Pressure-induced lattice collapse in tetragonal phase and structural phase transition in single crystalline Fe1.05Te
Chao Zhang, Wei Yi, Liling Sun, Wei Lu, Xiaoli Dong, Ligang Bai, Jing Liu, Genfu Chen, Nanlin Wang, Zhongxian Zhao
11 Pressure-induced lattice collapse in tetragonal phase and itsequation of state at 300K in single crystalline Fe Te Chao Zhang , Wei Yi , Liling Sun *, Xiao-Jia Chen , Russell J. Hemley , Ho-kwang Mao ,Wei Lu , Xiaoli Dong , Ligang Bai , Jing Liu , Antonio F. Moreira Dos Santos , Jamie J.Molaison , Christopher A. Tulk , Genfu Chen , Nanlin Wang, and Zhongxian Zhao * Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academyof Sciences, Beijing 100190, P. R. China Geophysical Laboratory, Carnegie Institution of Washington, Washington,DC 20015, U.S.A Department of Physics, South China University of Technology, Guangzhou 510640, P. R. China Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100039, P. R. China Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
Extensive measurements of X-ray diffraction, neutron diffraction, resistance, andmagnetization were performed on single crystalline Fe
Te under pressure. Apressure-induced lattice collapse was observed in tetragonal phase at pressure of 4GPa and at room temperature. The onset temperature of the structural phase transitionwas found to decrease with increasing pressure but increase upon further compressionafter passing through a minimum around the collapse. However, the onset ofantiferromagnetic transition scarcely changes with pressure. No superconductivity wasdetected at pressures up to 20 GPa.Corresponding author:[email protected]@aphy.iphy.ac.cn
The discovery of superconductivity with critical transition of 26 K in LaFeAsO F x [1] expedited the research for this new family of superconductor. Four types ofmaterials, ReFeAsO (Re= Ce, Pr, Nd, Sm etc), AFe As (A=Ba, Sr, Ca ), AFeAs (A=Li,Na) and FeSe(Te), have been discovered [2-4]. Superconductivity in the binary α -FeSe with transition temperature of 8 K was found [4] This compound has PbO-typetetrahedral structure, with stacking FeSe layers along the c -axis. Partial substitution ofTe for Se [5] enhances the Tc to 10 K. This superconducting transition temperature wasfurther increased by application of pressure, reaching 27 K [6] at 1.48 GPa and 36~37K with increasing pressure [7-8]. The compound of α -FeTe has very similar structureto tetragonal FeSe at ambient pressure. Theoretical calculations [9] demonstrated that α -FeTe adopts multiple Fermi surfaces, similar to that of ReFeAsO F x , with holepockets at the zone center and electron pockets at the zone corner. Theantiferromagnetic (AF) transition also can be observed in α -Fe Te, as ReFeAsO andAFe As parent compound exhibited. Neutron scattering studies at ambient pressure[10-11] found that the in-plane spin structure of Fe Te is completely different fromthat of ReFeAsO and AFe As whose moments form a collinear antiferromagnetic structure with spin direction along the a -axis in FeAs layer. Upon decrease temperature down to 65 K, the lattice distortion drives the Fe Te compound from thetetragonal (T) to the monoclinic (M) phase [10-11]. Since the non-superconductingparent compounds of LaFeAsO and BaFe As showed superconductivity under high pressure [12-14], the reported results motivate this investigation to explore pressure effect on structural and magnetic transition as well as pressure-induced potential superconductivity in Fe Te. Here we report high-pressure synchrotron x-raydiffraction, neutron diffraction, resistance and magnetization studies on Fe
Tecompound. A remarkable reduction in c axis has been observed. This is the firstobservation of pressure-induced lattice collapse in the tetragonal phase of Fe
