Superconductivity in efficient thermoelectric Cu$_3$Sb$_{0.98}$Al$_{0.02}$Se$_4$

3.2 MB·English

by  Xiao-Miao Zhao

Superconductivity in Efficient Thermoelectric Cu Sb Al Se
3 0.98 0.02 4
Xiao-Miao Zhao,1,2 Yong-Hui Zhou,3,1,4 Qiao-Wei Huang,1 Viktor V. Struzhkin,4 Ho-Kwang Mao,4,1
Alexander G. Gavriliuk,5,6 Xiao-Ying Qin,3 Di Li,3 Xi-Yu Li,3 Yuan-Yue Li,3 and Xiao-Jia Chen1,3,4,∗
1Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China
2Department of Physics, South China University of Technology, Guangzhou 510640, China
3Key Laboratory of Materials Physics, Institute of Solid State Physics,
Chinese Academy of Science, Hefei 230031, China
4Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, U.S.A.
5Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, Moscow 119333, Russia
6Institute for Nuclear Research, Russian Academy of Sciences, Troitsk, Moscow 142190, Russia
5
(Dated: January 15, 2015)
1
0
Bothsuperconductivityandthermoelectricityofferpromisingprospectsfordailyenergyefficiency
2
applications. The advancements of thermoelectric materials have led to the huge improvement of

n the thermoelectric figure of merit in the past decade. By applying pressure on a highly efficient
a thermoelectric material Cu Sb Al Se , we achieve dome-shape superconductivity developing
3 0.98 0.02 4
J at around 8.5 GPa but having a maximum critical temperature of 3.2 K at pressure of 12.7 GPa.

3 Thenovelsuperconductorisrealizedthroughthefirst-orderstructuraltransformationfromitsinitial
1 phasetoanorthorhombicone. Thesuperconductingphaseisdeterminedintheultimateformation
of the Cu-Al-Sb-Se alloy.

]
n
PACSnumbers: 73.63.Bd,64.70.Nd,74.70.Ad,74.62.Fj
o
c
-
r Both superconductivity and thermoelectricity are two effect,respectively. Theefficiencyofathermoelectricma-
p
important phenomena for energy efficiency applications. terialischaracterizedbythedimensionlessfigureofmerit
u
The discovery of superconductivity at remarkably high zT which is determined by the electrical conductivity,
s
. temperaturesinthelayeredcopperoxides[1]hasspurred Seebeck coefficient, and thermal conductivity at given
t
a manysubsequentdiscoveriesofnovelexoticsuperconduc- temperature. The recent achievement of high zT ma-
m
tors such as the two-band superconductor MgB with a terials benefited from the technique developments such
2
- critical transition temperature T as high as 40 K [2], as disorder within the unit cell [18], supplerlattices [19],
d c
exotic superconductivity in the heavy fermion supercon- complex unit cells [20], nanostructures [21, 22], distor-
n
o ductor PuCoGa5 with a Tc of 18.5 K [3] (which is an tion of the electronic density of states [23], and ultralow
c order of magnitude higher than previously reported for thermal conductivity materials [24]. The primary con-
[ this type of superconductor), and high-T superconduc- tribution to the zT as high as the record 2.6 [24] comes
c

1 tivity in iron pnictides and chalcogenides [4] in which fromthelowthermalconductivitybyscatteringphonons.
v many structural, magnetic, nematic orders coexist and ThezT valueofcommercialmaterialhasbeenlimitedto
2 compete with superconductivity. In addition, supercon- aboutunitinalltemperaturerangesoverearlyahalfcen-
8
ductivity has been discovered in carbon compounds like tury [17]. Cu SbSe , a narrow bandgap semiconductor
3 3 4
boron-doped diamond (11 K) [5], fullerides (33 K in with a bandgap of 0.1-0.4 eV [25], is being examined as
3
0 CsxRbyC60) [6], and borocarbides (up to 16.5 K with an efficient thermoelectric material due to the following
. metastablephasesupto23K)[7]aswellasinpolycyclic facts. First, it is Pb or Te-free thermoelectric material
1
aromatic hydrocarbons with a relatively high T of 37 K withlesstoxicandmucheasiertohandlecomparedwith
0 c
5 [8, 9]. Pressure was found to play an important role in otherthermoelectricmaterials. Second,elementalsubsti-
1 inducting superconductivity and enhancing T in these tutioninCu SbSe hasprovedtobethemosteffectivein
c 3 4
v: materials [10–15]. For examples, the first organic super- raising zT, which increases from 0.7 (Cu3Bi0.02Sb0.98Se4
i conductor was discovered in charge-transfer salts under at600K)to1.05(Cu Sn Sb Se at690K)[26,27].
