Superconductivity in an Organometallic Compound PCCP Physical Chemistry Chemical Physics Rsc.Li/Pccp

Showcasing research from the Group of Prof. Xiao-Jia Chen, As featured in: Center for High Pressure Science and Technology Advanced

Research, China Volume 21 Number 47 21 December 2019 Pages 25927–26264 Superconductivity in an organometallic compound PCCP Physical Chemistry Chemical Physics rsc.li/pccp

This work reports superconductivity with T c of 3.6 K in a potassium-doped organometallic compound – tri- o -tolylbismuthine – grown by a two-step method, i .e . ultrasound treatment and low temperature annealing. Using magnetic-susceptibility and electrical-resistivity measurements, the superconductivity in synthesized samples is supported by both the Meissner effect and the zero-resistivity ISSN 1463-9076 PAPER Jeremy K. Cockcroft, Adrian R. Rennie et al . Understanding the structure and dynamics of cationic state. The superconducting phase and its composition are surfactants from studies of pure solid phases determined by the combined studies of X-ray diffraction and See Yun Gao, Zhong-Bing Huang, theoretical calculations as well as Raman spectroscopy. The Xiao-Jia Chen et al ., present work enriches the physical properties of organometallic Phys . Chem . Chem . Phys ., 2019, 21 , 25976. compounds in superconductivity.

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Superconductivity in an organometallic compound† Cite this: Phys. Chem. Chem. Phys., 2019, 21, 25976 Ren-Shu Wang, ‡ab Liu-Cheng Chen, ‡b Hui Yang,a Ming-An Fu,a Jia Cheng,a Xiao-Lin Wu,a Yun Gao, *a Zhong-Bing Huang *a and Xiao-Jia Chen*b

Organometallic compounds constitute a very large group of substances that contain at least one metal-to- carbon bond in which the carbon is part of an organic group. They have played a major role in the development of the science of chemistry. These compounds are used to a large extent as catalysts (substances that increase the rate of reactions without themselves being consumed) and as intermediates in the laboratory and in industry. Recently, novel quantum phenomena such as topological insulators and superconductors were also suggested in these materials. However, there has been no report on the experimental exploration of the topological state. Evidence for superconductivity from the zero-resistivity state in any organometallic compound has not been achieved yet, though much effort has been made. Here we report the experimental realization of superconductivity with a critical temperature of 3.6 K in a potassium-doped organometallic compound, i.e. tri-o-tolylbismuthine, with evidence of both the Meissner effect and the zero-resistivity state through dc and ac magnetic susceptibility measurements. The obtained superconducting parameters classify this compound as a type-II superconductor. The benzene ring is identified to be the essential superconducting unit in such a phenyl organometallic compound. The Received 30th July 2019, superconducting phase and its composition are determined by combined studies of X-ray diffraction and Accepted 2nd October 2019 theoretical calculations as well as Raman spectroscopy measurements. These findings enrich the DOI: 10.1039/c9cp04227j applications of organometallic compounds in superconductivity and add a new electron-acceptor family of organic superconductors. This work also points to a large pool for finding superconductors rsc.li/pccp from organometallic compounds.