Te atroom temperature. The equations of state of T phase and cT phase were determinedfrom neutron diffraction results. A phase diagram of pressure dependence of the onsettemperature of structural transition was also established through resistancemeasurements.High pressure was created using a diamond anvil cell. Diamonds with lowbirefringence were selected for the high-pressure XRD measurements. Diamond anvilsused were cut with a 300 µ m culet, with a 100 µ m diameter sample hole in a stainlesssteel gasket. The single crystalline sample was loaded into the hole with silicon oil tomaintain the sample in a hydrostatic pressure environment. Pressure was applied in adirection normal to the a-b plane of the sample and determined by ruby fluorescencemethod at room temperature [15]. Angle-dispersive XRD experiments were carried outat the Beijing Synchrotron Radiation Facility (BSRF). A monochromatic x-ray beam with a wavelength of 0.6199 Å was used for all XRD measurements. The XRD images were collected using a charge-coupled device (CCD) detector, and the XRD geometrywas calibrated with CeO .Two individual resistance measurements were carried out in a diamond anvil cell made of Be-Cu alloy. The four-standard-probe technique was adopted in the experiments, as reported in Ref. [16]. No pressure medium was used in high-pressure resistance measurements. The high-pressure resistance as a function of temperaturewas measured using a CSW-71 cryostat. The magnetization measurements underambient pressure were carried out using Quantum Design Magnetic PropertyMeasurement System (MPMS-XL1), and those under hydrostatic pressure wereconducted using a commercial pressure cell ( Mcell 10) specialized for MPMS-XL1.The experimental details can be found in Ref. [17].Time-of-flight neutron diffraction experiments were performed at the SNAP(Spallation Neutrons and Pressure) beamline of the Spallation Neutron Source at OakRidge National Laboratory. To obtain a set of diffraction peaks, the powder sampleground from the same single crystal was used for the measurements. Pressure wasgenerated with diamond anvils in Paris-Edinburgh high-pressure cell. The sample wasloaded into TiZr gasket hole with pressure medium of a 4:1mixture of methanol andethanol. The neutron diffraction data were collected through three banks and averagedfrom them.The sample was synthesized by solid state reaction at ambient pressure, asdescribed in a previous report [18]. Characterization by x-ray diffraction indicated that the sample used in this study is single crystal with tetragonal symmetry, as shown in Fig.1 (a). Both of resistance and magnetization measurements at ambient pressureexhibited anomalies at the same temperature near 65 K, as shown in Fig.1 (b) to (c),consistent with our previous experimental results that the abrupt decrease in resistance and magnetization below 65 K is related to the first-order T-M phase transition and AF transition respectively.
Fig.2 shows x-ray diffraction (XRD) patterns of Fe . Te at ambient and highpressures at room temperature. Inasmuch as the wavelength of synchrotron x-ray beamemployed for high-pressure experiments is different from the wavelength (1.54 Å) ofCu K α radiation used for ambient pressure measurement, we applied d-spacing valuein Fig.2 for comparison, instead of two theta. Only the (003) reflection of the singlecrystalline sample can be detected over the angular range available at the experimentaldiffraction conditions. No new peaks were found in the diffraction under pressure up to11.5 GPa, however, remarkable shift of peak (003) was observed during compressionto 4 GPa, as displayed in Fig.2 (a). High-pressure neutron diffraction (ND)experiments were carried out for the polycrystalline Fe Te sample in order to furtherconfirm the observed lattice distortion. The ND patterns were shown in Fig.2 (b). Allpeaks collected at 0.7 GPa can be well indexed as tetragonal form. It was found thetetragonal phase persists up to 7 GPa, entirely consistent with our XRD results. Wecomputed lattice parameter c and a value on basis of ND patterns obtained at differentpressures and summarized the data together with the c value calculated from XRDpattern in Fig.3. Apparently, two sets of c values derived from