Superconductivity and Structural Conversion with Na and K doping of

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Superconductivity and Structural Conversion with Na and K doping of the Narrow-gap Semiconductor CsBiTe 4

6

Haijie Chen, Helmut Claus, Jin-Ke Bao, Constantinos C. Stoumpos, Duck Young Chung, Wai-Kwong Kwok, and Mercouri G. Kanatzidis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02030 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Chemistry of Materials

Superconductivity and Structural Conversion with Na and K doping of the Narrow-gap Semiconductor CsBi4Te6 Haijie Chen,1,2 Helmut Claus,1 Jin-Ke Bao,1 Constantinos C. Stoumpos,2 Duck Young Chung,1 Wai-Kwong Kwok,1 and Mercouri G. Kanatzidis*,1,2

1

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439,

United States 2

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United

States

Abstract: The monoclinic narrow-gap (~0.08 eV) semiconductor CsBi4Te6 is a unique layered system which can be doped to achieve high thermoelectric performance as well as superconductivity. Here, we report superconductivity and structure change induced by alloying CsBi4Te6 single crystals with Na and K. Substitution of Na in CsBi4Te6 with doping levels ≥ 0.39 and of K with ≥ 0.63, transforms the original monoclinic structure (p-type) to the orthorhombic RbBi3.67Te6-type structure (n-type). When the K level is ≤ 0.18 the monoclinic structure type of CsBi4Te6 is retained. Transport and magnetic measurements on all as-synthesized doped single crystals demonstrate type-II, bulk superconductivity. A maximal superconducting transition at 5.07 K, which is the highest temperature in bismuth chalcogenide-based superconductors, was obtained in Cs0.82K0.18Bi4Te6 with a high upper critical field of ~15 T. These findings suggest superconductivity may be induced by proper doping in narrow-gap semiconductors.

1 ACS Paragon Plus Environment

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Introduction The emergence of superconductivity in semiconductors with relatively low carrier density (1019 – 1020 cm-3), such as PbTe,1 Bi2Se3,2 Bi2Te3,3 IrTe2,4 TiSe2,5 SnTe,6 SrTiO3,7 and AxZrNCl (A = Li, Na, K),8 is an exceptional phenomenon because superconductors generally are metals with carrier densities above 1022 cm-3 and often have high density of states (DOS) at the Fermi level (EF). The observed superconductivity in semiconductors, which have very low DOS at EF, is counterintuitive and underscores the limited understanding we have on its mechanism and how to induce it. Therefore, it is important to discover and study new semiconductors that can be doped into superconductors to gain further insights into the evolution of this intriguing phenomenon. Thus far, studies suggest that superconductivity can be induced in narrow-gap semiconductors by aliovalent doping with elements such as Tl in PbTe (superconducting transition temperature (Tc ~1.5 K),9 In in SnTe (Tc ~4.5 K),10-13 Pt or Pd in IrTe2 (Tc ~3.1 K),14, 15 Tl in Bi2Te3 (Tc ~2.3 K),16 F in LaOBiS2 (Tc ~10.6 K),17 La in SrFBiS2 (Tc ~10.6 K),18 and Ag in (PbSe)5(Bi2Se3)6 (Tc ~1.7 K).19 Another approach is by intercalating Cu in TiSe2 (Tc ~4.2 K)5, 20 or Cu, Sr, and Nb in Bi2Se3 (Tc ~2.5 K) in which potential topological superconductivity can emerge with fully gapped state in the bulk and gapless Majorana state at the surface.21-28 Moreover, high pressure is also a powerful technique to induce superconductivity in semiconductors, such as the topological parent material Bi2Te329 and the layered In2Se3,30 Previously, we reported superconductivity in narrow-gap layered semiconductors RbBi3.67Te6 and CsBi4Te6 with low carrier densities of 1019 cm-3.31-34 The monoclinic


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CsBi4Te6 consists of unique Bi−Te slabs featuring Bi−Bi bonds, whereas RbBi3.67Te6 forms regular Bi−Te layers in the orthorhombic space group with Bi vacancies to maintain charge balance. In CsBi4Te6, superconductivity was induced via hole doping resulting in Tc of 4.4 K, while RbBi3.67Te6 is a superconductor below 3.2 K.35 Subsequently, superconductivity was also found in the homologous quaternary series AMmBi3Q5+m (m = 1, 2) (A = Rb, Cs; M = Pb, Sn; Q = Se, Te) semimetals with relatively lower Tc < 3 K.36, 37 The above results, which demonstrate that the ternary semiconducting families of bismuth tellurides can harbor superconductivity despite their low carrier densities, have prompted the study of isovalent Na and K doping in the form of Cs1-xNaxBi4Te6 and Cs1yKyBi4Te6

solid solutions. Our goal was to understand how isovalent doping on the Cs

site, which does not alter the carrier concentration, transforms the crystal structures and charge-transport dynamics in this family of superconductors. Detailed single crystal Xray diffraction and transport measurements demonstrate two completely different behaviors for Na and K. Specifically, Na does not act as an isovalent dopant but instead prefers the Bi site which has a nearly identical ionic radius (rBi = 1.03 Å, rNa = 1.02 Å).38 Instead, introducing Na fractions of 0.39 and 0.45 induces a structural transformation from the monoclinic CsBi4Te6-type to an orthorhombic RbBi3.67Te6-type structure with a change in carrier type from p- to n-type. In contrast, K fraction < 0.18, substitute only into the Cs site retaining the CsBi4Te6-type structure and induces superconductivity without changing the carrier type and density. With increasing K fraction in the range of 0.63 to 0.83, both the Cs sites and Bi sites are affected, prompting a change from monoclinic CsBi4Te6-type to the orthorhombic RbBi3.67Te6-type structure. A noteworthy



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Tc of 5.07 K (highest in the bismuth chalcogenide class of superconductors) with an upper critical field (Hc2(0)) of ~15 T were obtained in the monoclinic Cs0.82K0.18Bi4Te6.

