Effects of ordered structure on superconductivity in Ti70−xZr30Nbx (15≤ x ≤ 60)

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2012 EPL 97 17013

(http://iopscience.iop.org/0295-5075/97/1/17013)

Download details:

IP Address: 128.143.22.132

The article was downloaded on 19/03/2013 at 07:13

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

January 2012

EPL, 97 (2012) 17013 www.epljournal.org

doi: 10.1209/0295-5075/97/17013

Effects of ordered structure on superconductivityin Ti70−xZr30Nbx (15 x 60)Z. W. Wang

1, Y. Li

2, C. Ma

1, Z. Wang

1, H. X. Yang

1, H. F. Tian

1, H. L. Shi

1, L. Jia

1, F. Zhang

2

and J. Q. Li1(a)

1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of SciencesBeijing 100190, PRC2 School of Materials Science and Engineering, Beihang University - Beijing 100191, PRC

received 17 September 2011; accepted in final form 25 November 2011published online 4 January 2012

PACS 74.62.Bf – Effects of material synthesis, crystal structure, and chemical compositionPACS 74.20.Pq – Electronic structure calculationsPACS 61.05.J- – Electron diffraction and scattering

Abstract – Superconductivity and structural properties of Ti70−xZr30Nbx have been systemati-cally investigated for x ranging from 0 to 60. Superconductivity is observed in the cubic β-phasewith 10 x 60. Moreover, evident modifications in superconductivity and superstructure, beinginterpreted as Nb and Ti local orders, are discovered in samples with 25 x 50. This super-structure phase in general coexists with the cubic β-phase and yields two superconducting transi-tions in the superconducting materials. Electronic structure calculations reveal a relatively higherdensity of states for the superstructure phase at the Fermi level, therefore a strong electron-phononcoupling and higher superconducting Tc are expected in agreement with the experimental data.

Copyright c© EPLA, 2012

Superconductivity and the high critical fields (Hc) forthe Nb-based binary solution alloys have been of techno-logical and academic interest for decades, as alloy of thiskind generally have excellent mechanical properties andgood fabricability and therefore facilitate the manufac-ture of conductors for large superconducting devices [1–4].On the other hand, these alloys, especially the Ti-basedmaterials, have been also widely used because of theiroutstanding shape memory effect, superior corrosion resis-tance, good biocompatibility, and rich structural transfor-mation behaviors [5–8]. In structural point of view, thesealloys mostly exhibit two stable structures, i.e. the β phasewith body-centered-cubic (bcc) structure and the α phasewith hexagonal closed-pack (hcp) structure. Occasionally,certain metastable phases can be also observed in thesematerials. In previous publications, superconductivity andthe high-pressure behaviors of binary (Nb, Ti)-based alloyshave been extensively studied [9–12]. Certain notable dataon the transport properties and structural transformationspresented evident experimental supports to the theoreti-cal idea based on the electronic band-structure calcula-tions for transition metals. For instance, Bashkin et al. [13]reported that the structural transition in TiZr alloy to the

(a)E-mail: ljq@aphy.iphy.ac.cn

β phase is in connection with an increase of the super-conducting Tc under a high pressure. However, only afew studies have focused on the superconductivity in Ti-Zr-Nb ternary alloys [14] in correlation with microstruc-ture features and structural transformations. Recently, ourstudy revealed that the Ti70−xZr30Nbx system adopts acubic structure (the β-phase) and shows up clear super-conductivity for x within a large range. In this paper,we report the superconducting and microstructure proper-ties observed in Ti70−xZr30Nbx (10 x 60) materials, inparticular, effects of structural order on superconductivityare discussed based on microstructure analysis.All Ti70−xZr30Nbx (x= 5, 10, 15, 20, 25, 30, 35, 40, 45,

50, 55, and 60) alloys were prepared under an ultra-pureargon atmosphere as reported previously in ref. [15]. Thetemperature dependence of electrical resistivity ρ(T ) wasmeasured by a standard four-point probe method, and theapplied magnetic field effect on ρ was measured using aquantum design physical property measurement system.The temperature dependence of AC magnetic susceptibil-ity χ(T ) measurements were performed by using QuantumDesign SQUID magnetometer at a magnetic field of 1Oewith the frequency of 333Hz. Samples for TEM observa-tions were prepared by the conventional method includingcutting, mechanical polishing, dimpling and finalized by