XRD and ND measurements are in good agreement. A large reduction in c axis was observed under applied pressure, as shown in Fig.3(a). The c value is reduced about 5% at ~4 GPa. Thestriking reduction in c direction suggested that applied pressure drives a latticecollapse in the tetragonal phase of Fe Te. To distinguish pressure-induced collapsed tetragonal phase from the tetragonal (T) phase, here we defined the high-pressure phase as cT phase. Pressure dependence of parameter a is plotted in Fig.3 (b). We noted that the response of a to pressure is different from parameter c , a value decreaseswith pressure linearly. The unit cell volumes (V) as a function of pressure (P) in the Tand cT phase were plotted in Fig.4. No visible discontinuity was found, furthersupporting that no first-order phase transition occurred under high pressure at least to7 GPa. The data was fitted by the third order Birch-Murnaghan equation of state [19]: ⎥⎦⎤⎢⎣⎡ −−+⎥⎦⎤⎢⎣⎡ −= −−− )1)/)((4(431)/()/(23 VVBVVVVBP
Where B is the isothermal bulk modulus at zero pressure, B ' is the pressurederivative of B evaluated at zero pressure, and V/V is the ratio of high-pressurevolume and zero-pressure volume of the sample. The resulting parameters are listed inTable 1. We obtained B = 31.3 ± B ' = 6.6 ± B =86.7 ± V = 88.7 ± for the cT phase when B ' = 4 was fixed. Theseresults provide the crucial information for future theoretical and experimentalinvestigations of the high pressure-behavior of FeTe parent compound.To probe the electronic properties in the T and cT phase, and track the evolution ofthe onset temperature of structural transition with pressure, two separate high-pressureresistance measurements were carried out in a diamond anvil cell. Fig. 5 shows therepresentative electrical resistance (R) of the Fe Te sample as a function oftemperature (T) under high pressure up to 20 GPa. It was seen that the breadth of thetransition of the compressed sample became broadened with increasing pressure. Tomake the data comparable, we define the maximum of dR/dT as the onset transitiontemperature. The onset temperature of the phase transition is pressure sensitive, shiftsto low temperature side during compression, as shown in Fig.5 (a). Interestingly, the onset temperature shifted back to higher temperature side at 4 GPa where is the criticalpressure of the T-cT phase transition. This upward shift of the onset is obvious in theR-T plot as displayed in Fig.5 (b). With further applying pressure up to 14.7 GPa, thesample lost its metallic character, behaving like a semiconductor. This dramatic changein electronic properties may be related to an additional phase transition. Polycrystallinex-ray diffraction at higher pressure (~14 GPa) is necessary to clarify this phenomenon.No superconductivity was observed upon uploading, in agreement with high-pressureresistance measurements for FeTe recently reported by Okada et al [20].Downloading from the maximum pressure investigated down to 4.7 GPa, we found thesample still hold semiconductor feature, as illustrated in Fig.5 (c). One potential reasonfor the observed irreversibility is due to a hysteretic effect from the proposedadditional phase transition at the pressure where the sample lost its metallic behavior,as mentioned above. Another reason may be from the stress effect on the sample whichcan not recover back during releasing pressure down to 4.7 GPa.Inasmuch as the structural and magnetic transition in Fe
Te take place at thesame temperature (65K), we can not conclusively determine if the change in the onset transition temperature with pressure corresponds to the onset of T-cT phase transition.
In order to distinguish pressure effect on the onset of T-cT phase and AF transitionseparately, we performed magnetization measurements under pressure for the Fe