1 Introduction were suggested theoretically as candidates for topological insulators8 due to their lattice symmetry and strong spin–orbit coupling. In Organometallic compounds, first discovered by Zeise of general, many topological insulators can become superconductors Denmark in 1827, have been an active frontier in organic, by chemical doping9 or the application of external pressure.10 It is inorganic, coordination and biological chemistry.1 These com- interesting to examine whether such organometallic compounds pounds usually contain at least one chemical bond connecting a could eventually exhibit superconducting properties through carbon (C) atom of an organic molecule and a metal element (M). some external modifications. Their extensive prospects have been found in applications as As a typical organometallic compound, tri-o-tolylbismuthine catalysts2,3 (most famously, Ziegler–Natta catalysts), organic (o-TTB) with the structure of three methyls connected to triphe- synthesis reagents4,5 (most commonly, Grignard reagents), nylbismuth (TPB) in the ortho-position of the C–Bi bond is one anticancer drugs (e.g. metallocene dihalides6), and other special example of inexpensive and nontoxic organobismuth reagents. reagents7 (such as antiseptics, sterilizers, and insecticides). These reagents are widely used in chemical synthesis such as in However, the applications of their physical properties have not the preparation of transition metal complexes or as catalysts for been widely addressed as expected. Recently, these compounds polymerization reactions in medicinal chemistry.11 Very recently, the Meissner effect was reported in potassium (K)-doped TPB,12 a School of Materials Science and Engineering, Faculty of Physics and Electronic though the evidence for superconductivity from electrical transport Technology, Hubei University, Wuhan 430062, China. measurements is still absent. Meanwhile, tri-p-tolylbismuthine E-mail: [email protected], [email protected] (p-TTB), a methyl derivative of TPB, was found to possess super- b Center for High Pressure Science and Technology Advanced Research, conductivity at 3.6 and/or 5.3 K after K doping.13 Here, we choose Shanghai 201203, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ an o-TTB molecule to examine possible superconductivity by c9cp04227j doping an alkali metal. Differing from TPB, one surrounding ‡ These authors contributed equally to this work. hydrogen in each phenyl is replaced by methylphenyl in o-TTB.

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Although both compounds still share similar physicochemical Monkhorst–Pack k-point mesh were adopted for geometry opti- properties, adding CH3 in o-TTB affects the molecular geometry mization. A finer 5 5 5 k-point sampling scheme was used due to the different steric hindrance. This in turn induces addi- for calculating the density of states. The convergence criteria for tional C–H intermolecular interactions. Previous studies14 have the energy and max force are set to 104 eV and 0.015 eV Å1, shown that the Bi–Bi distance increases from 5.11 to 5.71 Å from respectively. TPB to o-TTB. This may facilitate metal-doping in o-TTB molecules from a spatial arrangement perspective. On the other hand, the addition of methyl generally reduces the symmetry because of the 3 Results and discussion introduction of the lattice distortion. As a result, the symmetry space group changes from C2/c for monoclinic TPB to P1for The Meissner eﬀect and zero-resistance are two essential char- triclinic o-TTB. This degradation in structural symmetry as well as acters for superconductivity. The Meissner eﬀect in K-doped intercalation of metal atoms can lead to an increase of the carrier o-TTB with the original stoichiometric ratio x of 3 (labeled by A) concentration, which helps to enhance the electrical conductivity. is confirmed by both dc and ac magnetic susceptibility (w) Therefore, o-TTB is expected to exhibit better electrical transport measurements. Fig. 1(a) shows the temperature dependence properties by simple addition of functional groups. of the dc magnetic susceptibility (wdc) with zero-field cooling (ZFC) and field cooling (FC) runs at a magnetic field of 10 Oe.

