Pressure effect and electron diffraction on the anomalous transition
in ternary superconductor Bi
2
Rh
3
Se
2
C.Y. Chen
a, C.L. Chan
a, S. Mukherjee
a, C.C. Chou
a, C.M. Tseng
b,1, S.L. Hsu
b, M.-W. Chu
b,
J.-Y. Lin
c, H.D. Yang
a,na
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
b
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
c
Institute of Physics, National Chiao-Tung University, Hsinchu 300, Taiwan
a r t i c l e i n f o
Article history: Received 12 July 2013 Received in revised form 16 September 2013 Accepted 25 September 2013 by E.V. Sampathkumaran Available online 3 October 2013 Keywords:
A. Bi2Rh3Se2ternary compound
D. Structural phase-transformation E. Effects of pressure
E. Transmission electron microscopy
a b s t r a c t
The effect of external hydrostatic pressure up to 22.23 kbar on the temperature-dependent transport properties of the ternary compound Bi2Rh3Se2 is investigated. Interestingly, the resistive anomaly
at Ts250 K, previously proposed as a charge-density-wave (CDW) transition, is shifted to higher
temperature with increasing pressure, in distinct contrast to an established knowledge for CDW. Using temperature-dependent electron-diffraction characterizations, we have unraveled that this transition is, in effect, of a structural phase-transformation nature, experiencing the symmetry reduction from a high-symmetry C-centered monoclinic lattice to a low-high-symmetry primitive one below Ts. A more elaborately
determined room-temperature C-centered lattice was also proposed.
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1. Introduction
Over the past several decades there has been considerable interest in the coexistence of superconductivity (SC) and charge-density-wave (CDW) transition in transition metal di- and tri-chalcogenides, MX2and MX3 (M¼transition metal, X¼S, Se, Te)
[1–5]. The nature of SC is to decrease material's resistivity to zero,
while CDW is to increase material's resistivity becoming semi-conductor or insulator. It is an interesting behavior that a sig-nificant competition between SC and CDW coexists in the same material[6,7], which can be clearly explored by tuning physical or chemical pressure. Similar situation occurs in the ternary rare-earth-transition-metal silicides R5T4Si10(R¼rare-earth elements;
T¼Co, Ir, Rh, and Os). For instance, Lu5Ir4Si10 has been known
to enhance the superconducting transition temperature under applied pressure and doping at the expense of suppressing the resistive CDW anomaly[8,9]. Currently, the number of iron-based superconductors is a real surprise and has generated tremendous interest. Specially FeSex[10–12] could provide the most
appro-priate site of understanding the iron-based superconductors because of its simple structure similar to the Fe–As layer. At high pressure, there is a subtle interaction between SC and spin density
wave (SDW) in FeSex[12]. On the other hand, the high resolution
electron microscopy reveals that a structural transition from tetragonal to orthorhombic in Fe1.01Se is a driving force for
super-conducting phase[13]. Thus, Se deficiency might be a key factor for stabilizing the superconducting phase in FeSex. This also
sug-gests that the SC, SDW and structure are strongly correlated in the Fe–Se systems.
Compared to R5T4Si10 and FeSex, the recently discovered
parkerite-type superconductors [14,15] could also represent an intriguing class of materials for tackling the intricate interactions among the SC, CDW, SDW, and/or structural transitions. Sakamoto et al. [15] have reported a superconducting transition (0.7 K) along with CDW state at Ts250 K in parkerite-type Bi2Rh3Se2.
In the report, the room-temperature crystal structure has been considered as C-centered monoclinic (C12/m1, β¼ 89.153(3)1; a¼11.414(10) Å, b¼8.3709(9) Å, c¼11.989(1) Å), and the CDW formation was proposed through observations of superlattice peaks below Tsusing X-ray powder diffraction[15]. Nevertheless,
the indicated transition might not direct to an unambiguous CDW onset in the current absence of detailed band-structure knowledge and more extensive structural characterizations for Bi2Rh3Se2[16].