X 3 0.02 0.98 4
pressure[10]. Theapplicationofpressurehasalsodriven Third,nanostructuredCu SbSe [26]possessesthelower
3 4
r
Cs C from insulator to superconductor, with the high- thermalconductivityandthushashigherzT,threetimes
a 3 60
estT of38Kinfullerides[11]. TherecordhighT sof29 as large as that of the bulk material. Finally, this com-
c c
K in element superconductors [12] and 164 K in copper poundhashighelectricalconductivity,whichcontributes
oxides [15] were achieved at high pressures. The recent to its zT as high as 1.2 at 550 K [28].
breakthrough in discovering superconductivity at 190 K InthisLetter,weshowthatpressuremanipulateselec-
inH-Ssystem[16]furtherhighlightstheroleofpressure. tronicbehaviorofanaluminium-dopedCu SbSe ,abulk
3 4
All these discoveries serve as springboards for the search materialwithhighthermoelectricefficiency,anddrivesit
for new superconducting materials. to be a superconductor. The electrical transport, struc-
Thermoelectricity is about converting heat into elec- tural, and vibrational properties of this material are in-
tricity and vice-versa using the Seebeck and the Peltier vestigated by the combination of resistivity, synchrotron
2

X-raydiffraction(XRD),andRamanscatteringmeasure-
8 0
ments. Wedemonstratethatthepressure-inducedsuper- R /110 (a ) (b )
conductivity is accompanied by the first-order structural
C o m p r e ssio n 2 0 D e c o m p r e ssio n
7 0
transition. We thus obtain a novel superconductor from
a dense high-zT thermoelectric material.
6 0 3 .2 G P a 8 .2
The powder mixture from the very pure ele-
) 3 .6 G P a 1 5 4 .2 1 0 .2
ments with the weighted according to the formula of W 5 0 7 .1 6 .2 1 3 .7
Cpsluoumw3Slpybe0dh.9e8uaAtneldd0e.0rb2ySvae24c0uuowCma/sholffoo1ar0d−4ed52 hPionaut.orsTuqhupeatrsotaz9m0ap0mleospCowuaelnerdes istance (m 4 0 8911 ..0152..11 1121 6804 ....1541 1 0
then held for 12 hours followed by the cooling to 500 es 3 0 1 2 .7 2 4 .5
R
oC (1 oC/min) and the quench in water at room tem- )1 0
pfgooertrsa4t8wurehero.eurpTsuhltveoerspiarzmoemdploientsetowheoprmoewotdgheeenrnseitianyn.naeTnahlaeegdaretasetuml3t0eo0drtaionCr-. 12 00 5 (mResistance W05 0 1 4 . 91 0C1.2o0 m0D pecreosms2pio0rn0ession 300
The bulk samples were obtained by spark plasma sinter- T (K )
0 0
ingfor5minattemperatureof673Kandpressureof50 0 6 1 2 1 8 2 4 3 0 0 6 1 2 1 8 2 4 3 0
MPa. Thesingle-phasestructurewasconfirmedfromthe T (K ) T (K )
XRD measurements. The fractographs were observed by
field emission scanning electron microscopy. The typical FIG. 1: (Color online) Electrical resistance of
Cu Sb Al Se as a function of temperature at var-
grain size of the samples is ∼20 µm. Al substitution for 3 0.98 0.02 4
ious pressures up to 24.5 GPa in both the compression (a)
Sb shrinks the hostlattice and 2%substitution shifts zT
anddecompression(b)runs. Insetshowsthewholemeasured
increase by 30% over wide temperatures up to 600 oC.