17013-p1

Z. W. Wang et al.

0.00

0.05

0.10

0.15

6 8 10 12 14-2.5

-2.0

-1.5

-1.0

-0.5

0.0x = 35

χ / 1

0-3 e

mu

/mo

l

T (K)

x = 45

(d)10 20 30 40 50 60

6

8

10

12

Tc1

Tc2

Tc /

K

Nb content (at%)

(b)

3 6 9 12 15

0.0

0.2

0.4

0.6

0.8

1.0

R(T

) / R

(15K

)

T (K)

x = 5x = 15x = 25x = 45

Tc

(a)

6 8 10 12 14

0.0

0.1

0.2

0.3

0.4

0.5

0.6 x = 45

ρ / m

Ω.c

mT (K)

x = 35T

c2 = 10.4K

Tc1

= 11.7K

(c)

Fig. 1: (Colour on-line) (a) Temperature dependences of resis-tivity for typical Ti70−xZr30Nbx samples, illustrating supercon-ducting transition temperature from 4.5K to 11.7K. (b) Therelationship between the Tc and Nb content of Ti70−xZr30Nbxsamples. (c) Temperature dependence of resistivity and (d) ACmagnetic susceptibility for x= 35 and 45, clearly exhibiting twosuperconducting transitions.

Ar+ ion beam thinning in the liquid nitrogen condition.Structural investigations were carried out on a FEI Tecnai-F20 transmission electron microscope (TEM) operating at200 kV.X-ray diffraction measurements were firstly used to

characterize the structure of Ti70−xZr30Nbx samples withx= 5, 10, 15, 25, 45, and 55, respectively. The resultsdemonstrate that the crystal structure shows clear changeswith the increase of Nb content. The sample with x= 5has a hexagonal structure (the α′ phase), and the x= 10sample shows certain complex structural features due tothe coexistence of a cubic structure (the β phase) andan orthorhombic structure (the α′′ phase). Samples withx ranging from 15 to 50 have the cubic structure and noimpurity phases are observed. These facts suggest that theNb substitution for Ti is of benefit for the stability of theβ phase, a similar phenomenon was also observed in thebinary Ti-Nb alloys in which Nb atoms play a key rolefor the stability of the cubic β-phase [16]. On the otherhand, samples with x 55 show visible diffraction peaksfrom impurities. Therefore, in the following study we willmainly focus on the structural and physical properties forsamples with 15 x 55.In fig. 1(a) we present the temperature dependence of

resistivity for Ti70−xZr30Nbx samples with various Nbconcentrations at zero magnetic field, demonstrating clearchanges of superconducting transition temperatures insamples with x= 5, 15, 25, and 45. Our careful investiga-tions show that the alloys with the cubic structure (15x 55) exhibit clear superconductivity with Tc ranging

from 4.5K to 11.7K, we herein choose Tc as the onsettemperature of the downturn in ρ(T ) curves. The magneticsusceptibility of the superconducting samples has beenalso measured under a field of 1Oe, sharp drops indicatingthat the magnetic onset of superconductivity transitionsare in agreement with the data from resistivity measure-ments, the Tc in these samples increases gradually with therise of Nb content as clearly illustrated in fig. 1(b). It isalso noted that these superconductors above Tc often showa metallic behavior in the normal state and the supercon-ducting transitions in general are very sharp with the tran-sition widths less than 0.5K for most samples. Sampleswith a low Nb content (x 5) have the hexagonal struc-ture and no superconductivity is found down to 4.2K. Onthe other hand, samples with a high Nb content of x 60often contain an impurity phase and show complex struc-tural and transport properties. Another striking phenom-enon revealed in our study is the appearance of two super-conducting transitions in samples with 25 x 50 as typi-cally illustrated in fig. 1(c) for two well-characterizedsamples with x= 35 and 45. Figure 1(d) presents thetemperature dependence of AC magnetic susceptibility forsamples of x= 35 and 45, two superconducting transitionsfrom both the real part (χ′) and the imaginary part (χ′′)of the susceptibility data are clearly recognizable. Theseresults are in good agreement with what observed fromresistivity measurements as shown in figs. 1(a) and (b).In addition, the superconducting volume fractions of thehigh-Tc phase have been estimated to be about 11% and9% for samples of x= 35 and 45, respectively. Our forth-coming study suggests that this phenomenon is essentiallyin correlation with the appearance of a local structuralorder as discussed in the following context based on TEMobservations. Moreover, it is noted that Ralls et al. haveperformed extensive studies on the high-field supercon-ductivity of the Nb-Zr-Ti ternary system; it was pointedout that the critical field as a function of ternary compo-sition has a saddle point between the Ti-Nb and Zr-Nbbinary alloys, and the ternary system was not expectedto show relatively higher critical fields than the binarysystems [17]. Considering the two superconducting tran-sitions in correlation with the ordered structure in oursamples, we have been carrying out a further study onthe critical fields and related properties in certain well-characterized samples as will be reported in a paper inpreparation.TEM observations and selected-area electron diffrac-