In this paper, we report the pressure effect on the resistive anomaly occurred at Ts250 K compared to conventional CDW
and systematic temperature-dependent electron diffraction[16]to investigate the possible satellite spots resulted from CDW forma-tion below Tsand the structural transformation across Ts.
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Solid State Communications
0038-1098/$ - see front matter& 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ssc.2013.09.025 nCorresponding author. Tel.:þ886 752 537 32.
E-mail address:[email protected] (H.D. Yang).
1Present address: Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
2. Material and methods
Crystalline Bi2Rh3Se2 was prepared with stoichiometric
mix-tures (Bi:Rh:Se¼2:3:2) of high-purity Bi, Rh, and Se, sintered in a quartz tube isolating from air at the maximum temperature 1320 K for 6 h and then slowly cooled with the rate of 2 K/h to 1020 K. Finally, it was water quenched to room temperature. The crystal structure of the obtained product was identified by powder X-ray diffraction (XRD) measurements using both in-house (wave-length, 1.54 Å) and synchrotron radiation facilities (wave(wave-length, 0.62 Å; National Synchrotron Radiation Research Center, Taiwan). The magnetic susceptibility was performed in Superconducting Quantum Interference Device (SQUID) (Quantum Design, MPMS XL-7). The low-temperature specific heat was measured from 0.5 to 2 K by using a3He heat-pulsed thermal relaxation
calori-meter [17]. High temperature (2–300 K) specific heat measure-ment was done in Physical Property Measuremeasure-ment System (PPMS) (Model: 6000, Quantum Design). Pressure dependent electrical resistivity measurements were executed using a four-terminal probe method in PPMS with the temperature 2–340 K under the hydrostatic pressure up to 23 kbar using the piston-cylinder self-clamped technique [18]. An inert fluid, Daphne-7373, was
used as the pressure transmitting medium along with a man-ometer superconducting tin. The electron diffraction characteriza-tions were performed on a transmission electron microscopy (TEM; JEOL 2000FX) operated at 200 kV and equipped with a low-temperature sample stage. The TEM specimens were pre-pared byfirstly crashing a sintered pellet into powders and then collecting the micro-crystalline materials using a copper grid. The temperature homogeneity and accuracy upon TEM characteriza-tions have been faithfully controlled as reported previously [16]. An illuminating area of 280 nm in diameter was always used upon selected-area electron diffraction (SAED).
0 100 200 300 0 100 200 0.5 1.0 1.5 2.0
H = 10 kOe
Bi
2Rh
3Se
2C
(J/mol K)
T (K)
ZFC FCχ
(10
-4emu/mol)
0.0 0.5 1.0 1.5 2.0 0 15 30 T (K) Tc = 0.68 K C (mJ/mol-K) 0 200 400 10 20 30 40 (-4,-2,2) (022) (-4,0,3) (-4,0,2) (-2,2,2) (220) (021) (020) (001) 2θ (degrees) Intensity (Counts) T = 300 KFig. 1. (Color online) Left label: Temperature dependence of specific heat C. Right label: Temperature dependence of magnetic susceptibilityχ in the presence of magneticfield 10 kOe. Lower inset: X-ray powder diffraction data at 300 K. Upper inset: C vs. T curve, where Tc¼0.68 K.
0 100 200 300 0 1 2 1 bar 6.26 kbar 12.75 kbar 17.64 kbar 22.23 kbar
T (K)
ρ
(μΩ
m)
Bi
2Rh
3Se
2I = 1 mA
P
100 150 200 250 300 1.8 2.0 2.2 ρ (μΩ m) T (K) 0 T 0.5 TFig. 2. (Color online) Temperature dependence of electric resistivity at different applied pressure (maximum 22.23 kbar). Inset: Temperature dependence of electric resistivity under different magneticfields at ambient pressure.
0 5 10 15 20 25 0.08 0.09 0.10 0.11 260 280 300
P (kbar)
Δ
ρ / ρ
T
s(K)
d
T
s/d
P ~ 2.22 K/kbar
150 200 250 300 1.6 2.0 2.4T
sρ
T (K)
ρ
(μΩ
m)
Δρ
P= 1 bar
Bi
2Rh
3Se
2Fig. 3. (Color online) (a) Transition temperature at different pressure appears as a linear relationship, and the slope is about dTs/dP¼2.22 K/kbar. (b) The relative
anomaly amplitude Δρ/ρ is suppressed by applied pressure. Inset: Schematic diagram of how to determine the value ofΔρ and ρ.
