Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Valence State of Eu and Superconductivity in Se-Substituted EuSr2Bi2S4F4 and Eu2SrBi2S4F4 Zeba Haque,† Gohil Singh Thakur,†,‡ Ganesan Kalai Selvan,§ Theresa Block,∥ Oliver Janka,∥ Rainer Pöttgen,∥ Amish G. Joshi,⊥ Rangasamy Parthasarathy,† Sonachalam Arumugam,§ Laxmi Chand Gupta,†,∇ and Ashok Kumar Ganguli*,†,# †
Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India Max-Planck-Institute for Chemical Physics of Solids, Dresden 01187, Germany § Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirapalli 620024, India ∥ Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany ⊥ CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India # Institute of Nano Science & Technology, Habitat Centre, Mohali 160062, India ‡
S Supporting Information *
ABSTRACT: Recently, we reported the synthesis and investigations of EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have now been able to induce superconductivity in EuSr2Bi2S4F4 by Se substitution at the S site (isovalent substitution) with Tc = 2.9 K in EuSr2Bi2S2Se2F4. The other compound, Eu2SrBi2S4F4, shows a significant enhancement of Tc. In Se-substituted Eu2SrBi2S4−xSexF4, we find Tc = 2.6 K for x = 1.5 and Tc = 2.8 K for x = 2, whereas Tc = 0.4 K in the Se-free sample. In addition to superconductivity, an important effect associated with Se substitution is that it gives rise to remarkable changes in the Eu valence. Our 151Eu Mössbauer and X-ray photoemission spectroscopic measurements show that Se substitution in both of the compounds Eu2SrBi2S4F4 and EuSr2Bi2S4F4 gives rise to an increase in the Eu2+ component in the mixed-valence state of Eu.
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extra EuF2 layer. Superconductivity in EuBiS2F (Tc ≈ 0.3 K)11 and Eu3Bi2S4F4 (Tc ≈ 1.5 K)17 arises without any substitution. This behavior of the Eu-based BiS2 compounds make them unique and different from other BiS2-based superconductors. Both of them exhibit mixed valence of the Eu ions, giving rise to electron doping into the conduction layer and making them intrinsic superconductors. There have been successful attempts to modify the conducting BiS2 layers by substituting isovalent selenium atoms18−22 at the S sites in both the 1121 and 3244 series. In EuBiS2F11 crystallographically there is only one Eu site. The 151Eu Mössbauer spectrum shows Eu ions in a divalent state at 388 K. However, well-resolved resonance absorption signals corresponding to Eu2+ and Eu3+ ionic states are observed at low temperatures (200 and 90 K). This clearly implies that the Eu ions exhibit valence fluctuations that are much slower than the 151Eu Mössbauer probing time. From the intensities of the lines corresponding to the two valence states one can easily estimate the average valence υ of Eu as a function of temperature. For example, υ is ∼2.20 at 90 K in EuBiS2F.11 We observe similar effects of mixed-valence fluctuation in our
INTRODUCTION After the discovery of superconductivity in layered BiS2 compounds as in Bi4O4S3,2 a tremendous amount of work on BiS2-based compounds has been carried out in the past 4−5 years.3,4 Soon after the realization of superconductivity in Bi4O4S3, superconductivity was observed in LaO0.5F0.5BiS2 at 2.5 K (ambient pressure) and 10.6 K (high-pressure annealing).5 Subsequently several groups studied various other BiS2-based materials, namely LnBiS2O and ABiS2F (Ln = La−Nd, Yb, Bi; A = Sr, Eu).6−11 The LnBiS2O and ABiS2F family is abbreviated as 1121-type BiS2 superconductors. All of these compounds share the same basic crystal structure having the conducting BiS2 bilayers alternating with a blocking layer of edge-sharing [Ln2O2]/[A2F2] tetrahedra. Superconductivity is induced in LnBiS2O by electron doping either by substitution of tetravalent ions at the rare-earth site or by substitution of oxygen by fluorine ions,12−14 while the rare-earth doping in SrBiS2F creates an analogous effect.10,15,16 EuBiS2F is an intrinsic superconductor with Tc = 0.3 K,11 due to the mixed valence state of the Eu ions. The attraction and interest of researchers toward the BiS2 superconductors resulted in the design and synthesis of the new layered structure Eu3Bi2S4F4.17 This material has a layered tetragonal structure (SG I4/mmm, Z = 2), consisting of two EuBiS2F subunits interspersed with an © XXXX American Chemical Society
Received: June 19, 2017
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DOI: 10.1021/acs.inorgchem.7b01555 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Powder X-ray diffraction patterns of (a) EuSr2Bi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2) and (b) Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2), confirming the formation of the tetragonal 3244 phase. Insets show the expansion of the cell volume upon Se substitution in both systems.
