppressed under hydrostatic pressure, as many iron-basedsuperconductors do, and, if the transition is really related toa certain magnetic transition, whether superconductivity canbe induced with the suppression of the transition.2. Experimental DetailsThe polycrystalline sample of EuPdAs was synthesized bysolid state reaction method, using EuAs as the precursormade by reaction of Eu and As (purity 99.9%, Alfa Aesar).High purity EuAs and Pd powder (99.99% purity, Cuibolin)were weighed in stoichiometric ratio, mixedwell, and pressedinto a pellet in an argon filled glove box.The pellet was loadedinto an alumina crucible, which was sealed in an evacuatedquartz ampule and then kept inside a PID controlled furnace.The temperature of the furnace was raised to 1000∘C witha heating rate of 1.2∘C/min and maintained for 24 hours.The sintered sample was cooled down naturally to roomtemperature with the furnace power turned off from 1000∘C.X-ray diffraction (XRD) measurements were performedon a Bruker D8 advanced diffractometer with the Cu-K𝛼radiation. Dc magnetization measurements were carried outwith a SQUID-VSM-7T (Quantum Design). Measurementsof resistivity under pressure were performed up to 2.3GPa ona physical property measurement system (PPMS-16T, Quan-tum Design) by using HPC-33 piston type pressure cell withthe Quantum Design dc resistivity and ac transport options.For the resistive measurements, silver leads with a diameterof 50𝜇m were glued to the EuPdAs sample in a standardfour-probe method by using silver epoxy, and the sample wasimmersed in the pressure transmitting medium (Daphne7373) in a Teflon cap with a diameter of 4mm. Hydrostaticpressure was generated by a BeCu/NiCrAl clamped piston-cylinder cell. The pressure upon the sample was calibratedHindawi Publishing CorporationAdvances in Condensed Matter PhysicsVolume 2015, Article ID 575421, 4 pageshttp://dx.doi.org/10.1155/2015/5754212 Advances in Condensed Matter Physics20181614121086420×103Intensity (counts)10 20 30 40 50 60 70 80 902𝜃 (deg)ExperimentCalculationDifferenceEuPdAsImpurity: Eu5As3Impurity: Pd2AsEuPdAsa bcFigure 1: Powder X-ray diffraction patterns and the Rietveldrefinement profile of the EuPdAs sample at room temperature. Allmain diffraction peaks can be indexed well by the P63/mmc spacegroup (hexagonal structure) with 𝑎 = 4.262 A˚ and 𝑐 = 8.535 A˚ withEu5As3andPd2As as the impurity phases.The ratio betweenEuPdAsand the impurities is found to be 90 : 10.with the shift in 𝑇𝑐of a high purity Sn sample by measuringthe temperature dependence of resistivity.3. Results and DiscussionFigure 1 shows the powder XRD patterns of the EuPdAssample at room temperature, along with the result of theRietveld structural refinement using the TOPASprogram.Onthe whole, it is clear that the main diffraction peaks can beindexed well by a hexagonal cell structure with the P63/mmcspace group. In addition to a principal phase, some weakpeaks arising from the impurity phases of Eu5As3and Pd2Ascan also be seen. A detailed fitting to the structural data showsthat the ratio between EuPdAs and the impurity phases isaround 90 : 10 for this sample. The refined lattice parametersare extracted to be 𝑎 = 4.262 A˚ and 𝑐 = 8.535 A˚.Figure 2(a) shows the temperature dependence of zero-field-cooled (ZFC) andfield-cooled (FC) dcmagnetic suscep-tibility for EuPdAs under 20Oe. An anomaly can be clearlyseen around 8K, where the ZFC curve shows some deviationfrom the FC curve; this may be an indication of long rangemagnetic ordering of Eu ions. We can also observe a kinkat around 5.4 K, which has not been reported before. Weconjecture that the 5.4 K kink may be a secondary transitionof the Eu ions ordering, but we cannot rule out the possibilitythat this anomaly arises from the impurity phases of Eu5As3or Pd2As. Figure 2(b) shows the temperature dependence ofZFC magnetic susceptibility under 1𝑇. In addition to thesusceptibility saturation below 10K, corresponding to the 8Kanomaly in Figure 2(a), there is another kink around 180K,which can be easily observed in the inset of Figure 2(b). Andabove 180K, the magnetic susceptibility of EuPdAs followsCurie-Weiss law behavior very well.The temperature dependence of electrical resistivity forEuPdAs with temperature ranging from 2.3 K to 300K isshown in Figure 3. The 8K and 180K kinks also clearlyappear in these resistivity curves, which is consistent with themagneticmeasurements (while the 5.4 K anomaly is