Photon Factory Activity Report 2018 #36 (2019)

BL-8B/2017G671

Hydrostatic Compression Effects on Fifth-Group Element Superconductors Nb and Ta
Subjected to High-Pressure Torsion
Masaki MITO1,*, Takayuki TAJIRI2, Terukazu NISHIZAKI3, Kaveh EDALATI4,5, andZenji HORITA1,4,5
1
Graduate School of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
2
Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan3
Department of Electrical Engineering, Kyushu Sangyo University, Fukuoka 813-8503, Japan
4
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University,Fukuoka 819-0395, Japan
5International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University,
Fukuoka 819-0395, Japan
Severe plastic deformation by high-pressure torsion (HPT) brings about strain at the unit-cell level as well as reduction in
the grain size. We investigated the crystal structure of severely strained Nb and Ta materials under the hydrostaticcompression, and discussed the origin of the change in superconducting transition temperature Tc. The large increase in
Tc for HPT-processed Nb is triggered by the structural deformation in the unit-cell level, while the small increase in Tc for
HPT-processed Ta originates from strengthening intergrain-contact.
1 Introduction
In the periodic table containing 118 elements, there are
29 superconductors at ambient pressure, and 11 elementsof them exhibit the increase in superconducting transition
temperature Tc under hydrostatic pressure (HP) [1, 2].
Three fifth-group element superconductors such as V, Nb,
and Ta have nominal Tc of 5.3 K, 9.2 K, and 4.5 K,
respectively. All of three are considered to belong to agroup that exhibits the increase in Tc under the HP
circumstances [2]. Indeed, smooth lineac-increase in Tc isobserved only in V [3]. According to the study ofStruzhkin et al., nonstrained Nb shows the increase in Tc
after its slight decrease, and has the optimal Tc of 9.9K ataround P = 10 GPa [4]. Indeed, the initial dTc/dP for Nb
depends on experimental conditions such as the sample
condition and kind of pressure transmitting medium(PTM) [3, 5]. Contrary to the above two, Ta exhibits thedecrease in Tc over wide pressure range [3], and there is
only one plot suggesting the increase in Tc at 43 GPa [4].
Thus, there are much variety in the pressure dependenceof Tc among V, Nb, and Ta (see Fig. 1(a)). In the present
study, we compare the aforementioned HP effects on
non-strained V, Nb, and Ta (see Fig. 1(a)) and the HP
effects on severely strained V, Nb, and Ta (see Fig. 1(b))
considered as the so-called Josephson arrays of smallgrains.
Recently by using severe plastic deformation (SPD) [6,7], the improvement of functionality in superconductors
has been successfully conducted [8, 9]. The SPD process
plays the role of accumulating dislocations, resulting in
the grain refinement at the submicrometer or nanometer
ranges. It is known to be effective in increasing Tc ofsuperconductors such as Nb [8] and Re [9]. There wehave to consider both the intergrain network via grainboundary and intragrain crystallinity as factors toinfluence Tc. The strain installed in the SPD process isrelatively evaluated via semi-experimental manner: In thecase of high-pressure torsion (HPT) [10, 11] which is oneof the SPD techniques, the revolution number N is aparameter to pursue the grain refinement.
We are exploring the observation of new pressure
response using the SPD materials. In the present study,
we conduct the HP compression to strained materials
subjected to the HPT process. Figure 2 shows a strategy
the present study. There are two parameters such as N
related with the initially installed strain [Fig. 2(a)] andpressure (P) related with additional strain [Fig. 2(b)].
Both HPT-Nb (N = 10) and HPT-Ta (N = 10) exhibit themaximum Tc at around P = 2 GPa.