One can see a sudden drop in wdc around 3.6 K in both the ZFC

2 Experimental and computational and FC runs. The sudden drop of wdc, originating from the methods well-defined Meissner effect, indicates the occurrence of superconductivity in the studied organometallic compound. T is 2.1 Experimental details c defined as the temperature where a sharp drop takes place. The Synthesis of potassium-doped o-TTB is the same method as that of shielding fraction extracted from wdc at a temperature of 1.8 K is 12 intercalated triphenylbismuth, completed by a two-step synthesis about 17%. This is around five times higher than the reported method, i.e. ultrasound treatment and low temperature annealing. one (3.74%) in K-doped TPB.12 This significant improvement Alkali-metal potassium (99% purity, Sinopharm Chemical Reagent) could benefit from the contributions of the functional group was cut into patches and mixed with o-TTB (498% purity, Tokyo –CH3. Fig. 1(c) shows the magnetic hysteresis (M–H) loop with Chemical Industry). The original stoichiometric ratio (K:o-TTB) is scanning the magnetic field up to 2 kOe in the temperature x :1(x = 3 and 2). The mixture was sealed in quartz tube with a high range of 2–3 K. The clear diamond-like hysteresis loop shrinks 4 vacuum (10 Pa). Then novel ultrasonic processing at 90 1Cfor with increasing temperature. This behavior coincides with the 10 h has the advantage of sufficient mixing. The low temperature feature of a typical type-II superconductor. M decreases linearly annealing can avoid KH to yield quality doped crystals. To measure with increasing H in the initial stage until the applied field the physical properties of the doped samples, the obtained material H exceeds the lower critical field (Hc1). The temperature-dependent was placed into different airtight containers to prevent the dissocia- Hc1 is shown in Fig. 1(d). The inset of Fig. 1(d) shows the method tion of the sample. A nonmagnetic capsule, diamond anvil cell for determining the value of Hc1 at 2 K, namely the field deviation (DAC), capillary and mylar film were used for sample protection to from linear behavior. An Hc1(0) value of 62 2Oeisthusobtained take magnetization, resistivity, Raman scattering and X-ray diffrac- from the extrapolation to zero temperature based on the empirical tion measurements, respectively. The magnetization measurements 2 law Hc1(T)/Hc1(0) = 1 (T/Tc) . In Fig. 1(b), the diamagnetic signal were completed using a Magnetic Properties Measurement System in the Meissner state is gradually suppressed until being comple- (Quantum Design MPMS3). The resistivity was measured using a tely faded by the applied magnetic field up to 2 kOe. This is an Physical Properties Measurement System (Quantum Design PPMS) intrinsic property of a type-II superconductor. with the help of the standard four-probe method. The X-ray diffrac- The Meissner effect was also observed below 3.6 K in one tion patterns for pure and doped o-TTB were collected using an X-ray sample with a mole ratio of 2 : 1 (labeled as sample B, see Fig. S1 diffraction spectrometer using Cu Ka radiation (Bruker D8). The in the ESI†). The observation of superconducting transitions Raman spectra were collected on an in-house system with a spectro- with a near-unanimous Tc in the samples with different starting meter (Princeton Instruments) at a wavelength of 660 nm and a stoichiometric ratios indicates that the actual mole ratio of power less than 1 mW to avoid any damage of the samples. synthetical K-doped o-TTB is independent of the original stoichiometric value. The superconducting shielding fraction 2.2 Computational details of sample B estimated from wdc at 1.8 K is 8.9%, almost half of Our theoretical calculations were performed by using the plane- sample A. wave pseudopotential method as implemented in the Vienna ab The ac magnetic susceptibility wac measurements provide initio simulation package (VASP) program.15,16 The generalized further evidence for the occurrence of the Meissner effect, as gradient approximation with the Perdew–Burke–Ernzerhof shown in Fig. 1(e). As a common technology for the identifi- formula17 for the exchange–correlation potential and the projector- cation of the existence of superconductivity, the real compo- 18 0 augmented wave method for the ionic potential were used to nent wac of the ac susceptibility reveals the magnetic shielding, 00 model the electron–electron and electron–ion interactions. Plane and the imaginary part wac is a measure of the magnetic 19 wavebasissetswithanenergycutoffof450eVanda3 3 3 irreversibility. Both components of wac exhibit exquisite