Experimental Section Reagents. The following chemicals were used as purchased: cesium metal (99.9%, Strem Chemicals, Inc.); potassium metal (98%, Sigma Aldrich), sodium metal (98%, Sigma Aldrich), bismuth metal (99.9%, Strem Chemicals, Inc.), and tellurium shots (99.999%, American Elements). Bi2Te3. Stoichiometric mixture (~10 g) of elemental Bi and Te corresponding to Bi2Te3 was sealed in a silica tube (15 mm O.D. × 13 mm I.D.) at a residual pressure of 100%), generating unstable refinements. Herein, all Bi and Te atoms were set with full occupancies. Hall effect. We performed Hall effect measurements to obtain the carrier concentration in carefully selected single crystals of these compounds. For the orthorhombic parent compound, the ideal charge balanced formula is ABi3.67Te6 (A = Cs, Rb, K, Na) achieved through Bi vacancies.34 The fact that Na occupies only Bi octahedral sites while K substitutes both Cs and Bi sites48-51 raises the following consideration: if K substitutes for Cs, in principle the carrier density should remain unaffected because Cs and K are isovalent. On the other hand, when Na or K substitutes into the Bi sites, they are not isovalent and can act either as an acceptor or donor depending on whether they substitute Bi atoms or fill the vacancies in the Bi sites. To probe this behavior, we conducted Hall measurements to determine the carrier type and concentration, on all as-synthesized single crystals (see Figure S2 − S4). For CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6, the Hall resistivity (ρxy) is negative at positive magnetic field, demonstrating n-type behavior. The carrier concentration n increases from ~1019 cm-3 in CsBi4Te6 to ~1020 cm-3 at 300 K for both compounds (see Figure 2a and Figure 2b). As shown in Figure S3c and d, for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6, ρxy is positive at positive magnetic fields, indicating holes as the dominant carriers. The carrier density is 2.3 × 1019 and 2.1 × 1019 cm-3 for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6 respectively at 300 K which is nearly unchanged from 2.4 × 1019 cm-3 for the undoped CsBi4Te6. Since there is no significant effect on the dominant carrier type and carrier density, this result is consistent with isovalent K doping in Cs sites as



obtained

from

the

single

crystal

X-ray

refinement.

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For

Cs0.78K0.63Bi3.54Te6,

Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6, the Hall coefficients (ρH = dρxy/dH) are negative (n-type) and n is also ~ 1020 cm-3 at 300 K, which is similar as CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6. Na/K occupation in Bi sites in the orthorhombic structure can create holes when Na/K substitutes for Bi atoms or electrons when Na/K fills only the Bi vacancies. Judging from the results of the Hall measurements, vacancy filling appears to be more dominant than substitution of Bi with Na/K. Superconductivity. Superconductivity was observed in all compounds from resistivity measurements on single-crystals. Figure 3a shows temperature dependence of the normalized resistance for CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6. Tc increases with Na doping, going from Tc ~3.13 to 3.22 K. Figure 3b shows the temperature dependence of the

normalized

resistance

(ρ/ρ6K)

for

Cs0.91K0.09Bi4Te6,

Cs0.82K0.18Bi4Te6,

Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6, corresponding to nominal y = 0.1 – 0.7, respectively. Generally, Tc of the monoclinic phases (~5 K) are higher than the orthorhombic phases (~3 K) with the highest Tc of 5.07 K obtained for Cs0.82K0.18Bi4Te6 (y = 0.2). In each group of monoclinic and orthorhombic phases, Tc increases with K doping. For comparison, superconductivity appears below 3 K in a series of AMmBi3Q5+m (m = 1, 2) (A = Rb, Cs; M = Pb, Sn; Q = Se, Te) semimetals with n ~1021 cm-3 where electrons are the dominant carriers.33, 34 This implies that introduction of more carriers can lead to a deterioration of superconductivity in the orthorhombic phases, which is contrary to the normal behavior of superconductivity in narrow-gap semiconductors,13,