temperature range for the electrical resistance at 10.2 GPa
High-pressure electrical resistance measurements were (decompression) and 16.1 GPa (compression).
performed by means of standard four-probe method in a
miniature nonmagnetic diamond anvil cell [29]. A thin GPa. However, a sharp transition with a narrow width
BN layer acted as an electric insulator between the elec- ofaround0.2Kwassoonreacheduponfurthercompres-
trodes leads and the gasket. Diamond anvil cells with sion. The zero resistance is clearly observed, evidencing
T301 stainless steel gasket were used with the anvils in superconductivity in this material. These features indi-
300 µm culet for both XRD and Raman spectroscopy catethegoodhomogeneityofthesuperconductingphase.
measurements. The XRD experiments at high pres- The value of T exhibits a pronounced rise as pressure
c
sures with synchrotron radiation were conducted at the increases from 8.5 GPa, reaches a maximum of 3.2 K at
Beijing Synchrotron Radiation Facility (BSRF) with a 12.7GPa,andthendecreasesgraduallywiththeincrease
wavelength 0.6199 ˚A at room temperature. Silicone oil of pressure. What is truly striking is that the resistance
was loaded as pressure transmitting medium to main- exhibits a semiconducting behavior over the whole tem-
tain quasi-hydrostatic pressure environment in two runs perature range at low pressures and keeps this behavior
ofXRDexperiments. Pressurewasmeasuredbycombin- in the normal state when the material becomes super-
ing the ruby fluorescent method [30] and the equation of conductive below 10.1 GPa. This result follows that the
states of Au [31]. The sample-to-detector distance and materialturnsfromasemiconductortoasemiconducting
experimental parameters were calibrated with standard superconductorwiththeapplicationofpressure. Aspres-
CeO powder diffraction. The two-dimensional diffrac- sure is further increased the resistance exhibits a nearly
2
tion images were converted to 2θ versus intensity data linearmetallicdecrease,contributingthemetallicnormal
plots using the FIT2D software. The crystal structures state. Forthedecompressionrun,itisratherunexpected
wererefinedusingGSASpackage[32]. TheRamanspec- tofindthatthesuperconductingphasecansustainto4.2
tra were measured in backscattering geometry with visi- GPa [Fig. 1(b)], which is much lower than the critical
ble laser wavelength of 633 nm. pressure of emergence of superconductivity in the com-
We have measured the electrical resistance of pressionrun. Anobvioushysteresisisthusobtainedfrom
Cu Sb Al Se at high pressures and low tempera- theresistivemeasurements. TheinsetinFig. 1(b)shows
3 0.98 0.02 4
tures. The results are shown in Fig. 1 for both the an expanded graph of resistance-temperature from 2 to
compression and decompression runs. We observed that 300Kforboththecompressionanddecompressionruns.
theelectricalresistanceexhibitsasemiconductingbehav- We have performed high-pressure Raman scat-
ior in the low pressure region. For pressures above 8.5 tering and synchrotron XRD measurements on
GPa,aclearsuperconductingtransitionemergeswithan Cu Sb Al Se up to 40.1 GPa. The Raman
3 0.98 0.02 4
onsetT at2.3K[Fig. 1(a)]. Atthebeginning,pressure- spectra of this material at different pressures are shown
c
induced fraction of the the superconducting phase is not in Fig. 2(a). When the pressure is increased to 8.2 GPa,
large enough for exhibiting zero resistance at 8.5 and 9.2 there is an abrupt change of the Raman spectra, provid-
3
structure remains the high-pressure phase at 4.6 GPa
and completely returns to the low-pressure phase at 0.4
(a ) (b )
1 b a r P (G P a) GPa,suggestingatinybutnoticeabletraceofhysteresis.