Tesample. Fig.6 shows the temperature dependence of magnetization under fixed pressures. It was seen that the onset temperature of the AF transition (T AF ) remains nearly unchanged within resolution with increment of pressure to 0.8 GPa which is the highest pressure (~1GPa) of our magnetization measurement available. Next, wecompared pressure-induced R-T and M-T shift at the similar pressure level and foundthere is 6.3 K shift of R-T curve at 0.6 GPa while little shift of the M-T curve at 0.8GPa, strongly implying that the onset detected by resistance measurements is assignedto structural phase transition rather than magnetic transition. Here we use T STR torepresent the onset temperature of structural phase transition.The T
SRT , together with resistance data obtained at 150 K, as a function ofpressure were plotted in Fig.7. Clearly, a notable change can be seen upon traversingof the T-cT phase. Firstly, the pressure dependence of T
STR suddenly changed at the T-cT phase boundary. The T
STR decreases with the pressure in the T phase, and in turnincreases in the cT phase. Secondly, the pressure dependence of the resistance has asimilar sign change at the boundary. The collapse tetragonal phase was also found inCaFe As under applied pressure to 2.5 GPa where c parameter is reduced about ~1 (Å)[21-22]. Theoretical calculation [23] and inelastic neutron scattering experimentalresults under pressure [24] explained this strange behavior by that strong suppressionof Fe-spin state reduced Fe-As bonding and enhanced corresponding As-As bonding. A striking increase in conductivity of CaFe As upon traversing from T phase to cT phase has been observed experimentally [25-28] which is consistent with our results inwhich the resistance decreases from T phase to cT phase with pressure. Theenhancement of conductivity upon travelling from T phase to cT phase of CaFe As is considered to be relevant to Fe-spin state in FeAs layer [25-28]. Weak spin state of Fe atom diminishes electron scattering which is directly responsible for the conductivity. We found there is a different manner in the cT phase. The resistance increases withpressure. We consider this change may be attributed to impurity of excess Fe inFe
Te compound. In fact, there is a competition of spin state between excess Fe andFe in FeTe after the lattice collapse. Under high pressure, remarkable reduction in c axis also shortened the distance of the excess Fe atoms. It is most likely that the spinstate of excess Fe was not suppressed by increment of pressure. In contrast, reductionin distance between excess Fe atoms enhances the electron scattering, which results inincrease in resistivity. Further study is needed to identify these important problems,particularly neutron scattering experiments under high pressure and low temperature. AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements
We acknowledge interesting discussions with Prof. Pengcheng Dai and Prof. XiDai. This work was supported by the NSFC Grants No. 10874230, 10874211,10804127, and 10874046, by the 973 project and Chinese Academy of Sciences. Thework at Carnegie was supported by the U.S. DOE-NNSA (DEFC03-03NA00144).
SNAP was supported by the scientific user facilities division of the U.S. DOE-BES at the Spallation Neutron Source. We acknowledge the support from EU under theproject CoMePhS. ReferencesReferencesReferencesReferences [1] Y. Kamihara, T. Watanabe, M. Hirano and H. Hosono, J. Am,Chem. Soc. , , 2215(2008); G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P. Zheng, J. L.Luo, N. L. Wang , Phys. Rev. Lett. , 247002 (2008); X. H. Chen, T. Wu,G. Wu, R. H. Liu, H. Chen, D. F. Fang, Nature, , 7196 (2008).[2] M. Rotter, M. Tegel, I. Schellenberg, W. Hermes, R. Pöttgen, D. Johrendt,Phys. Rev. B , 020503(R) (2008); K. Sasmal, B. Lv, B. Lorenz, A. Guloy, F. Chen, Y. Xue, C. W. Chu, Phys. Rev. Lett. , 107007 (2008). [3] X. C. Wang, Q. Q. Liu, Y. X. Lu, W. B. Gao, L. X. Yang, R. C. Yu, F. Y. Li, C. Q.Jin, arXiv: 0806.4688; D. R. Parker, M. J. Pitcher, P. J Baker, I. Franke, T.Lancaster, S. J. Blundell, S. J. Clarke,Chemical Communications, 2189 (2009)or arXiv: 0810.3214; M. J. Pitcher, D. R. Parker, P. Adamson, S. J. C.Herkelrath, A. T. Boothroyd, S. J. Clarke, Chemical Communications,5918(2008) or arXiv: 0807.2228; C. W. Chu, F. Chen, M. Gooch, A. M.Guloy, B. Lorenz, B. Lv, K. Sasmal, Z. J. Tang, J. H. Tapp and Y. Y. Xue,arXiv: 0902.0806.