absent inthe resistive curve).The overall transport behavior, especiallythe 180K anomaly, reminds us of some parent phases of iron-based materials, like BaFe2As2, which exhibits antiferromag-netic order originated from Fe ions around 140K. Whenwe apply pressure or do some chemical substitutions onBaFe2As2, the 140K transition will be gradually suppressedto lower temperature, eventually, the antiferromagnetic tran-sition will totally disappear and superconducting state will beinduced [2, 10].Inspired by the knowledge in the parent phase of ironbased superconductors, we suspect that the 180K anomalyfor EuPdAs may be related to some kind of magnetic orderof the Pd ions. If we can find some way to suppress thistransition, we may expect some interesting properties, likesuperconductivity. To realize this purpose, we try to apply ahydrostatic pressure to EuPdAs.The temperature dependence of resistivity for EuPdAsat various pressures with temperature ranging from 2.3 K to250K is illustrated in Figure 4. We find that the transitionat 180K is totally suppressed with a pressure of 0.48GPa,and the resistivity value is depressed monotonously uponincreasing pressure. However, no superconducting transitionhas been observed with pressure up to 1.90GPa, while the8K anomaly is always preserved. Since the 180K transitionis so sensitive to pressure, we deem that it is merely a valencechange transition and may not relate to the formation of anylong rangemagnetic order. In addition, the 180K anomaly cancompletely recover when pressure is removed (see Figure 3,the red curve); this reminds us of some pressure-inducediron-based superconductors, like CaFe2As2, in which thecrystal structure will completely recover with pressureremoved [11].It is reported that the transition at 180K is accompaniedwith a giant volume collapse of 1.9% caused by a strongcontraction of the 𝑐-axis (2.3%) between 180K and 150K [3–5]; on the other hand, the lattice parameters will experience astrong but continuous decrease with increasing pressure [12].Under the above considerations, it is reasonable to assumethat the pressure we applied can result in the volume collapseat room temperature, which may induce the valence shiftto 2.4 to occur at much higher temperature. Thus whenapplying pressure, the behaviour of the resistivity curve is wellpreserved below 180K, while the 180K transition disappearsand the curve above 180K extends smoothly from that atlower temperature.The resistivity of the system is generally about 100–300 𝜇Ωcm, which manifests that the sample can be catego-rized as a badmetal. Since Euhas usually an ionic state of Eu2+and/or Eu3+, while the As can stay at several anionic statessuch as As3− and As1−, in this case the ionic state of Pd can beAdvances in Condensed Matter Physics 30 10 20 30 40 50T (K)12345678910𝜒(emu/mol)ZFC 20OeFC 20Oe(a)T (K)500 100 150 200 250 300100 200 300T (K)010234×103𝜒(emu/mol)1T0.0000.0010.0020.0030.0040.0051/𝜒(mol/emu)(b)Figure 2: (a) Temperature dependence of dc magnetic susceptibility for EuPdAs as measured at an applied magnetic field of 20Oe in thetemperature range of 4.5 K to 50K. Both magnetic susceptibilities measured in zero-field-cooled (ZFC) and field-cooled (FC) modes areshown. (b) Temperature dependence of dc magnetic susceptibility for EuPdAs under 1𝑇 in the temperature range of 4.5 K to 300K.The insetshows 1/𝜒 versus 𝑇 to clarify the 180K anomaly.50 100 150 200 250 300T (K)00.10.20.30.40.50.60.70.80.9𝜌(𝜇Ωcm)0GPa0GPa-pressure removedFigure 3: Temperature dependence of electrical resistivity forEuPdAs in the temperature range 2.3 K to 300K (the black curve).The red curve is the result measured after pressure is removed(sample still immersed in the pressure transmitting medium).widely tuned.This provides a good way to tune the electronicstates of thematerials.Thismay provide an explanation to thetransition at around 180K, which is most probably inducedby a valence change effect. From our recent study, one can seethat the magnetic moment detected in the material is quiteweak, either from the Pd or Eu ions. Nor has the long rangemagnetic order formation been foundon the Pd ions. It wouldbe interesting to use chemical doping to tune the electronicstate 4d orbital in order to achieve a good balance betweenthe itinerancy and local magnetism.0.90.80.70.60.50.40.30.20.10.00.00GPaGPaGPaGPaGPa0.481.051.501.9050 100 150 200 250T (K)0𝜌(𝜇Ωcm)Figure 4: Temperature dependence of