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Fig. 1 Magnetic susceptibility measurements for sample A. (a) Temperature dependence of the dc magnetic susceptibility wdc with an applied magnetic field of 10 Oe in field cooling (FC) and zero-field cooling (ZFC) runs. (b) The temperature dependence of wdc in the ZFC run at various magnetic fields. (c) The magnetic hysteresis loops with a magnetic field up to 2 kOe at temperatures of 2, 2.5, and 3 K. (d) The temperature dependence of the lower critical 2 field Hc1(T). Error bars represent estimated uncertainties in determining Hc1. The cyan zone represents the empirical formula Hc1(T)/Hc1(0) = 1 (T/Tc) .The inset shows the method for determining Hc1 at a temperature of 2 K based on the deviation from linear behavior. (e) The ac magnetic susceptibility with 00 0 imaginary wac (top) and real wac (bottom) components as a function of temperature. The applied amplitude and frequency in the wac measurements are 5 Oe and 234 Hz, respectively. changes at around 3.6 K. This temperature is exactly the same Fig. 2(a) shows the enlarged view of the low temperature as that detected from the temperature dependence of wdc. resistivity. Zero-resistivity behavior with Tc B 3.6 K can be Due to the absence of magnetic flux in the normal state, the clearly observed. Therefore, K-doped o-TTB is identified as a values of both parts are close to zero. Below Tc, the diamag- new organic superconductor from both the Meissner eﬀect and 0 12 netic signal in wac drops fast with lowering the temperature zero-resistivity state. In comparison with TPB, adding the due to the exclusion of magnetic flux. In the imaginary part, –CH3 group in o-TTB does not make much difference for Tc thefluxpenetratingthesamplefallsbehindtheappliedflux rather than bringing about the increase of the superconducting 00 to form a positive peak signal. Such a peak signal in wac shield fraction. This indicates that the phenyl is the essential implies a tendency to form zero-resistivity in the supercon- unit for holding superconductivity in phenyl organometallic ducting state at a qualitative level. compounds. The enhancement of the superconducting shield Now we check the existence of the zero-resistivity in K-doped fraction is a key factor for the realization of the zero-resistivity o-TTB. Fig. 2(a) displays the resistivity measurements on this state in K-doped o-TTB. Therefore, introducing the functional compound at ambient pressure. It can be seen that the change groups offers a simple but effective means to improve the of the resistivity with temperature is not regular. There see- superconducting properties in organometallic compounds. mingly exists a hump at around 120 K. Above that, the resis- The superconducting parameters can be obtained from the tivity shows a non-metallic feature. It changes to metallic-like field dependences of both the magnetic susceptibility and behavior at low temperatures. This phenomenon is probably resistivity. Fig. 2(b) shows the temperature dependence of the due to the weak linkage inside the sample or the poor contact resistivity at a pressure of 5 kbar and at various magnetic fields between the sample and electrode. However, as the tempera- up to 1.3 Tesla. The suppression of superconductivity can be ture is decreased, the resistivity suddenly shows a sharp drop found by the application of magnetic fields. The temperature- and then reaches zero at a certain temperature. This observa- dependent resistivity curve systematically shifts toward lower tion serves as solid evidence for supporting the existence of temperatures with increasing magnetic fields. Meanwhile, the superconductivity in this organometallic compound. The onset temperature span of the superconducting transition broadens temperature, below which a sudden drop of the resistivity is significantly as the magnetic field is increased. Superconductivity observed, is just the critical temperature for this new super- is completely destroyed at a magnetic field of 1.3 Tesla in the conductor. Its value is exactly the same as that detected from studied temperature range. The temperature-dependent resistivity the magnetic susceptibility measurements (Fig. 1). The inset of curves at various magnetic fields allow the determination of an

25978 | Phys. Chem. Chem. Phys., 2019, 21, 25976--25981 This journal is © the Owner Societies 2019 Paper PCCP

The obtained superconducting parameters in K-doped o-TTB are reasonably comparable to those for low-dimensional organic salts22 and metal-doped fullerides.23 One may wonder what the superconducting phase of K-doped o-TTB could be. The crystal structures of pristine and K-doped o-TTB are shown in Fig. 3. The powder X-ray diﬀraction (XRD) patterns of pure o-TTB (Fig. 3(a)) can be indexed well as a triclinic class with the space group P1(1).

There are eight molecules of C21H21Bi in a unit cell with lattice parameters a = 38.3100 Å, b = 5.2500 Å, and c = 20.2200 Å together with angles a = 90.001, b = 121.001, and g = 90.001,as shown in Fig. 3(c). The crystal structure of this organometallic compound changes dramatically after doping with the alkali metal (Fig. 3(b)), implying the formation of a distinct structure after adding potassium. The crystal structure of K-doped o-TTB is obtained on the basis of K-doped TPB.12 Both compounds show very similar XRD patterns (Fig. 3(b)). While replacing TPB with o-TTB in the unit cell of K-doped TPB, we performed the full relaxation of atomic positions for mole ratios of 4 : 1 and 3 : 1. The optimized results showed that the XRD pattern of 3 : 1 fairly matches with the measurements. For such a structure,

