Manuele D.
Balestra
ab,
Omargeldi
Atanov
c,
Robin
Lefèvre
b,
Olivier
Blacque
b,
Yat Hei
Ng
c,
Rolf
Lortz
c and
Fabian O.
von Rohr
*a
aDepartment of Quantum Matter Physics, University of Geneva, CH-1211 Geneva, Switzerland. E-mail: fabian.vonrohr@unige.ch
bDepartment of Chemistry, University of Zürich, CH-8057 Zürich, Switzerland
cDepartment of Physics, The Hong Kong University of Science and Technology, Clear Water Bay Kowloon, Hong Kong
First published on 29th July 2022
We report on the discovery, structural analysis, and the physical properties of Nb4SiSb2 – a hitherto unknown compound crystallizing in the V4SiSb2-type structure with the tetragonal space group I4/mcm and unit cell parameters a = 10.3638(2) Å and c = 4.9151(2) Å. We find Nb4SiSb2 to be a metal undergoing a transition to a superconducting state at a critical temperature of Tc ≈ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of ΔC(Tc)/γTc = 1.33 mJ mol−1 K−2. We find that for Nb4SiSb2, the unoccupied sites on the 4b Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at Tc ≈ 1.2 K for Nb4Cu0.2SiSb2, and Tc ≈ 0.8 K for Nb4Pd0.2SiSb2 as well as for Nb4Pt0.14SiSb2. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.
The results presented in this paper refer to structures, crystallizing in a defect variant of the W5Si3-type structure, commonly known as the V4SiSb2 structure. The W5Si3 structure itself exhibits the tetragonal space group I4/mcm8 and is a bulk superconductor with a critical temperature of Tc = 2.7 K.9 Other compounds crystallizing in the same structure-type and exhibiting superconducting properties are Nb5Si3 with a critical temperature of Tc = 0.7 K,10 and the ternary W5Si3-type compounds Nb5Sn2Ga, Ta5SnGa2, and Zr5Sb2.36Ru0.36 with critical temperatures of Tc ≈ 1.8 K, 1.8 K, and 5 K, respectively.11–13 In the V4SiSb2 structure, the 4b Wyckoff position of the W5Si3 structure is unoccupied, forming void channels along the c-direction. These channels are filled by Sb centred, essentially unhybridized 5p orbitals forming a 2D net stacking along the c-direction leading to “electron-filled” voids.14 The prospect of intercalating these voids with electrophilic species has been theoretically proposed by Rytz et al.14
To date, only six compounds have been reported to crystallize in this structure-type, namely V4SiSb2 and the compound series of Ti4TBi2 with (T = Cr, Mn, Fe, Co, Ni). All of these compounds are known to be non-magnetic metals.15,16 Furthermore, 5 pseudo-quaternary antimonides with the general formula Nb4Pd0.5ZSb2 with Z = Cr, Fe, Co, Ni, Si have been reported.17 These compounds contain three transition metals in an ordered arrangement; hence they are isostructural to each other and crystallize in substitutional variants of the W5Si3-type structure, or alternatively, they can be interpreted as V4SiSb2-type compounds with half occupied channels.
Here, we report on the discovery of the compound Nb4SiSb2, which crystallizes in a V4SiSb2-type structure with the tetragonal space group I4/mcm. We show that this material exhibits bulk superconductivity at a critical temperature of Tc ≈ 1.6 K. Furthermore, we find that the 4b Wyckoff void position can be partially occupied by the transition metals Cu, Pd or Pt, leading to the compounds Nb4Cu0.2SiSb2, Nb4Pd0.2SiSb2, and Nb4Pt0.14SiSb2. All three compounds are bulk superconductors with critical temperatures of Tc ≈ 1.2 K, 0.8 K, and 0.8 K, respectively.
Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku SmartLab diffractometer using a Cu X-ray source (Kα1 = 1.540600 Å, Kα2 = 1.544430 Å) with CuKβ filter and collected using a 2θ range of 5–100°. The machine is equipped with a 3 kW sealed X-ray tube, CBO optics and a D/teX Ultra 250 silicon strip detector. Data was recorded using the SmartLab Studio II software. Rietveld refinements were performed using the FULLPROF software package23 and fitting of the diffracted data was done using the Thompson-Cox-Hastings pseudo-Voigt function with asymmetry correction.24
Specific heat measurements were performed from 300 mK to 2 K in a He-3 15 T magnet cryostat with a custom-developed modulated-temperature AC calorimetry technique using an SR830 digital lock-in amplifier, and from 2–10 K with a long relaxation technique in a He-4 cryostat. For the latter, each relaxation provides about 1000 data points over a temperature interval of 30–40% of the base temperature, which has been varied between 2 K and 10 K. The relaxation technique provides a high precision up to 1% while the AC technique is less accurate but provides high resolutions of ΔC/C of 10−5 at a high density of data points.25 Temperature-dependent magnetization measurements were performed using a Quantum Design Magnetic Properties Measurement System (MPMSXL) equipped with a reciprocating sample option (RSO) and a 7 T magnet.
We find Nb4SiSb2 to crystallize in the tetragonal space group I4/mcm with the lattice parameters a = b = 10.3638(2) Å and c = 4.9151(2) Å with the corresponding calculated cell volume of V = 527.92(3) Å3. Hence, it is found to adopt the same centrosymmetric structure type that was previously reported for V4SiSb2.15,16 The crystallographic data and the details of the structure refinement are summarised in Table 1. All crystallographic positions as well as the anisotropic displacement parameters are presented in Table 2.
Parameters | Nb4SiSb2 | Nb4Cu0.2SiSb2 | Nb4Pd0.2SiSb2 | Nb4Pt0.14SiSb2 |
---|---|---|---|---|
Crystal system | Tetragonal | Tetragonal | Tetragonal | Tetragonal |
Structure-type | V4SiSb2 | W5Si3(defect) | W5Si3(defect) | W5Si3(defect) |
Space group | I4/mcm (No. 140) | I4/mcm (No. 140) | I4/mcm (No. 140) | I4/mcm (No. 140) |
Absorption correction method | Analytical | Analytical | Spherical | Analytical |
Temperature [K] | 160(1) | 160(1) | 160(1) | 160(1) |
Lattice parameters [Å] | a = 10.3638(2) | a = 10.3954(2) | a = 10.3991(2) | a = 10.3803(2) |
c = 4.9151(2) | c = 4.9233(2) | c = 4.93619(16) | c = 4.9348(2) | |
Cell volume [Å3] | 527.92(3) | 532.03(3) | 533.81(3) | 531.73(3) |
Formula unit/cell | 4 | 4 | 4 | 4 |
ρ calcd [g cm−3] | 8.093 | 8.189 | 8.268 | 8.376 |
μ [mm−1] | 149.393 | 532.03(3) | 153.021 | 155.001 |
Crystal size [mm] | 0.018 × 0.016 × 0.013 | 0.005 × 0.003 × 0.002 | 0.01 × 0.01 × 0.01 | 0.015 × 0.015 × 0.01 |
F(000) | 1120.0 | 1143.0 | 1157.0 | 1164.0 |
Radiation type | Cu Kα (λ = 1.54184) | Cu Kα (λ = 1.54184) | Cu Kα (λ = 1.54184) | Cu Kα (λ = 1.54184) |
2Θ range [°] | 12.078 to 146.58 | 12.04 to 148.58 | 12.036 to 147.576 | 12.058 to 147.716 |
Index range | h[−9,12] | h[−11,9] | h[−11,12] | h[−12,12] |