Fig. 4. (Color online) The refinement of the powder XRD pattern acquired at room temperature using synchrotron radiation source (wavelength, 0.62 Å). Red curve, the calculated powder pattern. Black crosses, the experimental data. Green curve, the background. Blue curve, the difference between the experimental and calcu-lated intensities. Vertical bars, the Bragg reflections. Inset, blowup of the room- and low-temperature XRD patterns with the systematic appearance of addition reflec-tions such as the one indicated by the black arrow.
3. Results and discussions
The data obtained for the temperature-dependent specific heat (C) and magnetic susceptibility (χ, after diamagnetic correction) measurements upon an applied magnetic field of 10 kOe for Bi2Rh3Se2are shown inFig. 1. Below 250 K, C andχ drop gradually.
In the lower and upper insets present the part of in-house, room-temperature XRD pattern and low-room-temperature (0.5–2 K) C–T curve, revealing a superconducting transition (Tc) at 0.68 K. The
C–T measurements are in fair agreement with those in previously published report[15]and the detailed structural characterizations by synchrotron X-ray diffraction and SAED are shown later in
Figs. 4and5.
Fig. 2 shows the temperature dependence of the resistivity
between 2 and 340 K under various pressures up to 22.23 kbar. At ambient pressure a hump-like resistive anomaly is observed around Ts250 K, which agrees with the previous result and was
previously attributed to a CDW transition [15]. However, the increasing Ts with increasing pressure in the material is
incon-sistent with that observed for many conventional CDW super-conductors [1,6,7]. The temperature dependence of resistivity under magnetic field is also displayed in the inset of Fig. 2. No detectable shift of the resistive anomaly is observed in the presence of magnetic field, indicating that the origin for the resistive anomaly occurred at Ts is not related to SC and
magnetism.
The transition temperature Tsderived fromFig. 2under applied
different pressures is illustrated in Fig. 3(a). The Ts increases
linearly with P at a rate of dTs/dP¼2.22 K/kbar and the relative
peak intensityΔρ/ρ declines gradually. The increase of Tsthrough
the application of P in Bi2Rh3Se2is quite intriguing. In comparison
with other CDW systems, the negative pressure coefficients dTs/dP¼ 0.3 K/kbar in 2H–NbSe2[4]and 6.25 K/kbar in NbSe3
[2] have been reported. Therefore, the effective origin for the
Fig. 5. SAED patterns of Bi2Rh3Se2obtained (a) at room temperature and (b) 149 K along [100]-projection, and at (c) room temperature and (d) 149 K along [1̄01]-projection.
All reflections observed in (a) and (c) are fully explained within the monoclinic C12/m1 lattice, featuring the otherwise β angle of 1341 (see text). In (b) and (d), the systematic presence of symmetry-forbidden reflections compared to (a) and (c), respectively, unravels a structural phase transition below Tsinto monoclinic P12/m1. (e) and
(f), the BF and DF images of an individual crystal taken at 149 K. The DF imaging has been acquired using 0k0 (k¼odd) type reflections and comparable diffraction contrasts were observed exploiting other symmetry-forbidden reflections.
C.Y. Chen et al. / Solid State Communications 177 (2014) 42–45 44
transition upon the resistive anomaly at Ts prompts for further
elaborations. We have then characterized the pressure-dependent change in resistivity (Δρ/ρ) at the transition, as shown inFig. 3(b). TheΔρ/ρ decreased with pressure P is shown inFig. 3(b) indicat-ing the suppression of sharpness of transition with the applied pressure.