17, 19, 52

suggestive of a potentially


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unusual superconductivity mechanism in the ternary semiconductors A-Bi-Te (A = Rb, Cs). To see the resistivity behavior in the normal state, the temperature dependence of the resistivity for monoclinic Cs0.82K0.18Bi4Te6 and orthorhombic Cs0.78K0.63Bi3.54Te6 above Tc was measured, Figure 3c and 3d. The monoclinic phase displays a concave curve in the resistivity data as a function of temperature, whereas the orthorhombic phase shows a convex curve. This is consistent with a normal single band degenerately doped metal-like behavior for the former and possibly a multiband based behavior for the latter, similar to the behavior of the resistivity in Ba1-xKxFe2As2.53 Below 50K the resistivities exhibit a Fermi liquid behavior,54 characterized by a T2 temperature dependence for both compounds, and can be well fitted with ρ = 0.84 + 1.5×10-4T2 for x = 0.2 and ρ = 0.2 + 4.4×10-5T2 for x = 0.3, respectively. It is known that superconductivity exists in Bi thin films (Tc ~6 K)55 and the very air and moisture sensitive CsBi2 (Tc ~4.8 K).56 In order to determine if the samples exhibit bulk superconductivity and exclude the possible superconducting filament arising from Bi thin film or CsBi2, we conducted magnetic susceptibility measurements on Cs0.82K0.18Bi4Te6 and Cs0.78K0.63Bi3.54Te6 as representative single crystals. For Cs0.82K0.18Bi4Te6, the magnetization observed at 2 K in the zero-field cooled (ZFC) measurement is estimated to be about 10% of that expected for full diamagnetism, Figure 4a. A higher superconducting volume fraction is expected below 2 K because the diamagnetism is still increasing steeply with decreasing temperature. From the low-field dependence magnetic susceptibility data (Figure 4b), a zero-temperature lower critical field Hc1(0) ~4 mT was obtained (Figure 4c).



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Detailed temperature dependent resistivity curves under various applied magnetic fields were measured to evaluate the upper critical field Hc2(0). As shown in Figure 4d, Tc decreases monotonically with increasing field from 0 − 7 T. Ginzburg-Landau (GL) theory43 and Werthamer-Helfand-Hohenberg (WHH) formula45 fittings give a high Hc2(0) ~15.6 and 14.1 T, respectively. The calculated GL coherence ξ(0) ~ 4.7 nm and the penetration depth λ(0) ~460 nm. The corresponding GL parameter κ (λ(0)/ξ(0)) of 98 identifies this material as an extreme type-II superconductor, similar to Nb doped Bi2Se3.28, 57 Cs0.9K0.1Bi4Te6 has a similar value of Hc2(0) ~15.5 (GL) and 13.6 T (WHH) based on similar analysis, Figure 4f (and Figure S5 and S6). This demonstrates that the introduction of more K in the structure has little effect on Hc2(0) in the monoclinic structure. As shown in Figure 5a, the superconducting volume fraction at 2 K for Cs0.78K0.63Bi3.54Te6 is almost 100%, confirming full bulk superconductivity at this temperature. The temperature and field dependent magnetic susceptibility (Figure 5b) suggests a value of ~2.5 mT for Hc1(0) (Figure 5c). Tc also decreases monotonically with increasing field from 0 to 0.4 T, Figure 5d. GL and WHH models give Hc2(0) 1.2 and 1 T, respectively. From similar analysis as above, we obtain ξ(0) ~33 nm, λ(0) ~450 nm and κ ~14, suggesting also a type-II superconductor. For the other orthorhombic compounds, field dependent resistivities are shown in Figure S7 − S10. All Hc2(0) are plotted in Figure 5f. Generally, GL theory gives minor larger Hc2(0) than the values calculated by the WHH formula. Hc2(0) of CsBi3.52Na0.45Te6 is determined to be 1.02 T from GL theory and 0.89 T from WHH formula which is smaller compared to that in CsBi3.55Na0.39Te6 (1.3 T from GL theory and 1.08 T from


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WHH formula). This demonstrates that increasing the Na fraction into CsBi4Te6 has negative effect on Hc2(0). As for the orthorhombic K doped compounds, an increased Hc2(0) of 1.54 T (GL theory) and 1.25 T (WHH formula) was obtained in Cs0.72K0.67Bi3.54Te6. When more K is introduced, Hc2(0) is much decreased, with 0.9 and 0.86 T for Cs0.64K0.74Bi3.55Te6, 0.86 and 0.8 T for Cs0.63K0.79Bi3.54Te6, and 0.86 and 0.71 T Cs0.58K0.83Bi3.53Te6, calculated by GL theory and the WHH formula, respectively. The effect on Hc2(0) of K doping shows a dome-like behavior in the orthorhombic crystals, which is different from that in the monoclinic crystals as discussed above. We note that WHH/GL extrapolations may give an over-evaluated Hc2(0) because it is valid when only the orbital effects are considered. Most superconducting materials with a spin-singlet pairing have a Pauli paramagnetic limiting field (Hc2(0) ~1.85*Tc) which breaks the singlet Cooper pair . However, the Pauli limiting field value only considers the simple one-band BCS superconductor case. There are other factors such as multiband and strong spin-orbit coupling which can also affect the upper critical field of a superconductor. To more precisely determine the upper critical fields for these samples need to be further investigated at even lower temperatures and higher magnetic fields. Heat Capacity. To evaluate the electronic contribution to the superconductivity, we conducted specific heat measurements on Cs0.63K0.79Bi3.54Te6 single crystals. The data confirms the bulk superconducting transition around 3.4 K, which is consistent with the resistivity results presented above, Figure 6. The normal-state data below 4 K (in blue dash line) is well fitted by C(T) = γT + β1T3 + β2T5, where γT and β1T3 + β2T5 are the electron and phonon contributions to the specific heat, respectively. The calculated coefficients are γ = 8.49 mJ mol-1 K-2, β1 = 8.37 mJ mol-1 K-4, and β2 = 0.19 mJ mol-1 K-