k[−12,12] | k[−12,12] | k[−12,12] | k[−12,12] | |
l[−5,6] | l[−6,6] | l[−6,5] | l[−6,5] | |
Observed reflections | 1466 | 838 | 2368 | 2381 |
Independent reflections (2σ) | 165 | 166 | 166 | 167 |
R int | 0.0278 | 0.0385 | 0.0312 | 0.0298 |
R σ | 0.0127 | 0.0314 | 0.0107 | 0.0117 |
Refined parameters | 14 | 16 | 16 | 17 |
GOF | 1.363 | 1.142 | 1.252 | 1.240 |
R 1 (all data) (%) | 1.69 | 3.33 | 1.64 | 1.60 |
wR1 (≥2σ) (%) | 1.69 | 2.96 | 1.62 | 1.57 |
wR2 (all data) (%) | 4.32 | 7.53 | 3.71 | 3.67 |
wR2 (≥2σ) (%) | 4.33 | 7.37 | 3.71 | 3.66 |
Max/min residual electron density [e Å−3] | 1.41/−0.94 | 1.12/−1.52 | 0.97/−0.98 | 1.13/−0.85 |
Atom | Wyckoff symbol | x | y | z | U(eq) [Å2] | U 11/U22 | U 33 | U 12 | Occ. |
---|---|---|---|---|---|---|---|---|---|
Nb4SiSb2 | |||||||||
Nb | 16k | 0.29305(6) | 0.58530(6) | 1/2 | 0.0111(3) | 10.6(4)/10.9(4) | 11.8(4) | 0.2(2) | 4.00 |
Si | 4a | 1/2 | 1/2 | 3/4 | 0.0122(3) | 10.1(11)/10.1(11) | 16(2) | 0 | 1.00 |
Sb | 8h | 0.14037(5) | 0.35963(5) | 1/2 | 0.0119(9) | 11.8(3)/11.8(3) | 13.0(5) | −1.5(3) | 2.00 |
Nb4Cu0.2SiSb2 | |||||||||
Nb | 16k | 0.29297(9) | 0.41603(9) | 1/2 | 0.0080(4) | 6.2(6)/6.4(6) | 11.4(6) | 0.3(4) | 4.00 |
Cu | 4b | 0 | 1/2 | 3/2 | 0.021(7) | 24(8)/24(8) | 16(12) | 0 | 0.199(16) |
Si | 4a | 1/2 | 1/2 | 3/2 | 0.0063(12) | 3.2(17)/3.2(17) | 12(3) | 0 | 1.00 |
Sb | 8h | 0.14385(8) | 0.35615(2) | 1/2 | 0.0110(4) | 8.8(5)/8.8(5) | 15.3(7) | 2.3(4) | 2.00 |
Nb4Pd0.2SiSb2 | |||||||||
Nb | 16k | 0.29305(4) | 0.58369(4) | 1/2 | 0.0125(2) | 12.1(3)/12.8(3) | 12.7(3) | −0.01(17) | 4.00 |
Pd | 4b | 0 | 1/2 | 1/4 | 0.0147(15) | 13.6(16)/13.6(16) | 17(2) | 0 | 0.199(5) |
Si | 4a | 1/2 | 1/2 | 3/2 | 0.0131(6) | 13.1(8)/13.1(8) | 13.0(14) | 0 | 1.00 |
Sb | 8h | 0.14470(4) | 0.35530(4) | 1/2 | 0.0170(2) | 15.8(2)/15.8(2) | 19.5(3) | −3.5(2) | 2.00 |
Nb4Pt0.14SiSb2 | |||||||||
Nb | 16k | 0.58429(5) | 0.29284(5) | 1/2 | 0.0063(2) | 6.3(3)/6.2(3) | 6.5(4) | −0.08(18) | 4.00 |
Pt | 4b | 1/2 | 0 | 3/2 | 0.0122(5) | 12.5(17)/12.5(17) | 12(3) | 0 | 0.140(3) |
Si | 4a | 1/2 | 1/2 | 3/2 | 0.0020(6) | 2.7(8)/2.7(8) | 0.7(16) | 0 | 1.00 |
Sb | 8h | 0.35697(4) | 0.14303(4) | 1/2 | 0.0101(3) | 9.0(3)/9.0(3) | 12.5(4) | −3.1(2) | 2.00 |
In the structure of Nb4SiSb2 each atom occupies one atomic site: the niobium atoms are located at the 16k Wyckoff position, silicon occupies the 4a and antimony the 8h Wyckoff positions. Silicon forms thereby columns which can be interpreted as ∞[Nb8/2Si] chains along the c-direction as shown in Fig. 1(c). The Si–Si bonding distance in Nb4SiSb2 within the columns is 2.4576(1) Å, which is in good agreement with the ones found in V4SiSb215 and comparable to Si–Si bond distances in similar structures.17,26 Each Si atom is surrounded by eight Nb atoms with a distance of 2.6252(6) Å forming antiprisms with the surrounding neighbour atoms. Nb has a coordination number (CN) of 13 consisting of six Nb neighbours located in the ∞[Nb8/2Si] column, one Nb in the adjacent ∞[Nb8/2Si] column, two Si, and four Sb neighbours located in between the two columns. The Nb–Nb distances range from 3.0275(8) to 3.2807(9) Å. These distances, together with the relatively short intercolumn distance between two Nb