Fig. 1: Overview on pressure dependence ofsuperconducting transition temperature Tc for (a) nonstrained and (b) severely strained V, Nb, and Ta [3].
2 Experiment
As illustrated in Fig. 2(a), the disks of Nb, and Ta were
subjected to HPT processing at room temperature under aselected pressure of P = 6 GPa and revolutions of N up to20 with a rotation speed of 1 rpm. Specimens with

Photon Factory Activity Report 2018 #36 (2019)

dimensions of approximately 0.8 × 0.8 × 0.5mm3 were
cut from an HPT-processed disk using a wire-cutting
electric discharge machine at the position of 2.5mm fromthe disk center. The grain size at steady state for Nb andTa is 240 nm and 180 nm, respectively [12]Fig.2: Strategy of the present study in the case of Nb [3,5]. (a) In high-pressure torsion (HPT) for controlling theinitial condition, pressurization and revolution areconducted by using two metal anvils. As the revolution
number (N) increases, dislocations are accumulated (a-1),
and the grain size largely changes. The small-angle grainboundaries gradually transform to large-angle grainboundaries with increasing N ((a-2)→(a-3)). Finally due
to the balance between generation and annihilation ofdislocations, the grain size remains unchanged despite theincrease in N. (b) A few pieces of sample cut from theHPT-processed materials are placed in diamond anvil cell(DAC) to conduct the hydrostatic pressure experiment.
There, the specimen is pressed in a quasi-hydrostaticpressure condition using the DAC.
We performed X-ray diffraction (XRD) analyses for
Nb and Ta under high pressure up to P = 6.8 GPa at room
temperature using a synchrotron radiation XRD systemwith a cylindrical imaging plate at BL-8B. The energy ofthe incident X-rays was 16 keV. Pressure was applied
using a DAC that consisted of two diamond anvils withflat tips having a diameter of 0.8mm and a 0.3-mm-thick
CuBe gasket. The small pieces of HPT-Nb or HPT-Ta
were placed in a randomly oriented manner in a sample
cavity with a diameter of 0.4mm along with a ruby ofmanometer and transparent PTM such as fluorinated oil(FC77, Sumitomo 3M Ltd.). Transparent FC77 undergoes
solidification below 1 GPa at room temperature [13]. All
the atomic positions in the body-centered cubic (bcc)
structure are special positions, and therefore, thestructural parameters can be evaluated just with the use of
the diffraction peak angle. The lattice parameters were
estimated using all of observed diffraction peaks: For Nb,
we used five diffraction peaks of the plane indices (110),(200), (211), (220), and (310). For Ta, the diffraction
peak of the plane index (310) was not used.

3 Results and Discussion
Figures 3 shows the pressure dependence of interplanar
distance of (110), (200), (211), (220), and (310) for theHPT-Nb materials for N = 1 (c) and 10 (d). For reference,the data for both arc-melting (ARC) and AR materials arealso shown in Figs. 3(a) and 3(b), respectively. At first,
the AR material exhibits anisotropic transformation of theunit cell structure at around P = 2 GPa, suggesting thatstrain accumulated in the cubic lattice with highsymmetry is released at a threshold pressure (Pc). Thisphenomenon does not occur in the ARC material without
residual strain, suggesting the importance of residualstrain. When the HPT procedure with N = 1 is subjectedto the AR material, the grain refinement enhances thecapacity of accumulating the additional strain, resulting in
the increase in Pc to approximately 3.4 GPa. However,
HPT with the level of N = 10 does not permit toaccumulate any additional strain, so that anisotropic unitcell deformation occurs even at quite small pressure.
Comparing with ARC, AR and HPT (N = 1) exhibits
the one-direction shrinkage and two-direction expansion.
On the other hand, the HPT (N = 10) material exhibits
two-direction shrinkage for P < 2 GPa and two-direction
expansion for P > 2 GPa. At around 2 GPa, thedeformation manner changes, suggesting complex
manner of strain release. In particular, the HPT (N = 10)
material with heavy residual strain has a deformed bcc
structure, which is close to the body-centered tetragonal(bct) structure for P > 3 GPa.
Figures 4 shows the pressure dependence of interplanar
distance of (110), (200), (211), and (220) for the HPT-Ta
samples for N = 0 (a), 2 (b), and 10 (c). In all of three, allof interplanar distances exhibit the shrinkage, and there
are no anisotropic deformation of the bcc unit cell. For N= 0, at around P = 1~2 GPa, the shrinkage magnitude issuppressed similarly to that observed in AR-Nb. The
above pressure region is consistent with that possessing
the maximum of Tc. On the other hand, both N = 2 and 10
shows that the uniform contraction continues
monotonously. Both HPT (N = 2) and HPT (N = 10) do
not show anomalous change in a at around P = 1~2 GPa,suggesting that the increase in Tc cannot be explained
only with the change in lattice constant. The normalized
lattice constant exhibits no prominent N dependence, andit continues to decrease monotonously in the consideredpressure region.
In fifth-group element superconductors V, Nb, and Ta,
artificial material manipulation using the severe plastic
deformation and additional hydrostatic compression wasattempted to increase the superconducting transition
temperature Tc. In HPT-Nb with the small N and HPT-Ta
for N = 0, 2, and 10, the increase in Tc due tostrengthening the intergrain contacts was observed.
Further, the prominent increase in Tc for HPT-Nb for N =10 originates in the anisotropic deformation of the unit
cell. When the quasihydrostatic compression is yielded tothe material with heavy residual strain in the unit cell,severe strain accumulated in the bcc unit cell with highsymmetry is released at a small amount of additionalpressure. In the case of Nb, this manner of lattice
deformation is favorable for increasing Tc. The