tion analysis on samples with single superconducting tran-sition show a well-defined cubic structure, and, on theother hand, materials with double superconducting transi-tions often reveal certain complex microstructure features.Figures 2(a)–(d) show the electron diffraction patterns forx= 45 with two sharp superconducting transitions, takenrespectively along the [101] and [111] zone axis direc-tions; all diffraction spots in figs. 2(a) and (b) can bewell indexed by a cubic β-phase with lattice parametera= 0.35 nm in good agreement with the X-ray diffraction

17013-p2

Effects of ordered structure on superconductivity in Ti70−xZr30Nbx (15 x 60)

Fig. 2: (Colour on-line) Electron diffraction patterns (a) and(b) for the cubic β-phase and (c) and (d) for the superstructurephase taken, respectively, along (a) the [110] and (b) the[111] zone axis directions. (e) High-resolution TEM imagetaken from a Ti25Zr30Nb45 crystal showing coexistence ofthe β-phase (area A) and the superstructure phase (area B).(f) High-resolution TEM image shows the atomic structure forthe area A. (g) Atomic structure for the superstructure phase;the inset displays the theoretical image in good agreement withthe experimental ones. (h) A simple structural model for thesuperstructure phase.

data. In addition to the cubic phase, a superstructurephase also appears in the materials with double supercon-ducting transitions as observed in x= 25 and 45 samples.Figures 2(c) and (d) show two diffraction patterns withclear superstructure spots in contrast with the diffractionpatterns of figs. 2(a) and (b); these weak superstructurespots commonly appear at the systematic (h, k, l)+ q posi-tions characterized by the wave vector q= (1/2, 1,−1/2).Though we have made numerous attempts to prepare thesingle-phase sample of the superstructure with relativelyhigher Tc, however, our experimental results show that

the superconducting samples often show up as a mixtureof conventional cubic structure phase and superstructurephase, and no pure phase with high Tc is achieved in ourexperiments, as discussed in the following context.Figure 2(e) shows a high-resolution electron micrograph

of Zr25Ti30Nb45 crystal taken along the [111] zone axisdirection, exhibiting the coexistence of the conventionalcubic structure (area A) and the superstructure phase withq= (1/2, 1,−1/2) (area B). This high-resolution TEMimage was obtained from a thin region in the crystal;therefore, in combination with the results of theoreticalsimulations, the atomic structure in this superstructurephase could be identified. Image calculations, based onthe schematic model for the Nb-Ti order together withthe random distribution of Zr atoms, were carried outby varying the crystal thickness and the defocus value.A calculated image with the defocus value of −65 nm,the spherical aberration of 1.2mm, and the thickness of∼ 10 nm is superimposed on the image, and appears to bein good agreement with the experimental one.It is also noted in our study that the ratio of the rela-

tively high and low Tc phases in the samples with 15x 55 depends visibly on the conditions for synthesizingmaterials. In order to enhance the fraction of higher Tcphase, we performed a variety of experiments with differ-ent annealing temperatures on the Ti25Zr30Nb45 sample;the results demonstrate that the material annealed at800 C under high vacuum can increase visibly the portionof high Tc phase. Figure 3(a) shows the XRD pattern forthe annealed sample, a clear diffraction peak appears at2θ= 32.7 which is much stronger than that in the as-prepared samples; this reflection corresponds well withthe double interplanar distance of the (112) plane with aperiodicity of L= 2d112, which is in good agreement withthe electron diffraction results. Figure 3(b) presents thetemperature dependence of the AC magnetic susceptibil-ity, exhibiting two clear superconducting transitions. Andthe superconducting volume fraction of the high Tc phasewas estimated to be about 30%, which is notably largerin comparison with that in the annealed sample as shownin fig. 1(d). The temperature dependence of resistivity isshown in the inset of fig. 3(b), a clear drop of resistanceoccurs at around 11K. Actually, we have also made numer-ous attempts to adjust the fraction of the superstructurephase in our experiments, however, no single phase mate-rials with Tc above 11K were obtained.Theoretical analysis of the superconductivity in this