6

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. The Debye temperature is low, ΘD = (12π4NR/5β)1/3 ~135.9 K, and consistent with the

heavy element nature of the compounds. The electron m* is estimated to be 11.4 m0 from the expression γ = π2/3κB2N(EF) = 1.36 × 10-4 × Vmol2/3nγ1/3m*/m0,51 where Vmol is the molar volume, nγ is the carrier concentration per atom, and m*/m0 is the effective mass. As shown in Figure 6b, the electronic contribution Cel = C(T) − β1T3 − β2T5 is normalized by the normal state contribution γC(T). The dimensionless specific-heat jump (Cel/(γTc)) is evaluated to be ~1.64 which is close to the theoretical value (1.43) from the wellknown Bardeen−Cooper−Schrieffer (BCS) theory. Phase Diagram. All of the characteristic parameters (Tc, structure, carrier types and Na/K ratios) are summarized in a phase diagram plotted in Figure 7a as a function of Na and K content. Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6 have the p-type monoclinic CsBi4Te6-type structure with Tc ~5 K. For CsBi3.55Na0.39Te6, CsBi3.52Na0.45Te6, Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6, the structure changes to n-type RbBi3.67Te6 structure with Tc ~3 K. Within the same structure type, Tc increases with increasing Na and K fractions. For comparison, the maximal Tc and Hc2(0) with the previous well-known bismuth chalcogenide-based superconductors of SrxBi2Se3,25 CuxBi2Se3,2 NbxBi2Se3,28,

57

Tl0.6Bi2Te3,16 CsBi4Te633 and RbBi6.67Te634 are reported to be 2.9 K, 2.1 T; 3.8 K, 3 T; 3.1 K, 1.8 T; 2.3 K, 1.1 T; 4.4 K, 9.7 T; and 3.2 K, 0.5 T; respectively. As shown in Figure 7b, it is obvious that both values obtained from Cs0.82K0.18Bi4Te6 are much higher. For CuxBi2Se3,2 NbxBi2Se3,28,

57

and Tl0.6Bi2Te3,16 the carrier density is determined to be

around 1020 cm-3 which is a magnitude higher than that in Cs0.82K0.18Bi4Te6. This further


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points to an unusual nature of the superconducting mechanism in CsBi4Te6, which suggests additional experimentation needed in the future.

Conclusions CsBi4Te6 is a narrow-gap, low crystal symmetry semiconductor that exhibits superconductivity through an extensive range of alloying. With the exception for low K (y ≤ 0.18), the monoclinic CsBi4Te6 structure is destabilized and transforms to the very different orthorhombic RbBi3.67Te6-type structure with Na or K doping with a concomitant change in dominant carriers from holes (p-type) to electrons (n-type). Superconductivity is present in all compositions and Tc increases with increasing level of alkali metal fractions. Moreover, the effect of Na and K alloying on the upper critical field Hc2(0) results different behaviors in the two different structures. Magnetization and heat capacity results demonstrate bulk, type-II superconductivity. The Na and K doped ternary CsBi4Te6 system provides an excellent framework for investigation of complex multinary phases with respect to superconductivity arising from narrow-gap semiconductors.

ASSOCIATED CONTENT Supporting Information. Crystal data, structure refinement, and atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2×103) with estimated standard

deviations

in

parentheses

for

CsBi3.52Na0.45Te6,

Cs0.82K0.18Bi4Te6,

Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6 at 293 K. Powder X-ray diffraction patterns (PXRD) for K-doped CsBi4Te6. Temperature 17 ACS Paragon Plus Environment


dependence

of

Hall

Cs0.78K0.63Bi3.54Te6,

resistivity

for

CsBi3.55Na0.39Te6,

Cs0.72K0.67Bi3.54Te6,

Page 18 of 35

CsBi3.52Na0.45Te6

Cs0.64K0.74Bi3.55Te6,

and

Cs0.63K0.79Bi3.54Te6,

Cs0.58K0.83Bi3.53Te6. Calculation of upper critical field (Hc2(T)) for CsBi3.55Na0.39Te6, CsBi3.52Na0.45Te6

and

Cs0.78K0.63Bi3.54Te6,

Cs0.72K0.67Bi3.54Te6,

Cs0.64K0.74Bi3.55Te6,

Cs0.63K0.79Bi3.54Te6, Cs0.58K0.83Bi3.53Te6 using the Werthamer-Helfand-Hohenberg (WHH) theory (red solid line) and the Ginzburg-Landau (GL) theory (blue solid line).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The authors declare no competing financial interest. Acknowledgements This research was primarily performed in the Materials Science Division of Argonne National Laboratory and supported by the U.S. Department of Energy, Ofﬁce of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. SEM/EDS was conducted by the use of the EPIC, Keck-II, and/or SPID facility(ies) of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.