atoms of 3.0449(13) Å are in good agreement with distances found in comparable structures.27,28 Also, the Nb-Sb distance ranging from 2.8238(7) Å to 2.9781(4) Å is in good agreement with the distances found in the related compounds, such as e.g. in Nb5Sb4.28 Each Sb has eight Nb neighbours and therefore a CN of 8. Another feature of this structure are the voids at the 4b Wyckoff position. These void positions are surrounded by four Sb atoms. These form void channels along the c-direction. If these void positions were fully occupied, then the V4SiSb2 structure would be equivalent to the W5Si3 structure.15
The validity of the structural model, the phase purity, and the homogeneity of the sample were confirmed by means of PXRD at ambient temperature and SXRD at 160 K. The reliability factors of the SXRD refinement can be found in the ESI.† In Fig. 3(d) the PXRD pattern of the polycrystalline sample is shown, with its respective Rietveld refinement. We find the lattice parameters of a = b = 10.3686(4) Å, and c = 4.9193(2) Å, as well as a calculated cell volume of V = 528.86(3) Å3. Hence, the SXRD and PXRD refinements and structural solutions are in excellent agreement with each other (ESI†).
The bulk nature of the superconductivity in Nb4SiSb2 is confirmed by low-temperature specific-heat measurements. Temperature-dependent specific-heat measurements are of particular importance to prove the bulk nature of a superconductor.29,30
In Fig. 2(b), we present the temperature-dependent specific heat C(T)/T of Nb4SiSb2 in a temperature range between T = 600 mK and 2 K. We find a clearly pronounced discontinuity in the specific heat, resulting from the superconducting transition. The data was fitted using the α-model.31,32 Thereby, an entropy conserving construction was used to determine the critical temperature, Tc ≈ 1.6 K. This value is in good agreement with the critical temperature from the resistivity measurement. From the α-model fit, we obtained α = 1.7 and the Sommerfeld constant of γ = 9.00 mJ mol−1 K−2. We find a ratio for the normalized specific-heat jump of ΔC/γTc = 1.33 mJ mol−1 K−2, which confirms the bulk nature of the superconductivity, as this value is close to the weak-coupling BCS ratio of 1.43. This corresponds to a value of the superconducting gap of 2Δ(0) = 3.4kBTc.
Under the assumption of a degenerate electron gas of non-interacting particles, the electronic contribution to the heat capacity in a solid at low temperatures is proportional to the density of states at the Fermi level D(EF) and linear in T. With the previously determined value of γ = 9.00 mJ mol−1 K−2, the density of states at the Fermi level can be calculated as described by F. Heiniger et al.33 according to
![]() | (1) |
We obtain for Nb4SiSb2 a density of states at the Fermi level of D(EF) = 3.8 states eV−1.
Magnetic susceptibility measurements of Nb4SiSb2 were conducted in the normal-state, i.e. in a temperature range between T = 10 K to 300 K, in an external field of μ0H = 1 T. The observed temperature-independent positive magnetic moment corresponds to a Pauli-paramagnet (see ESI†). A summary of all obtained physical parameters can be found in Table 3.