kind of alloys in correlation with microstructure featuresand structural transitions has been performed in a varietyof systems based on the weak-coupling BCS mechanism[18–20]. The fundamental relationship between crystallattice instabilities and superconductivity is described bythe McMillan equation [18], in which the superconduct-ing critical temperature (Tc) associated with the electron-phonon coupling parameter λ depends strongly on thedensity of states (DOS) at the Fermi energy (EF). Forinstance, in the NbN material, the superconductivity and

17013-p3

Z. W. Wang et al.

Fig. 3: (a) XRD pattern of an annealed Ti25Zr30Nb45 sample, aclear superstructure peak indexed with the twice planar spaceof (112) is indicated by an arrow. (b) Temperature dependenceof AC magnetic susceptibility for this annealed sample, theportion of relatively high Tc phase is enhanced; the insetdisplays the ρ(T ) curve for this annealed sample.

the electron-phonon coupling have been discussed basedon the electronic and phonon stiffness by Chen et al. [21];it is demonstrated that the pressure effects on the super-conducting transition temperature can be understood byalternations of electronic stiffness in niobium nitride.In the present study, our electron structure calcu-

lations were performed by means of the full relativis-tic Korringa-Kohn-Rostocker (SPRKKR) method [19,20].The random distribution of different atoms is treated bythe coherent-potential approximation (CPA). The totaldensity of states (TDOS) for Ti30Zr30Nb40 is presentedin fig. 4(a). A maximum in TDOS, the so-called the VanHove singularity [22], appears nearby the Fermi level EF.When the Zr concentration keeps constant, an increasein the Nb concentration, i.e. the substitution of Nb forTi atoms, results in the visible shift of TDOS towardsthe low energy, because Nb atoms can provide more elec-trons than Ti atoms. Our analysis reveals that the maxi-mum in the DOS for the alloy Ti35Zr30Nb35 occurs around

Fig. 4: (Colour on-line) (a) Total and atom projected density ofstates of the random alloy Ti30Zr30Nb40, calculated by CPA.The dashed line denotes the Fermi energy. (b) Total densityof states for the random alloys Ti70−xZr30Nbx with x being55, 45, 40, 35, 25 and 15. As x decreases, the maximumshifts downward across EF. The inset shows the comparisonof the TDOS for Ti35Zr30Nb35 with a cubic structure andsuperstructure.

EF, suggesting that the maximum Tc could possibly beobtained at this composition, as shown in fig. 4(b). Thisfeature can be used fundamentally to explain the increaseof Tc for x 35 (fig. 1(b)). According to our theoreticalresults on the electronic structures, it is also expected thatTc could show a decrease tendency for x> 35, however,the experimental results demonstrate that Tc almost keepsconstant for x> 35. This discrepancy is considered aris-ing from the coexistence of ordered and disordered struc-tures in samples with x larger than 35. In order to studythe effect of atomic ordering on the superconductivity,we have constructed a 1× 1× 2 supercell in which thedistribution of Zr atoms is random, but the Ti and Nbatoms form a long-range order as discussed in the abovecontext. Compared with the random alloy, the intensity ofthe DOS maximum for the superstructure has a certain