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(34) Malliakas, C. D.; Chung, D. Y.; Claus, H.; Kanatzidis, M. G., Superconductivity in the narrow gap semiconductor RbBi11/3Te6. J. Am. Chem. Soc. 2016, 138, (44), 14694−14698. (35) Prakash, O.; Kumar, A.; Thamizhavel, A.; Ramakrishnan, S., Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure. Science 2017, 355, 52−55. (36) Hsu, K. F.; Chung, D. Y.; Lal, S.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis, M. G., CsMBi3Te6 and CsM2Bi3Te7 (M = Pb, Sn): New thermoelectric compounds with low-dimensional structures. J. Am. Chem. Soc. 2002, 124, (11), 2410−2411. (37) Malliakas, C. D., Chung, D. Y., Claus H., Kanatzidis, M. G., Superconductivity in the 2-dimensional homologous series AMmBi3Q5+m (A = Cs, Rb; M = Pb, Sn; Q = Se, Te). Chem. Eur. J. 2018, 24, 1−6. (38) Shannon, R. D., Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. A 1976, 32, (5), 751−767. (39) X-RED;, X.-A. X.-S., STOE & Cie GMbH: Darmstadt,Germany 2009. (40) Palatinus, L.; Chapuis, G., SUPERFLIP–a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J. Appl. Crystallogr. 2007, 40, (4), 786−790. (41) Petříček, V.; Dušek, M.; Palatinus, L., Crystallographic computing system JANA2006: general features. Z. Krist-Cryst Mater 2014, 229, (5), 345−352. (42) Sheldrick, G. M., A short history of SHELX. Acta Crystallogr. A 2008, 64, (1), 112−122. (43) Ginzburg, V.; Landau, L., On superconductivity theory. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki 1950, 20, 1064. (44) Tinkham, M., Introduction to superconductivity. Courier Corporation: 1996. (45) NR, W.; HELFAND, E.; PC, H., Temperature and purity dependence of superconducting critical field Hc2. 3. electron spin and spin-orbit effects. Phys. Rev. 1966, 147, (1), 295. (46) Yamana, K., Kihara, K., Matsumoto, T., Bismuth tellurides: BiTe and Bi4Te3. Acta Cryst. 1979, B35, 147−149. (47) Kalpen, H., Honle, W., Somer, M., Schwarz, U., Peters, K., Schnering, H. G., Bismut(II)-chalkogenometallate(III) Bi2M4X8, Verbindungen mit Bi24+-Hanteln (M = Al, Ga; X = S, Se). Z. Anorg. Allg. Chem. 1998, 624, 1137−1147. (48) McCarthy, T. J.; Ngeyi, S. P.; Liao, J. H.; DeGroot, D. C.; Hogan, T.; Kannewurf, C. R.; Kanatzidis, M. G., Molten salt synthesis and properties of three new solid-state ternary bismuth chalcogenides,. β-CsBiS2, γ-CsBiS2, and K2Bi8Se13. Chem. Mater. 1993, 5, (3), 331−340. (49) Chung, D. Y.; Choi, K. S.; Iordanidis, L.; Schindler, J. L.; Brazis, P. W.; Kannewurf, C. R.; Chen, B.; Hu, S.; Uher, C.; Kanatzidis, M. G., High Thermopower and Low Thermal Conductivity in Semiconducting Ternary K−Bi−Se Compounds. Synthesis and Properties of β-K2Bi8Se13 and K2.5Bi8.5Se14 and Their Sb Analogues. Chem. Mater. 1997, 9, (12), 3060−3071.



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(50) Zhang, G.; Li, G.; Huang, F.; Liao, F.; Li, K.; Wang, Y.; Lin, J., Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3. J. Alloy. Compd. 2011, 509, (41), 9804−9808. (51) Pei, Y.; Chang, C.; Wang, Z.; Yin, M.; Wu, M.; Tan, G.; Wu, H.; Chen, Y.; Zheng, L.; Gong, S., Multiple converged conduction bands in K2Bi8Se13: A promising thermoelectric material with extremely low thermal conductivity. J. Am. Chem. Soc. 2016, 138, (50), 16364−16371.(52) Chen, H. J.; Zhang, G. H.; Hu, T.; Mu, G.; Li, W.; Huang, F. Q.; Xie, X. M.; Jiang, M. H., Effect of local structure distortion on superconductivity in Mg-and F-codoped LaOBiS2. Inorg. Chem. 2013, 53, (1), 9−11. (53) Shen, B.; Yang, H.; Wang, Z. S.; Han, F.; Zeng, B.; Shan, L.; Ren, C.; Wen, H.H., Transport properties and asymmetric scattering in Ba1−xKxFe2As2 single crystals. Phys. Rev. B 2011, 84, (18), 184512. (54) Chen, H. J.; Narayan, A.; Stoumpos, C. C.; Zhao, J.; Han, F.; Chung, D. Y.; Wagner, L. K.; Kwok, W. K.; Kanatzidis, M. G., Semiconducting Ba3Sn3Sb4 and Metallic Ba7–xSn11Sb15–y (x = 0.4, y = 0.6) Zintl Phases. Inorg. Chem. 2017, 56, (22), 14251−14259. (55) Haviland, D. B.; Liu, Y.; Goldman, A. M. Onset of superconductivity in the twodimensional limit. Phys. Rev. Lett. 1989, 62, 2180. (56) Roberts, B. W. J. Phys. Chem. Ref. Data. 1976, 5, 581. (57) Smylie, M.; Claus, H.; Welp, U.; Kwok, W. K.; Qiu, Y.; Hor, Y. S.; Snezhko, A., Evidence of nodes in the order parameter of the superconducting doped topological insulator NbxBi2Se3 via penetration depth measurements. Phys. Rev. B 2016, 94, (18), 180510.