two samples.1 We should point out that in Eu3Bi2S4F417 and in our two compounds1 there are two Eu sites, 2a and 4e. Mixedvalence effects are only shown by Eu atoms at the 2a sites. Eu atoms occupying the 4e sites are in stable divalent state. Extensive work has been carried out to modify the structure and number of charge carriers by means of external pressure and by various chemical substitutions in EuBiS2F23−25 and Eu3Bi2S4F4.26,27 This has resulted in achieving a dramatic increase in Tc to a maximum of 10 K.15,27 The solid solutions Eu3−xSrxBi2S4F4 (x = 0.5, 1)28 exhibit superconductivity at 0.87 and 0.47 K for x = 0.5 and 1, respectively. No superconductivity is observed in EuSr2Bi2S4F4 (x = 2) down to 300 mK.28 Here we study the effects of Se substitution in EuSr2Bi2S4F4 and Eu2SrBi2S4F4 at the S sites through resistivity and magnetic measurements. Superconductivity could be induced in EuSr2Bi2S4−xSexF4 at Tc = 2.9 K, and enhancement in Tc to a maximum of 2.8 K is observed in Eu2SrBi2S4−xSexF4. The change in the ratio of Eu2+ and Eu3+ states due to Se substitution is noteworthy, which has been studied by 151Eu Mössbauer and XPS spectroscopic measurements. It is very well known that XPS is a technique widely used to study surface properties; however, it has been used to study bulk materials also.29−31
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Fitting of the spectra was performed with the Normos-90 program system.32 X-ray photoemission spectroscopy (XPS) experiments were carried out using an Omicron Multiprobe Surface Science System, equipped with a monochromatic source (XM 1000) and a hemispherical electron energy analyzer (EA 125). Throughout, the photoemission experiments were carried out at an average base pressure of ∼3.1 × 10−11 Torr with a power of 300 W. The total energy resolution, estimated from the width of the Fermi edge, was about 0.25 eV for the monochromatic Al Kα line with a photon energy of 1486.70 eV. The pass energy for the core level spectra was kept at 30 eV. Ar ion sputtering was performed at 2 keV by maintaining an extractor pressure of 25 mPa.
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RESULTS AND DISCUSSION X-ray Diffraction Studies. The powder X-ray diffraction data shown in Figure 1 confirm the formation of nearly single phase samples of the type Eu3Bi2S4F4, crystallizing in the tetragonal structure (space group I4/mmm). Small amounts of impurity phases Bi2S3, Bi2Se3, and EuF2.4 were also present, which were below 10%. The Rietveld fits to the powder X-ray diffraction patterns for polycrystalline EuSr2Bi2S3.5Se0.5F4 and Eu2SrBi2S3.5Se0.5F4 samples are given in Figure S1 in the Supporting Information. Substitution of Se atoms at the S sites has resulted in a systematic expansion of the cell volume (insets in Figure 1). This expansion in the cell volume indicates successful Se2− substitution, as Se2− (r = 1.98 Å, CN = 6) is larger than S2− (r = 1.84 Å).33 Like other layered Bi−S2 materials (Eu,Sr)3Bi2S4F4 also consists of Bi surrounded by five chalcogen (Ch) atoms in a slightly distorted square pyramidal geometry. The four Ch atoms (denoted as Ch2) in the ab plane form the base and one Ch at the top (denoted as Ch1) forms the apex of the square pyramid. We show in Figure 2 the pictorial representation of different types of Bi−Ch bonds. There are two types of Bi−Ch bonds, namely Bi−Ch2 (in-plane bond) and Bi−Ch1 (out-ofplane or apical bond), and correspondingly three types of Ch− Bi−Ch bond angles. From our Rietveld analysis we find for both compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4, that Se prefers the in-plane Ch2 site over the apical out-of-plane Ch1 site, consistent with previous reports in LaO0.5F0.5BiS2‑xSex34 and Eu0.5La0.5FBiS2‑xSex.35 We have fixed the occupancy of Se at the in-plane site (along with S) and the remaining S at the apical position as per the loaded composition and then refined their positions. We show in Tables 1 and 2 the variation of in-plane and out-of-plane
EXPERIMENTAL SECTION