electrical resistivity forEuPdAs at various pressures in the temperature range 2.3 K to 250K.4. ConclusionIn summary, we have synthesized polycrystalline samples ofEuPdAs by solid state reaction method. Through resistivityand magnetic susceptibility measurements, we find thatEuPdAs shows a phase transition at 180K, as well as a secondphase transition below 10K at ambient pressure. With appli-cation of hydrostatic pressure, the transition at 180K is totallysuppressed under a pressure of 0.48GPa, and the resistivitycurve is depressed monotonously upon increasing pressure.However, superconductivity has not been induced at lowtemperature with pressure up to 1.90GPa. In order to achieve4 Advances in Condensed Matter Physicssome novel electronic properties, such as superconductivity,we suggest that tuning the occupation of the 4d orbitals of Pdwith electrons would be essential.Conflict of InterestsThe authors declare that there is no conflict of interestsregarding the publication of this paper.References[1] L. J. Li, Y. K. Luo, Q. B. Wang et al., “Superconductivity inducedby Ni doping in BaFe2As2single crystals,” New Journal ofPhysics, vol. 11, Article ID 025008, 2009.[2] P. C. Canfield and S. L. Bud’ko, “FeAs-based superconductivity:a case study of the effects of transition metal doping onBaFe2As2,” Annual Review of Condensed Matter Physics, vol. 1,pp. 27–50, 2010.[3] G. Michels, S. Junk, N. Lossau et al., “Mixed valency andfirst order phase transition in EuPdAs,” Zeitschrift fu¨r Physik BCondensed Matter, vol. 86, pp. 53–58, 1992.[4] D. Johrendt and A. Mewis, “Darstellung und kristallstrukturender verbindungen SEPdAs (SELa-Lu),” Journal of Alloys andCompounds, vol. 183, pp. 210–223, 1992.[5] G. Michels, C. Huhnt, W. Scharbrodt et al., “Temperatureand pressure driven valence change in ternary Eu-pnictides,”Physica B: Physics of Condensed Matter, vol. 206-207, pp. 408–411, 1995.[6] H.-H. Klauß, M. Hillberg, W. Wagener et al., “Valence fluctua-tions in ternary europium compounds,” Hyperfine Interactions,vol. 104, no. 1–4, pp. 171–175, 1997.[7] V. Kataev, G. Khaliullin, G. Michels et al., “ESR study of theelectronic properties of ternary europium pnictides,” Journal ofMagnetism andMagnetic Materials, vol. 137, no. 1-2, pp. 157–166,1994.[8] J. Paglione and R. L. Greene, “High-temperature superconduc-tivity in iron-based materials,” Nature Physics, vol. 6, no. 9, pp.645–658, 2010.[9] F. Wang and D.-H. Lee, “The electron-pairing mechanism ofiron-based superconductors,” Science, vol. 332, no. 6026, pp.200–204, 2011.[10] H. Fukazawa, N. Takeshita, T. Yamazaki et al., “Suppression ofmagnetic order by pressure in BaFe2As2,” Journal of the PhysicalSociety of Japan, vol. 77, no. 10, Article ID 105004, 2008.[11] H. Lee, E. Park, T. Park et al., “Pressure-induced superconduct-ing state of antiferromagnetic CaFe2As2,” Physical Review B—Condensed Matter and Materials Physics, vol. 80, no. 2, ArticleID 024519, 2009.[12] C. Huhnt, G. Michels, W. Schlabitz, D. Johrendt, and A. Mewis,“Pressure-driven valence change in ternary Eu pnictides,” Jour-nal of Physics Condensed Matter, vol. 9, no. 45, pp. 9953–9960,1997.Submit your manuscripts athttp://www.hindawi.comHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014High Energy PhysicsAdvances inThe Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014FluidsJournal of Atomic and  Molecular PhysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Advances in  Condensed Matter PhysicsOpticsInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014AstronomyAdvances inInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014SuperconductivityHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Statistical MechanicsInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014GravityJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014AstrophysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Physics Research InternationalHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Solid State PhysicsJournal of Computational  Methods in PhysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Soft MatterJournal ofHindawi Publishing Corporationhttp://www.hindawi.comAerodynamicsJournal ofVolume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014PhotonicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Journal ofBiophysicsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014ThermodynamicsJournal of

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