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Table 1. Analyzed composition, approximate yield, carrier density (n) at 300 and 10 K, superconducting transition temperature (Tc), and upper critical field (Hc2(0)) of Na- and K-doped CsBi4Te6 single crystals doping level attempted x=0

Composition (EDS/SEM analysis) CsBi4Te6

composition (refined crystal structure) CsBi4Te6

Structure type

n at 300 K (1019 cm-3) 2.4

n at 10 K (1019 cm-3) 3.2

Tc (K)

Hc2(0) WHH fit (T)

Hc2(0) GL fit (T)

Monoclinic

Approx. yield (%) 100

x = 0.1

CsNa0.4Bi3.5Te6

CsBi3.55Na0.39Te6

Orthorhombic

70

14.5

7.5

3.13

1.1

1.3

x = 0.2

CsNa0.5Bi3.5Te6

CsBi3.52Na0.45Te6

Orthorhombic

60

16.3

5.9

3.22

0.9

1.0

y = 0.1

Cs0.9K0.1Bi4Te6

Cs0.91K0.09Bi4Te6

Monoclinic

100

2.3

2.7

4.92

13.6

15.5

y = 0.2

Cs0.8K0.2Bi4Te6

Cs0.82K0.18Bi4Te6

Monoclinic

100

2.1

2.6

5.07

14.1

15.5

y = 0.3

Cs0.8K0.55Bi3.5Te6

Cs0.78K0.63Bi3.54Te6

Orthorhombic

80

11.1

5.5

3.04

1.0

1.2

y = 0.4

Cs0.7K0.6Bi3.5Te6

Cs0.72K0.67Bi3.54Te6

Orthorhombic

70

18.7

7.3

3.30

1.2

1.5

y = 0.5

Cs0.65K0.65Bi3.5Te6

Cs0.64K0.74Bi3.55Te6

Orthorhombic

60

14.7

7.9

3.42

0.8

0.9

y = 0.6

Cs0.6K0.7Bi3.5Te6

Cs0.63K0.79Bi3.54Te6

Orthorhombic

50

16.4

8.0

3.48

0.8

0.9

y = 0.7

Cs0.56K0.8Bi3.5Te6

Cs0.58K0.83Bi3.53Te6

Orthorhombic

40

17.9

7.4

3.56

0.8

0.9


Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 24 of 35

Table 2. Crystal data and structure refinements for CsBi3.55Na0.39Te6, Cs0.91K0.09Bi4Te6 and Cs0.78K0.63Bi3.54Te6 at 293(2) K Empirical formula

CsBi3.55Na0.39Te6

Cs0.91K0.09Bi4Te6

Cs0.78K0.63Bi3.54Te6

Formula weight Temperature Wavelength Crystal system Space group

Volume

1648.25 293(2) K 0.71073 Å Orthorhombic Pnma a = 28.367(6) Å, α = 90° b = 4.4143(9) Å, β = 90° c = 12.767(3) Å, γ = 90° 1598.7(6) Å3

Z Density (calculated)

4 6.848 g/cm3

1725.52 293(2) K 0.71073 Å Monoclinic C2/m a = 51.535(10) Å, α = 90° b = 4.3805(9) Å, β = 101.47(3)° c = 14.424(3) Å, γ = 90° 3191.2(12) Å3 8

1633.26 293(2) K 0.71073 Å Orthorhombic Pnma a = 28.228(6) Å, α = 90° b = 4.4035(9) Å, β = 90° c = 12.720(3) Å, γ = 90° 1581.1(6) Å3 4

7.183 g/cm3

6.861 g/cm3

Absorption coefficient

51.856 mm-1

F(000) Crystal size

2662 0.9864 × 0.0753 × 0.0687 mm3

56.753 mm-1 5565

52.027 mm-1 2642

θ range for data collection

2.681 to 24.999° -33 ≤ h ≤ 33, -5 ≤ k ≤ 4, -15 ≤ l ≤ 14 9924 1604 [Rint = 0.1395]

0.6995 × 0.0685 × 0.0228 mm3 1.613 to 24.998° -60 ≤ h ≤ 60, -5 ≤ k ≤ 4, -17 ≤ l ≤ 17 9851

0.9621 × 0.0684 × 0.0873 mm3 1.756 to 29.303° -38 ≤ h ≤ 38, -6 ≤ k ≤ 5, -17 ≤ l ≤ 15 14862

3192 [Rint = 0.1317]

2397 [Rint = 0.1634]

96.9%

96.8%

99.9%

Full-matrix least-squares on F2

Full-matrix least-squares on F2 3192 / 0 / 136

Full-matrix least-squares on F2 2397 / 5 / 77

Unit cell dimensions

Index ranges Reflections collected Independent reflections Completeness to θ = 25.242° Refinement method Data / restraints /

1604 / 5 / 76


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parameters Goodness-of-fit Final R indices [I > 2σ(I)]

1.207 Robs = 0.0967, wRobs = 0.2187

1.277

1.202

Robs = 0.0838, wRobs = 0.2364

Robs = 0.0646, wRobs = 0.1774

R indices [all data]

Rall = 0.1095, wRall = 0.2287

Rall = 0.1001, wRall = 0.2426 0.000033(11)