To shed more light on the resistive character inFig. 3(b), we have performed extensive structural characterizations using XRD and SAED as shown inFigs. 4 and5, respectively. The XRD and SAED investigations have been performed at both room and low temperatures. Moreover, the SAED patterns inFig. 5(a)–(d) are the representative results over five thoroughly investigated single-crystalline materials with the typical bright-field (BF) image of a crystal shown inFig. 5(e). It has been broadly recognized that an individual crystal out of the powder ensemble can be considered as a single crystal[16], which is indispensable for understanding fundamental structural characteristics. The SAED results inFig. 5 (a)–(d) are, therefore, general characteristics, instead of specific features of given local regions.
Within the C12/m1 symmetry of the room-temperature struc-ture of Bi2Rh3Se2, it is to be pointed out that, in effect, there exist
two separate unit-cell choices, the well-known one with non-orthogonalβ of 89.153(3)1[15]and the other withβ of 1341
[19]. Our careful powder XRD refinements using either lattice
choice within the pattern matching of the Rietveld method surprisingly led to similar figures of merit (both at the scale of Rwp, 2.4%; Rp, 1.5%; χ2, 1.2). InFig. 4, we show the refined
result of the large-β cell with thus-obtained a¼11.413(1) Å, b¼8.368(2) Å, c¼8.336(1) Å, and β¼134.05(4)1. Notably, the lat-tice volume of this unit cell is practically half of the refined small-β counterpart (a¼11.416(1) Å; b¼8.365(1) Å; c¼11.982(2) Å; β¼ 89.10(3)1). Upon a given symmetry, a small-volume lattice always corresponds to an optimized periodicity of the structure [20]. To first approximations, the refined cell with β1341 would represent the optimal lattice choice for Bi2Rh3Se2, rather than
the reported unit cell with β891 [15]. Our proposed lattice is indeed affirmed by the room-temperature SAED patterns shown in Fig. 5(a) and (c), the indexing of which cannot be achieved without this large-β cell.
More intriguingly, additional reflections were systematically observed below Tsin both the SAED (Fig. 5(b) and (d)) and XRD
(inset,Fig. 4) investigations at 149 and 100 K, respectively. Due to the low monoclinic symmetry and large cell parameters charac-teristic to Bi2Rh3Se2, non-negligible overlaps of XRD peaks take
place and the additional XRD reflections can thus merge into the intensity of the pristine Bragg reflections. A further refinement of the XRD pattern at 100 K becomes tedious and the low-temperature SAED turns out to be an optimal solution for tackling the structural subtlety below the transition temperature. Indeed, a careful examination of the low-temperature SAED patterns revealed the systematic presence of reflections with 0k0 (k¼odd) and h0l (h,l¼odd), otherwise forbidden to C12/m1. Tilting the crystals away from the zone-axes did not lead to the disappear-ance of these symmetry-forbidden spots, indicating that they are not the product of electron multiple scattering, but real structural reflections [16]. The symmetry-breaking spots further direct to reflection conditions compatible with those of monoclinic P12/m1
[20]. Using these symmetry-forbidden reflections for dark-field
(DF) imaging (Fig. 5(f)), only ordinary diffraction contrasts can be observed, totally free from stripe-like domains due to phase
modulations in CDW phases such as those found in the DF investigations of Ho5Ir4Si10 and Lu2Ir3Si5 at low temperatures
[16,21]. It can be readily suggested that Tsin Bi2Rh3Se2does not
correspond to a CDW transition, rather a structural transition, and all our electronic characterizations in Figs. 1–3 do not seem to support the onset of a CDW transition, either. The increase of the structural transition temperature upon external pressures other-wise remains an open question.
4. Conclusions
In summary, the pressure effect up to 22.23 kbar on the elec-trical resistivity of Bi2Rh3Se2has been studied in the temperature
range 2–340 K. The resistive anomaly occurred at Ts250 K is
shifted to higher temperature with increasing pressure, which is inconsistent with results observed in conventional CDW forma-tion. The structural elaborations by XRD and SAED indicated that a structural transition from C12/m1 to P12/m1 takes place below Ts,
rather than the previously suggested CDW transition.
Acknowledgment
This work was supported by the National Science Council, Taiwan under Grant no. NSC 100-2112-M-110-004-MY3.
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