[4] F. C. Hsu, J. Y. Luo, K. W. Yeh, T. K. Chen, T. W. Huang, P. M. Wu, Y. C. Lee, Y.L. Huang, Y. Y. Chu, D. C. Yan and M. K. Wu, PNAS, , 14262 (2008).[5] M. H. Fang, L. Spinu, B. Qian, H. M. Pham, T. J. Liu, E. K. Vehstedt, Y. Liu andZ. Q. Mao, arXiv: 0807. 4775.[6] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. , 152505 (2008). [7] S. Medvedev, T. M. McQueen, I. Trojan, T. Palasyuk, M. I. Eremets, R.J. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, C. Felser, arXiv:0903.2143.[8] S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi, Y. Takano, T.Kagayama, T. Nakagawa, M. Takata, and K. Prassides, arXiv:0903.2204.[9] A. Subedi, L. Zhang, D. J. Singh, M. H. Du. Phys. Rev. B , 134514 (2008).[10] S. L. Li, C. Cruz, Q. Huang, Y. Chen, J. W. Lynn, J. P. Hu, Y. L. Huang, F. C.Hsu, K.W. Yeh, M. K. Wu, P. C Dai, Phys. Rev. B , 054503 (2009).[11] W. Bao, Y. Qiu, Q. Huang, M.A. Green, P. Zajdel, M. R. Fitzsimmons, M.Zhernenkov, M. Fang, B. Qian, E. K. Vehstedt, J. Yang, H. M. Pham, L. Spinu,
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Te obtained at ambientpressure and room temperature with Cu K α radiation. (b) and (c) resistance andmagnetization as a function of temperature of the sample at ambient pressure, showingstructural (T-M) phase transition and antiferromagnetic transition at same temperaturenear 65 K. Fig.Fig.Fig.Fig. 2222 (a) Representative high-pressure XRD patterns of Fe
Te obtained with amonochromatic beam ( λ = 0.6199 Å) at 300K; stars * indicate diffraction peaks fromthe metal gasket; labeled peak is from the sample. (b) high-pressure ND patterns of the polycrystalline sample at different pressures. Fig.Fig.Fig.Fig. 3333
Pressure dependence of lattice parameter c/c (a) and a/a (b), showing a largelattice distortion in c axis in T phase. Fig.Fig.Fig.Fig. 4444
Unit cell volume of T phase and cT phase as a function of pressure; solid circles,experimental data; curves, third order Birch-Murnaghan equation of state fit to the dataobtained from ND measurements.
Fig.Fig.Fig.Fig. 5555
Resistance (R) versus temperature (T) at different pressures. (a) R-T curvesshift to lower temperature side with pressure. (b) Inverse shift of R-T curves withincreasing pressure. (d) R-T curves upon downloading from the maximum pressuredown to 4.7 GPa. The inset of the figure (b) shows the definition of onset transitiontemperature.
Fig.Fig.Fig.Fig. 6666
Temperature dependence of magnetization of Fe
Te at four pressures (ambient,0.1, 0.5 and 0.8 GPa), showing onset T AF remains nearly unchanged with appliedpressure up to 0.8 GPa under 1 Tesla. Fig.Fig.Fig.Fig. 7777
Pressure-temperature phase diagram constructed from resistance, XRD and ND measurements. The red solid circle represents T STR and the purple represents the R .
20 40 60 80 I n t en s i t y ( a r b . un i t )
001 002 003 004 005
Two Theta (deg.) (a) Fe Te R e s i s t an c e ( O h m ) Temperature (K)
65 K65 K65 K65 K (b) M / H ( - e m u / g ) H // c H=1T FC ZFC (c)
65 K65 K65 K65 K Fe Te Fig. 1 ** * *** ** * ** * * ( ) * ** d-spacing (Angstrom) (a) Ambient ( ) (b) ( )( )( )( )( ) Fig.2 a / a c / c Pressure(GPa)
ND XRD (a) (b) ND Fig. 3 U n i t c e ll v o l u m e ( A ) Pressure(GPa) Fe Te at 300K
Fig.4 Ambient 0.6 GPa 1.1 GPa 1.7 GPa 2.8 GPa 3.3 GPa 4 GPa 7.6 GPa R / R T(K)
Uploading (a) d R / d T Uploading (b)
T/K
Downloading (c) Fe Te
58K @7.6GPa
Fig. 5
50 100 150 200 250 3000.000.060.120.180.240.30
Ambient 0.1GPa 0.48GPa 0.8GPa M - M K ( e m u / g ) Temperature (K)
Fig. 6 T S T R ( K ) Pressure (GPa) R e s i s t an c e a t K ( O h m ) T phase cT phase
Fig.7 TableTableTableTable 1111
Bulk modulus B , volume at ambient pressure V and pressure derivative B ' of tetragonal phase T and collapse tetragonal phase cT in Fe Te compound.P (GPa) Phase B V (Å ) B ' ± ± ± ±±