Rall = 0.0974, wRall = 0.1917 0.00055(7)

Extinction coefficient 0.00034(6) Largest diff. peak and hole 4.298 and -4.988 e·Å-3 8.960 and -3.890 e·Å-3 4.600 and -8.104 e·Å-3 R = Σ||Fo| − |Fc||/Σ|Fo|, wR = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[w(|Fo|4)]}1/2 and w = 1/[σ2(Fo2) + (0.0850P)2 + 1001.2255P] for Cs0.91K0.09Bi4Te6,w = 1/[σ2(Fo2) + (0.0825P)2] for Cs0.78K0.63Bi3.54Te6 and w = 1/[σ2(Fo2) + (0.1004P)2 + 129.1664P] for CsBi3.55Na0.39Te6 where P = (Fo2 + 2Fc2)/3.



Page 26 of 35

Table 3. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2×103) for CsBi3.55Na0.39Te6 at 293(2) K with estimated standard deviations in parentheses Label x y z Occupancy Ueq* Bi(1)

407(1)

2500

6256(2)

0.882(8)

Na(1) 407(1) 2500 6256(2) 0.110(15) Bi(2) 1310(1) 7500 3802(2) 0.917(8) Na(2) 1310(1) 7500 3802(2) 0.075(15) Bi(3) 463(1) 2500 1252(2) 0.801(8) Na(3) 463(1) 2500 1252(2) 0.191(15) Bi(4) 1277(1) 7500 8738(2) 0.921(8) Na(4) 1277(1) 7500 8738(2) 0.071(15) Te(1) 2017(2) 12500 3992(2) 1 Te(2) 1160(2) 7500 6262(2) 1 Te(3) 420(1) 2500 3753(2) 1 Te(4) 366(2) 2500 8743(2) 1 Te(5) 1246(2) 7500 1257(2) 1 Te(6) 1985(2) 2500 8552(2) 1 Cs(1) 2468(2) 2500 1267(2) 1 * Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

34(1) 34(1) 34(1) 34(1) 36(1) 36(1) 34(1) 34(1) 37(1) 37(1) 35(1) 36(1) 39(1) 38(1) 51(1)


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Table 4. Atomic coordinates (×104), equivalent isotropic displacement parameters (Å2×103), and occupancies for Cs0.91K0.09Bi4Te6 at 293(2) K with estimated standard deviations in parentheses Label x y z Occupancy Ueq* Bi(1)

8637(1)

0

2503(2)

1

Bi(2) 6143(1) 0 2780(2) 1 Bi(3) 7352(1) 0 2594(2) 1 Bi(4) 1627(1) 0 915(2) 1 Bi(5) 2876(1) 0 890(2) 1 Bi(6) 4115(1) 0 634(2) 1 Bi(7) 9849(1) 0 2128(2) 1 Bi(8) 394(1) 0 1361(2) 1 Te(1) 2249(1) 0 767(2) 1 Te(2) 6755(1) 0 2428(2) 1 Te(3) 1042(1) 0 999(2) 1 Te(4) 7396(1) -5000 4019(2) 1 Te(5) 3467(1) 0 730(2) 1 Te(6) 6236(1) -5000 4210(2) 1 Te(7) 9200(1) 0 2159(2) 1 Te(8) 7980(1) 0 2445(2) 1 Te(9) 5574(1) 0 2811(2) 1 Te(10) 8772(1) -5000 3914(2) 1 Te(11) 4678(1) 0 393(3) 1 Te(12) 4924(1) 0 3594(3) 1 Cs(1) 3129(1) 0 4838(3) 0.90(3) K(1) 3129(1) 0 4838(3) 0.10(3) Cs(2) 9455(1) 0 4915(3) 0.91(3) K(2) 9455(1) 0 4915(3) 0.09(3) * Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

18(1) 17(1) 17(1) 18(1) 19(1) 19(1) 22(1) 22(1) 13(1) 16(1) 14(1) 18(1) 13(1) 19(1) 18(1) 18(1) 19(1) 21(1) 19(1) 25(1) 31(2) 31(2) 32(2) 32(2)



Page 28 of 35

Table 5. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2×103) for Cs0.78K0.63Bi3.54Te6 at 293(2) K with estimated standard deviations in parentheses Label x y z Occupancy Ueq* Bi(1)

407(1)

2500

8753(1)

0.887(6)

K(2) 407(1) 2500 8753(1) 0.100(14) Bi(2) 462(1) 2500 3750(1) 0.858(6) K(3) 462(1) 2500 3750(1) 0.129(14) Bi(3) 1314(1) 7500 11200(1) 0.898(6) K(4) 1314(1) 7500 11200(1) 0.088(14) Bi(4) 1284(1) 7500 6268(1) 0.897(6) K(5) 1284(1) 7500 6268(1) 0.088(14) Te(1) 1992(1) 12500 6439(2) 1 Te(2) 1164(1) 7500 8741(2) 1 Te(3) 420(1) 2500 11246(2) 1 Te(4) 2023(1) 2500 11019(2) 1 Te(5) 366(1) 2500 6253(2) 1 Te(6) 1249(1) 7500 3747(2) 1 Cs(1) 2466(1) 2500 3736(2) 0.778(13) K(1) 2466(1) 2500 3736(2) 0.222(13) * Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

20(1) 20(1) 21(1) 21(1) 19(1) 19(1) 19(1) 19(1) 25(1) 22(1) 21(1) 24(1) 20(1) 26(1) 34(1) 34(1)


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Figure Captions

Figure 1. (a) An as-grown ingot obtained by opening the tube (left). Crushing the ingot produces needle-like single crystals (right). (b) Structure changes from monoclinic (CsBi4Te6-type) to orthorhombic (RbBi3.67Te6-type) with the introduction of Na and K.