three molecules of C21H21Bi and nine K atoms distribute in a nearly cubic unit cell with dimensions a =9.5450Å,b =9.5810Å, and c = 9.5530 Å togther with angles a =89.621, b =90.391,and g =89.851, as shown in Fig. 3(d). Here K atoms represented by blue balls are intercalated in the interstitial space of bismuth and the methylphenyl rings. In addition, a metal Bi trace is also observed in the XRD patterns of the doped sample (marked by the symbol *), indicating partial decomposition of o-TTB into Bi atoms and methylbenzene (colorless liquid, removed). A similar 12 Fig. 2 The electrical transport properties for sample A. (a) Temperature situation has been discussed for K-doped TPB. Therefore, dependence of the resistivity at ambient pressure in the temperature range K3o-TTB is the most possible superconducting phase from the of 1.8–300 K. The inset displays the enlarged view of the low temperature comparison of the experimental observation and theoretical resistivity. (b) Temperature-dependent resistivity at various magnetic fields calculations. The atomic positions of C and Bi in o-TTB and from 0.0 to 1.3 Tesla at pressure of 5 kbar. The inset shows the upper those of C, H, Bi, and K in K3o-TTB are given in the ESI.† critical field (Hc2) as a function of temperature. The color area represents A comparison of potassium-doped o-TTB, TPB,12 and p-TTB13 the calculated Hc2 from the Werthamer–Helfand–Hohenberg expression. indicates that the three materials have a nearly cubic unit cell with close lattice constants. However, the steric hindrance of important superconducting parameter – the upper critical field methyl groups reduces the interstitial space for accommodating

(Hc2). Hc2 is defined by using the onset Tc criterion, which is potassium atoms in the unit cell, leading to a smaller mole ratio determined by the first dropped point deviating from the linear (3 : 1) for potassium-doped o-TTB and p-TTB than the one (4 : 1) resistivity curve. The inset of Fig. 2(b) summarizes the temperature for potassium-doped TPB. In addition, diﬀerent spatial distribu- dependence of Hc2. Based on the Werthamer–Helfand–Hohenberg tions of methyl groups make potassium atoms occupy diﬀerent 20 equation: Hc2(0) = 0.693[(dHc2/dT)]TcTc,onecanobtainthevalue positions in potassium-doped o-TTB and p-TTB. of Hc2(0) for the compound at pressure of 5 kbar. The calculated Raman spectroscopy measurements were performed to under-

Hc2(0) is about 1.78 0.10 Tesla at 0 K. The colorful area is stand the formation of the superconducting phase. Fig. 4 shows 2 extrapolated by using the formula Hc2(T)=Hc2(0)[1 (T/Tc) ]/ Raman spectra of pristine and doped o-TTB in the frequency range 2 1 [1 + (T/Tc) ]. Since the Tc values at ambient pressure and at a of 0–1800 cm .Pureo-TTB (red curve) displays four regions of pressure of 5 kbar are almost the same, we can assume the same vibrational modes for the lattice and Bi-phenyl, C–C–C bending, 24 Hc2(0) value for the ambient condition. By using the equations for C–H bending, and C–C stretching, from low to high frequencies. 21 2 2 the critical fields, Hc2(0) = F0/2px0 and Hc1(0) = (F0/4plL )ln(lL/x0) By intercalating K into o-TTB, the lattice and C–C–C bending with F0 being the flux quantum, we obtained a zero-temperature regions vanish abruptly. In addition, the other zones (C–H bending superconducting coherence length x0 of 136 3ÅandaLondon and C–C stretching) change dramatically. In the C–H bending 1 pentration depth lL of 2840 4 Å. The Ginzburg–Landau para- region, three Raman peaks (637, 1010 and 1197 cm )foro-TTB meter k = 20.9 0.4 is thus obtained based on the expression exhibit distinct red shifts upon doping. This indicates a phonon- 21 k = lL/x0, supporting the feature of a type-II superconductor. mode softening effect, arising from charge-transfer between the

Fig. 3 Structures and molecular arrangements of pure and K-doped o-TTB. (a) Experimental observed (red curve) and calculated (violet curve) XRD patterns for pristine o-TTB. (b) Experimental observed (olive curve) and calculated (blue curve) powder XRD patterns of K-doped o-TTB (sample A).