Polycrystalline samples with nominal compositions EuSr2Bi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2) and Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2) were synthesized by solid-state reactions. The reactants EuF3, EuF2, SrF2, Bi2S3, Bi2Se3, and Bi metal powder were thoroughly mixed, pelletized, and heated in evacuated quartz ampules at 1123 K for 30 h. The dark black pellets so produced remained stable in air for several weeks. The precursors Bi2S3 and Bi2Se3 used in the reaction were presynthesized by heating stoichiometric amounts of Bi metal powder with S and Se powder, respectively, at 773 K for 12 h. A detailed experimental procedure for the synthesis of Se-free compounds has been described in our previous paper.1 The phase purity of all samples was checked by powder X-ray diffraction with Cu Kα radiation using a Bruker D8 Advance diffractometer. The temperature dependence of resistivity and magnetization measurements (2−300 K) were performed on a Cryogen-free Physical Properties Measurements System-Vibrating Sample Magnetometer (PPMS-VSM). The 21.53 keV transition of 151Eu with an activity of 130 MBq (2% of the total activity of a 151Sm:EuF3 source) was used for the Mössbauer spectroscopic studies. The measurements were conducted in transmission geometry with a commercial nitrogen-bath (78 K) cryostat, while the source was kept at room temperature. The samples were placed in a thin-walled PVC container at an optimized thickness. B
DOI: 10.1021/acs.inorgchem.7b01555 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
sharp superconducting transition with Tconset = 2.9 K (inset of Figure 3a); the Tconset value is taken as the temperature corresponding to 90% of the normal state resistivity. The field dependence of resistive Tc is shown in Figure 3b. Application of a field shifts Tc to lower temperature. By using the WHH formula36 (WHH = Werthamer, Helfand, and Hohenberg), the upper critical field Hc2(0) is estimated to be 5.85 kOe. This value of Hc2(0) is much lower than that in Eu3Bi2S4F4. As superconductivity has not been observed in EuSr2Bi2S2F4 down to 0.3 K, it is remarkable that Se substitution (isovalent substitution) induces superconductivity in EuSr2Bi2S2F4. Figure 4a shows the temperature dependence of resistivity in Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2). The Se-free compound Eu2SrBi2S4F4 exhibits a semimetallic behavior in the temperature interval 300 K > T > 150 K with an upturn in resistivity value below 100 K. Among the Se-substituted samples we observe that the normal state resistivity value decreases upon Se substitution. The broad humplike feature shifts to lower temperatures (shown by arrows in Figure 4a), and with increasing Se substitution it finally disappears for x = 1.5, 2. We observe nearly temperature independent resistivity above 20 K in Eu 2 SrBi 2 S 2.5 Se 1.5 F 4 and a metallic conductivity for Eu2SrBi2S2Se2F4. Both compounds exhibit superconductivity at Tconset = 2.6 and 2.8 K, respectively (Figure 4b). The fielddependent resistivity as a function of temperature for Eu2SrBi2S2.5Se1.5F4 is shown in Figure 4c. The superconducting transition is suppressed on applying a field ≥0.5 T. We estimated Hc2(0) ≈ 8 kOe in Eu2SrBi2S2.5Se1.5F4. Magnetic Studies. dc susceptibility as a function of temperature, in the range 2−300 K, in an applied field of 20 Oe for the solid solutions EuSr2Bi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2) and Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2) is shown in Figure 5. In EuSr2Bi2S4−xSexF4, the sample with x = 2.0 shows a sharp drop in χ at 2.3 K. This indicates the onset of superconductivity as observed in the resistivity data. The other samples of this series show paramagnetic behavior down to 2 K. In the second series Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2), all the samples show paramagnetic behavior down to 2 K. A diamagnetic signal is not seen in the x = 1.5, 2 samples. In the unsubstituted as well as Se-substituted compounds of Eu2SrBi2S4F4, one of the Eu ions is essentially divalent, as our 151 Eu Mössbauer spectroscopic results show.1 The paramagnetic contribution of divalent Eu ions possibly overrides the diamagnetic signal, which is consistent with the absence of superconductivity in Eu2SrBi2S2.5Se1.5F4 and Eu2SrBi2S2Se2F4. We show in Figure 6 our magnetization measurements carried out at 2.1 K for the samples EuSr2Bi2S2Se2F4, Eu2SrBi2S2.5Se1.5F4, and Eu2SrBi2S2Se2F4. The saturation magnetic moment in EuSr2Bi2S2Se2F4 in an applied field of 9 T is 3.4(2) μB/fu, which is much higher than 1.75 μB/fu obtained in the Se-free EuSr2Bi2S4F41 sample. We see a clear enhancement of the divalent Eu state due to Se substitution. The fraction of the Eu2+ state increases from 25% to ∼50% by substituting half of the S ions by Se ions. Thus, we see that the average valence of Eu changes from 2.75 to 2.50 by substitution of half of S ions by Se ions. In the case of Eu2SrBi2S2.5Se1.5F4 the saturation magnetic moment in an applied field of 9 T is 11.7(2) μB/fu, which is higher than that of the Se-free analogue (it has an ordered moment of 9.39 μB/fu). However, with a further increase in Se, i.e. in Eu2SrBi2S2Se2F4, the ordered moment decreases to
Figure 2. Representation of the Ch1 and Ch2 sites in the BiS2 layer.