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Figure 2. Temperature dependence of carrier density (n) for (a) p-type undoped CsBi4Te6, (b) n-type CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6, (c) p-type Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6, and (d) n-type Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6. The n increases slightly with decreasing temperature for the compounds with monoclinic structures, but for the compounds with orthorhombic structures, it shows an inverse trend with temperature. Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6 and Cs0.58K0.83Bi3.53Te6 behave the same way, indicating that n keeps consistent with increasing K doping.


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Figure 3. Temperature dependence of normalized resistance (ρ/ρ6K) for (a) CsBi3.55Na0.39Te6 and CsBi3.52Na0.45Te6 from 4 to 1.8 K, and (b) Cs0.91K0.09Bi4Te6 (K0.09), Cs0.82K0.18Bi4Te6 (K0.18), Cs0.78K0.63Bi3.54Te6 (K0.63), Cs0.72K0.67Bi3.54Te6 (K0.67), Cs0.64K0.74Bi3.55Te6 (K0.74), Cs0.63K0.79Bi3.54Te6 (K0.79) and Cs0.58K0.83Bi3.53Te6 (K0.83) from 6 K to 1.8 K. (c) Temperature dependence of resistivity for monoclinic Cs0.82K0.18Bi4Te6 from 300 to 1.8 K. The inset shows the data below 50 K is well fitted with ρ = 0.84 + 1.5×10-4T2 formula. (d) Temperature dependence of resistivity for orthorhombic Cs0.78K0.63Bi3.54Te6 from 300 K to 1.8 K. The inset shows the data below 50 K is well fitted with ρ = 0.2 + 4.4×10-5T2 formula.



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Figure 4. (a) Temperature dependence of magnetic susceptibility (4πχ) of Cs0.82K0.18Bi4Te6 under zero-field-cooled (ZFC) and field-cooled (FC) procedures with applied magnetic field of 0.5 mT below 6 K. (b) Small field (0 − 50 mT) dependent 4πχ at different temperatures (2.0 − 4.4 K). (c) Calculation of the lower critical field at zero temperature (Hc1(0)), which is determined to be ~4 mT. (d) Field dependent resistivity from 1.8 to 6 K. (e) Calculation of upper critical field (Hc2(T)) using the GinzburgLandau (GL) theory and the Werthamer-Helfand-Hohenberg (WHH) formula. The two models yield an upper critical field of ~15.6 and 14.1 T, respectively. (f) Comparison of Hc2(0) for Cs0.91K0.09Bi4Te6 and Cs0.82K0.18Bi4Te6. 32 ACS Paragon Plus Environment

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Figure 5. (a) Temperature dependence of magnetic susceptibility (4πχ) of Cs0.78K0.63Bi3.54Te6 under zero-field-cooled (ZFC) and field-cooled (FC) procedures with applied magnetic field of 0.5 mT below 5 K. (b) Small field (0 − 50 mT) dependent 4πχ at different temperatures (2.0 − 3.0 K). (c) Calculation of the lower critical field at zero temperature (Hc1(0)), which is determined to be ~2.5 mT. (d) Field dependent resistivity from 1.8 to 4 K. (e) Calculation of upper critical field (Hc2(T)) using the GinzburgLandau (GL) theory and the Werthamer-Helfand-Hohenberg (WHH) formula. The two 33 ACS Paragon Plus Environment


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models yield an upper critical field of ~1.2 and 1 T, respectively. (f) Comparison of Hc2(0) for CsBi3.55Na0.39Te6, CsBi3.52Na0.45Te6 (olive and violet colors) and Cs0.78K0.63Bi3.54Te6, Cs0.72K0.67Bi3.54Te6, Cs0.64K0.74Bi3.55Te6, Cs0.63K0.79Bi3.54Te6, Cs0.58K0.83Bi3.53Te6 (black and red colors).

Figure 6. (a) Specific heat for Cs0.63K0.79Bi3.54Te6 divided by temperature (C/T) as a function of T2. (b) Temperature dependent plot of electron heat capacity divided by γT (Cel/γT; Cel = C(T) − β1T3 − β2T5) where γT and β1T3 + β2T5 are the electron and phonon contributions to the specific heat, respectively. A Cel/γTc value of 1.64 was obtained.

Figure 7. (a) Phase diagram with superconducting transition temperature (Tc) as a function of Na and K content alloyed in CsBi4Te6. (b) Comparison of the maximal Tc and upper critical field Hc2(0) from this work with the other Bi-Q (Q = Se, Te)-based superconductors (SrxBi2Se3,25 CuxBi2Se3,2 NbxBi2Se3,28, 57 Tl0.6Bi2Te3,16 CsBi4Te633 and RbBi6.67Te634). 34 ACS Paragon Plus Environment

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Superconductivity and Structural Conversion with Na and K doping of

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