Parameter | Units | Nb4SiSb2 | Nb4Cu0.2SiSb2 | Nb4Pd0.2SiSb2 | Nb4Pt0.14SiSb2 |
---|---|---|---|---|---|
T c,resistivity | K | 1.65 | 1.16 | 0.76 | 0.84 |
T c,specificheat | K | 1.59 | — | — | — |
RRR | — | 14.96 | 4.54 | 1.56 | 1.70 |
ρ(300) | mJ Ω cm | 2.06 | 0.70 | 8.46 | 2.49 |
ρ 0 | mJ Ω cm | 0.13 | 0.15 | 5.43 | 1.46 |
Type of magnetism | — | Pauli-paramagnetic | Pauli-paramagnetic | Pauli-paramagnetic | Pauli-paramagnetic |
γ | mJ mol−1 K−2 | 9.00 | 7.5 | — | |
ΔC/Tcγ | — | 1.33 | 1.2 | — | |
2Δ(0) | meV | 0.47 | 12 | — | |
D(EF) | States eV−1 per f.u. | 3.82 | 3.18 | — |
All samples were found to be single phase by means of PXRD measurements and corresponding Rietveld refinements (ESI†). Atomic compositions were confirmed using EDX analysis (ESI†).
All three structures are in good agreement with the previously reported structure for Nb4Pd0.5ZSb2 with Z = Cr, Fe, Co, Ni, Si, where it was thought that a half-occupied Pd 4b site was necessary to stabilize these compounds.17 In contrary to this previous assumption, we found here that the channels were in our case either unoccupied or filled with 0.2 or 0.14 respectively (in case of Pt), independent of the initially used starting stoichiometry. These results indicate that, with improved synthesis methodologies, the continuous solid solution might be accessible in the future. All information regarding the lattice parameters, crystallographic data, and details of the structure refinements are summarized in Table 1.
We find all three compounds to undergo a transition to a superconducting state at low temperatures. The critical temperature midpoints are determined as Tc,mid ≈ 1.16 K for Nb4Cu0.2SiSb2, Tc,mid ≈ 0.76 K for Nb4Pd0.2SiSb2 and Tc,mid ≈ 0.84 K for Nb4Pt0.14SiSb2. All three compounds with atoms in the void position of Nb4SiSb2 have lower transition temperatures than the parent compound.
For comparison, we have performed specific heat measurements in the normal state of Nb4SiSb2 and Nb4Pt0.14SiSb2 (shown in the ESI†). For Nb4SiSb2 we find values for γn and β of 8.40 mJ mol−1 K−2 and 0.16 mJ mol−1 K−4, respectively. The γn value of this fit is in good agreement with the more accurate low-temperature value discussed above. For Nb4Pt0.14SiSb2 we find values for γn and β of 9.10 mJ mol−1 K−2 and 0.31 mJ mol−1 K−4, respectively. We note that the values for γn differ only slightly, indicating a small change of the electronic properties upon void position filling. We find, however, that β changes quite strongly. These findings indicate that the decrease of the superconducting transition temperature is likely caused by a change in the phonons, and the vibrations, respectively.
Nb4Pd0.2SiSb2 has the lowest critical temperature of the doped compounds, as well as the lowest RRR value of RRR = ρ(300 K)/ρ(1.8 K) = 1.56. Nb4Cu0.2SiSb2 with RRR = 4.54 and Nb4Pt0.14SiSb2 with RRR = 1.70 follow the descending trend observed for the critical temperatures accordingly. These low RRR values correspond to a poor metallic behaviour and are 3 to 24 times smaller than the RRR of the parent compound Nb4SiSb2. The pronounced effect on the physical properties on void position doping becomes clearly apparent in the large change of the RRR values. The nature of the change is, however, not only affected by the electronic states, but also by the phonons and by impurity state scattering.
These compounds crystallize in a tetragonal variant of the W5Si3-type structure with partially occupied channels, extending along the c-direction. All three compounds were found to be superconductors with transitions temperatures of Tc ≈ 1.2 K for Nb4Cu0.2SiSb2, Tc ≈ 0.8 K for Nb4Pd0.2SiSb2 and Tc ≈ 0.8 K for Nb4Pt0.14SiSb2. We find that the insertion of a host atom into the void positions strongly affects the electronic and superconducting properties of this material.
Hence, our results indicate that this and related compounds might be promising host structures for the discovery of new superconducting materials, as they allow for a controlled manipulation of the electronic and phononic properties by chemical manipulation.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2166026–2166029. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc01510b |
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