0.4 Peaks marked with asterisks are the diffraction signal of
P (G P a )
4.6 Au in Fig. 2(b). In order to accurately determine the
4 0 .1
crystal structure of high-pressure phase, we repeated
3 5 .3 * * 33.4 the XRD measurements of Cu Sb Al Se without
3 0.98 0.02 4
2 8 .3
28.9 Au, and the diffraction patterns were the same as the
25.6 first one. Both Raman scattering and XRD data under
2 1 .2
22.2
Relative intensity 11 378764 ......432178 Relative intensity 1118037..1..0407.4 apistniusrodepNanuseesfcrduxoceautrodternwa-dsfepooturfrlpcoduhtrvcieitivtgseduiherternn-yapta.clrrtoeyhtnsresdsauiednsrrteseievittnpaaitihotlineavedseveair.odnefCedfiZnnuncie3teSmSsebfe.oaSnArsets4foottihcfesiretachstopteinorrudsneicisdffstweuurrairretacedh--l
3 .6 6.6 transition,thereareonlythreediffractionpeakswhen2θ
5.7 isbelow26oforCu Sb Al Se . TheXRDpatternat
3 .0 3 0.98 0.02 4
4.6 8.1GPawasindexedwithastructuresimilartothecubic
2 .0 3.6 structure,butitisevidencednotastandardstrictlycubic
0 .6 2.8 structure. Threelatticeparametersareveryclosebutnot
1.8
0 .3 G P a completely equal and present different compression coef-
1.0
ficients with increasing pressure. This is further demon-
5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 6 1 0 1 4 1 8 2 2 2 6 strated by the gradual broaden diffraction peaks formed
R a m a n s h ift (c m -1 ) 2 (cid:1) (D e g re e ) from the overlap of multiple diffraction peaks. The fur-
ther analysis indicates that the structural arrangement
FIG. 2: (Color online) Representative Raman spectra (a)
of the high-pressure phase is similar to that of Fm3c or
and synchrotron x-ray powder diffraction patterns (b) of
Cu Sb Al Se atroomtemperatureandpressuresupto F¯43c, except for the very small difference of lattice pa-
3 0.98 0.02 4
40GPainboththecompressionanddecompressionruns. The rameters. Through indexing and refining these data, we
diffraction signal of Au are marked with asterisks. determined the high-pressure phase belonging to a space
group Ibca, which can be recognized as a deformation of
ing the evidence of phase transition. This observation the cubic structure to an orthorhombic unit cell. The
provides an indication that phase transition may play a theorists predicted that the similar structure material of
key role for superconductivity. The clear and separate ZnSeandZnTetransformintoorthorhombicphasefinally
Raman modes disappear and a big bump emerges. It is underpressurewiththeminimumtotalenergy,providing
worth noting that the big bump, which is corresponding further evidence to support our result [34].
to the overlap of multiple peaks, shifts gradually toward The fitted results of both the low-pressure phase with
higher frequency with increasing pressure. Upon decom- I¯42/m space group at 1.9 GPa and the high-pressure
pression, we observed that the sample recovers to its phasewithIbcaspacegroupat21.1GPaareshowninFig.
initial phase at ambient conditions. 3(a), respectively. We found that Cu Sb Al Se
3 0.98 0.02 4
Figure 2(b) presents the XRD patterns of formsanAl-Cu-Sb-Sesubstitutionalalloyafterthephase
Cu Sb Al Se for applied pressure ranging from 1 transition is complete, similar to the behavior of Bi Te
3 0.98 0.02 4 2 3
to 33.4 GPa. All the Bragg peaks shift to larger angles, at high pressures [35]. The position at (0,0,0) of space
showing the shrinkage of the lattice constant. Upon group Ibca was occupied by the Al-Cu-Sb-Se alloy, as
compression, as we expected, there are several changes obtained from powder X-ray data. The pressure de-
intheXRDpatternsinthenumber,intensity, andshape pendence of the unit-cell volume V is usually described
of the peaks, suggesting that an obvious structural by the three order Birch-Murnaghan equation of state
transition takes place above 8.1 GPa. Upon further [36]. Fitting our data points yields the bulk modulus
compression, the XRD patterns have no change up to K = 73.73±0.01 GPa and its pressure derivative K (cid:48)
0 0
33.4 GPa. The diffraction data in low-pressure region = 5.2±0.01 for low-pressure phase, and the ambient-
can be fitted using the space group I¯42/m, which is con- pressure volume V = 22.62±0.01 ˚A3. The fit to the
0
sistent with the crystal structure of Cu SbSe [33]. The data within the high pressure phase results in values
3 4
critical pressure of structural transition from XRD data of K = 89.30±0.24 GPa, K (cid:48) =6.18±0.02, and V =
0 0 0
shows an excellent agreement with the Raman spectra. 19.19±0.01 ˚A3. It is important to note that the value
This result provides an opportunity to better under- of K obtained from I¯42/m is smaller than the obtained
0
stand the underling mechanism of superconductivity in K inIbca, suggestingtheharderbondsathigh-pressure
0
Cu Sb Al Se . Upon decompression, the crystal phase. The structural transition leads to the contraction
3 0.98 0.02 4
4
I-42m Ibca
I-42m Ibca
Semiconductor Metal
SC
FIG. 4: (Color online) Phase diagram of Cu Sb Al Se
3 0.98 0.02 4
showing the evolution of T and crystal structure with pres-
c
sure in both the compression and decompression runs.