The symbol * represents the decomposed Bi metal. (c) The molecular arrangement in pristine o-TTB. There are eight molecules of C21H21Bi in a unit cell. (d) The arrangement of K and organobismuth for K-doped o-TTB in a 2 1 1 supercell.

accepted as an approach to determine actual doping concentration in these superconductors. For K-doped o-TTB, there are 21–23 cm1 redshifts in the frequencies for the mentioned three phonon modes. A downshift of 6 or 7 cm1 in Raman spectra usually corresponds to transfer of one electron. The redshift in our K-doped o-TTB gives a number of transferred electrons of about 3. This is in good agreement with the result determined from the analysis of the XRD data (Fig. 3). On the other hand, the frequencies in the C–C stretching region increase with K doping. This behavior results from the phenyl polarization when a benzene ring is connected to the metal.28 Therefore, the complicated behavior of the Raman spectra of this superconductor is the product of competition between the charge-transfer effect and benzene polarization. We would like to point out there exists an important difference in the Raman spectra of potassium-doped o-TTB and 13 1 Fig. 4 Raman spectra of pristine and K-doped o-TTB (sample A). The p-TTB. While the highest C–C stretching mode at 1573 cm in 1 1 upper panel displays four regions of Raman active modes, which are o-TTB shifts up by 7 cm , the similar mode at 1581 cm in p-TTB marked by vertical dotted lines. The inside numbers are the frequencies shifts down slightly. This difference can be understood by taking of the vibrational modes, which are indicated by the arrows. the effect of the methyl group into account. Owing to the similar electron-donating nature of the methyl group to Bi, its connection alkali metal and organobismuth molecule. This downshift effect to the ortho or para position of the C–Bi bond will result in an has been generally observed in C-bearing superconductors such increased or decreased polarization of the phenyl rings, which as alkali-metal doped hydrocarbons.25–27 This has been widely then causes different Raman shifts.

4 Conclusions 14 S. Kim, J. Koo and H. C. Choi, ACS Appl. Nano Mater., 2018, 1, 5419–5424. The discovery of superconductivity in K-doped o-TTB enriches 15 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. the physical properties and adds to the potential technological Phys., 1993, 47, 558–561. applications of organometallic compounds as superconductors. 16 G. Kresse and J. Furthu¨ller, Phys. Rev. B: Condens. Matter The introduction of methyl in pristine organometallic molecules Mater. Phys., 1996, 54, 11169–11186. leads to a nearly five-fold increase of the superconducting shield 17 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 12 fraction compared to K-doped TPB due to the possible delocaliza- 1996, 77, 3865–3868. tion effect and/or the increase of the carrier concentration. This 18 P. E. Blo¨chl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, offers an effective method to tune (super)conductivity by the simple 50, 17953–17979. addition of functional groups. Our findings add a new electron- 19 R. A. Hein, Phys. Rev. B: Condens. Matter Mater. Phys., 1986, acceptor family for organic superconductors distinguished from the 33, 7539–7549. 29 25,26 M(dmit)2 (M = Ni or Pd) system, polyaromatic hydrocarbons, 20 N. R. Werthamer, E. Helfand and P. C. Hohenberg, Phys. 30–36 and p-oilgophenyls. This work also points to a new pool for Rev., 1966, 147, 295–302. producing superconductors from organometallic compounds. 21 M. Tinkham, Introduction to superconductivity, McGraw-Hill, New York, 1975. Conflicts of interest 22 L. P. Le, G. M. Luke, B. J. Sternlieb, W. D. Wu, Y. J. Uemura, J. H. Brewer, T. M. Riseman, C. E. Stronach, G. Saito, There are no conflicts to declare. H. Yamochi, H. H. Wang, A. M. Kini, K. D. Carlson and J. M. Williams, Phys. Rev. Lett., 1992, 68, 1923–1926. Acknowledgements 23 G. Sparn, J. D. Thompson, R. 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