1.00(a) ARC

0.995

110
200
211
220
310

-0.15%/GPa

1.00(b) AR

0.995

110
200
211
220
310

AR
-0.26%/GPa

ARC

0.990

relative interplane distance(a) N = 0

0.995

(110)
(200)
(211)
(220)

1.00

0.995

(b) N = 2(110)
(200)
(211)

-0.12%/GPa(220)0.990

1.00(c) N = 1

relative interplane distance

relative interplane distance

0.990

1.00

0.995

0.990
0.995

110
200
211
220
310

0-0.13%/GPa
N=1AR

ARC

0.990

relative interplane distance

1.00

0.990

relative interplane distance

relative interplane distanceknowledge obtained there yields a new strategy toincrease Tc from the viewpoints of material science. There
would be further potentiality for increasing Tc in theborder region between material science using severeplastic deformation and extreme condensed-matterphysics using hydrostatic compression.

relative interplane distancePhoton Factory Activity Report 2018 #36 (2019)

1.00

0.995

(c) N = 10(110)
(200)
(211)
(220)

1-0.11%/GPa234567

P [GPa]Fig. 4: Pressure dependence of normalized interplanar
distance of (110), (200), (211), and (220) planes for HPTTa with N = 0 (a), 2 (b), and 10 (c) [3].

(d) N = 10110
200
211
220
310

N=1ARCAR

0.990

Fig.3: Pressure dependence of normalized interplanar
distance of (110), (200), (211), (220), and (310) planes
for ARC (a), AR (b), HPT-Nb (N = 1) (c), and HPT-Nb(N = 10) (d) [3]. The broken guide lines in all figures
represent the interplanar distance change for ARC (black),AR (blue), and N = 1 (red). These initial slope values in
the pressure regime where isotropic structure change
occurs are also indicated. The pressure regime
corresponding to anisotropic structure change is indicatedin lightblue hatching.

Acknowledgement
This work was supported by MEXT KAKENHI, a
Grant-in-Aid for Scientific Research on Innovative Areas''Bulk Nanostructured Metals'' No. 25102709, and Grantin-Aid for Scientific Research (S) (No. 26220909). This
work was also supported by JSPS KAKENHI Grant
Number 16H04338, 16K05460, and 17H03379. The HPTprocess was carried out at the International ResearchCenter on Giant Straining for Advanced Materials (IRCGSAM) in Kyushu University.
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*mitoh@mns.kyutech.ac.jp