pearance of superconductivity. Upon compression, an
interesting structural transition at 8.1 GPa from I¯42/m
symmetry to Ibca symmetry was observed, below which
thereisnosignatureofsuperconductingtransition. Note
thatT isenhancedsteeplyasthepressureincreasesfrom
c
8to13GPa,thenitdecreaseswithappliedpressureafter
reaching the maximum value of 3.2 K. After that, there
isamonotonousreductionofT initiallyatarateof0.12
c
K GPa−1 as pressure is increased, however, the rate of
T -decrease is slowed down with pressure above 16 GPa.
c
Upon decompression, the superconducting phase disap-
pears below 4.2 GPa with the effect of hysteresis, which
is the same as the behavior of structure.
Over the whole pressure range studied, superconduc-
tivity correlates well with crystal structure on both the
FIG. 3: (Color online) (a) XRD patterns of compression and decompression runs. Below 10 GPa, an
Cu3Sb0.98Al0.02Se4 at pressures of 1.9 and 21.1 GPa increase of resistivity with decreasing temperature is ob-
for the corresponding space groups. The open circles repre- served either in the measured temperature range or in
sent the measured intensities and the red lines the results
thenormalstate. ThisindicatesthatCu Sb Al Se
3 0.98 0.02 4
of profile refinements by the best LeBail fit with each space
undergoes the change from semiconductor to supercon-
group. The positions of the Bragg reflections are marked
ductor at around 8.5 GPa. Superconductivity is real-
by vertical lines and the difference profiles are shown at
the bottoms (blue lines). The R values are R = 1.7%, ized in this material from semiconductor rather than
p
R =2.7% for the fitting at 21.1 GPa, R =1.0%, R =1.5% metal. Such a behavior has been observed in many nar-
wp p wp
at 1.9 GPa. (b) Pressure dependence of the volume per row bandgap semiconductors [38–41]. The observed su-
atom of Cu3Sb0.98Al0.02Se4 in both low-pressure phase and perconductivityinCu3Sb0.98Al0.02Se4 mayhavethesim-
high-pressure phase. Solid lines correspond to the results
ilar origin. For semiconducting superconductors, T is
c
of a least-squares fit using equation of states. The insets
mainly controlled by carrier concentration [42]. Super-
illustrate the atomic arrangement of the low-pressure and
conductivity in these materials arises primarily from the
high-pressure structures.
attractive interaction resulting from the exchange of in-
of the volume per atom. It is established [37] that the travalley and intervalley phonons, which can be larger
volume is decreased by 21% for Zinc sulphide and the than the repulsive Coulomb interaction.
28% for Zinc selenide during the structural transition, In summary, we have reported a finding of pressure-
respectively, which is larger than the collapse in volume induced superconductivity in the thermoelectric mate-
about 15.1 % from our data for Cu Sb Al Se . rial Cu Sb Al Se . The detailed Raman scattering
3 0.98 0.02 4 3 0.98 0.02 4
We summarized both the variation of T and struc- and synchrotron XRD measurements revealed the close
c
tural evolution as a function of pressure up to 25 GPa relationship between the structural transition and the
in Fig. 5. The phase diagram clearly reveals the close emergence of superconductivity. The superconducting
correlationbetweenthestructuraltransitionandtheap- phase was determined to be a alloy with an orthorhom-
5
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