17013-p4

Effects of ordered structure on superconductivity in Ti70−xZr30Nbx (15 x 60)

increase as shown in the inset of fig. 4(b), thus an increaseof Tc could be expected. On the other hand, atomic order-ing could also result in charge redistribution and allowsmore d electrons nearby EF. We have also taken accountof other supercell models with different long-range orders.Compared with the random alloy, an increase in the TDOSat EF is commonly observed for all ordered structures.This result is consistent with our experimental observa-tions which suggest that the superstructure phase has arelatively higher Tc than the cubic β-phase.In conclusion, Ti70−xZr30Nbx shows a rich variety of

superconductivity and microstructure phenomena. Super-conductivity is observed in materials with cubic structurefor x ranging from 10 to 60. Moreover, evident modifica-tions in superconductivity and superstructure, being inter-preted as Nb and Ti orders, are discovered for sampleswith the increase of the Nb content. Coexistence of twosuperconducting phases commonly appears in the sampleswith 25 x 50. Systematical investigations suggest thatthe superstructure phase has a relatively higher Tc. Elec-tronic structure calculations demonstrate a higher TDOSat the Fermi energy for the superstructure phase, there-fore, a strong electron-phonon coupling and a relativelyhigher Tc could be expected arising from atomic orderingin agreement with the experiment data.

∗ ∗ ∗This work was supported by the NSF of China (Grant

Nos. 90922001, 11074292, 11190022), the NationalKey Projects for Basic Research of China from theMOST (Grant Nos. 2011CBA00101, 2010CB923002,2012CB821404, 2006CB921301) and CAS (KJCX-EW-W11).

REFERENCES

[1] Neuringer L. J. and Shapira Y., Phys. Rev., 148(1966) 231.

[2] Berlincourt T. G. and Hake R. R., Phys. Rev., 131(1963) 140.

[3] Vonsovsky S. V., Izyumov Y. A. and Kurmaev E. Z.,in Superconductivity of Transition Metals Their Alloy andCompounds, edited by Brandt E. H. and ZavarnitsynA. P. (Springer, New York) 1982, p. 231.

[4] Poole C. P., Farach H. A., Creswick R. J. andProzorov R., in Superconductivity, 2nd edition (Acad-emic Press, Amsterdam) 2007, p. 71.

[5] Baker C., Met. Sci. J., 5 (1971) 92.[6] Ping D. H., Mitarai Y. and Yin F. X., Scr. Mater., 52(2005) 1287.

[7] Hao Y. L., Li S. J., Sun S. Y., Zheng C. Y., Hu Q.M. and Yang R., Appl. Phys. Lett., 87 (2005) 091906.

[8] Kim H. Y., Ikehara Y., Kim J. I., Hosoda H. andMiyazaki S., Acta Mater., 54 (2005) 2419.

[9] Struzhkin V. V., Timofeev Y. A., Hemley R. J. andMao H. K., Phys. Rev. Lett., 79 (1997) 4262.

[10] Kawamura H., Wittig J., Bireckoven B. and BuckelW., Physica B, 126 (1984) 485.

[11] Akahama Y., Kawamura H. and LeBihan T., Phys.Rev. Lett., 87 (2001) 275503.

[12] Mclnturff A. D., J. Appl. Phys., 40 (1969) 2080.[13] Bashkin I. O., Fedotov V. K., Nefedova M. V.,

Tissen V. G., Ponyatovsky E. G., Schiwek A. andHolzapfel W. B., Phys. Rev. B, 68 (2003) 054401.

[14] DeSorbo W., Lawrence P. E. and Healy W. A., J.Appl. Phys., 38 (1967) 903.

[15] Cui Y., Li Y., Luo K. and Xu H. B., Mater. Sci. Eng.A, 527 (2010) 652.

[16] Kim H. Y., Satoru H., Kim J. I., Hosoda H. andMiyazaki S., Mater. Trans., 45 (2008) 2443.

[17] Ralls K. M., Rose R. M. andWulff J., J. Appl. Phys.,51 (1980) 3316.

[18] McMillan W. L., Phys. Rev., 167 (1968) 331.[19] Ebert H., in Fully Relativistic Band Structure Calcu-

lations for Magnetic Solids - Formalism and Applica-tion, in Electronic Structure and Physical Properties ofSolids, edited by Dreysse H., Lect. Notes Phys., Vol. 535(Springer, Berlin) 1999, p. 191.

[20] Ebert H., The Munich SPRKKR package, version 5.4,http://olymp.cup.uni-muenchen.de/ak/ebert/SPRKKR.

[21] Chen X. J., Struzhkin V. V., Wu Z., Cohen R. E.,Kung S., Mao H. K., Hemley R. J. and ChristensenA. N., Phys, Rev. B, 72 (2005) 094514.

[22] Hove L. V., Phys. Rev., 89 (1953) 189.

17013-p5