bond lengths as well as bond angles as a function of Se substitution in EuSr2Bi2S4F4 and Eu2SrBi2S4F4. Table 1. Variation of Bi−Ch Bond Lengths and Angles as a Function of Se Substitution in EuSr2Bi2S4−xSexF4 EuSr2Bi2S4−xSexF4 series in-plane Bi−Ch2 bond length (Å) out-of-plane Bi−Ch1 bond length (Å) Ch2−Bi−Ch2 (in-plane) angle (deg) Ch1−Bi−Ch2 angle (deg)
x = 0.5
x=1
x = 1.5
x=2
2.904(9)
2.919(4)
2.93(2)
2.940(7)
2.38(7)
2.55(4)
2.51(3)
2.51(7)
165.5(1)
163.4(5)
160.0(2)
161.4(8)
97.2(1)
98.3(6)
98.5(2)
99.3(1)
Table 2. Variation of Bi−Ch Bond Lengths and Angles as a Function of Se Substitution in Eu2SrBi2S4−xSexF4 Eu2SrBi2S4−xSexF4 series in-plane Bi−Ch2 bond length (Å) out-of-plane Bi−Ch1 bond length (Å) Ch2−Bi−Ch2 (in-plane) angle (deg) Ch1−Bi−Ch2 angle (deg)
x = 0.5
x=1
x = 1.5
x=2
2.883(3)
2.906(4)
2.911(4)
2.916(2)
2.54(7)
2.63(4)
2.52(7)
2.48(3)
174.4(1)
168.2(6)
166.4(6)
165.0(4)
92.8(1)
95.9(6)
96.8(8)
97.5(5)
From the refinement results we see that the in-plane bond length is increased by replacing the larger Se2− ions at the S2− sites. The in-plane Ch2−Bi−Ch2 angle shrinks, which causes more effective orbital overlap between the Bi and Ch atoms. Resistivity Studies. The temperature-dependent resistivity studies of the solid solutions EuSr2Bi2S4−xSexF4 (x = 0, 1, 1.5, 2) are shown in Figure 3a. The x = 0 (Se-free) compound EuSr2Bi2S4F4 shows an upturn in resistivity below 100 K, suggesting a semimetallic behavior as shown in Figure 3a. The value of resistivity is quite close to that of the parent compound Eu3Bi2S4F4.17 We see a decrease in normal state resistivity due to Se substitution. There is suppression in the semimetallic behavior along with an increase in metallic nature due to Se substitution. For x = 0, we observe a broad hump (marked by arrows) in resistivity at 250 K in Figure 3a. As a function of Se substitution this feature shifts toward lower temperature and vanishes for x = 2.0. Quite remarkably EuSr2Bi2S2Se2F4 shows a C
DOI: 10.1021/acs.inorgchem.7b01555 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 3. (a) Temperature-dependent resistivity ρ(T) of EuSr2Bi2S4−xSexF4 (x = 0, 1, 1.5, 2). The inset shows a sharp superconducting drop at 2.9 K in EuSr2Bi2S2Se2F4. (b) Resistivity ρ(T) of EuSr2Bi2S2Se2F4 under various applied fields. The inset shows a plot of Hc2 vs T.
Figure 4. (a) Temperature-dependent resistivity ρ(T) for Eu2SrBi2S4−xSexF4 (x = 0, 0.5, 1, 1.5, 2). (b) Onset of the superconducting transition for x = 1.5, 2 at low temperatures showing criteria for determining Tconset. (c) Field-dependent resistivity ρ(T) for Eu2SrBi2S2.5Se1.5F4 in the superconducting region. The inset shows a plot of Hc2 vs T.
9.1(2) μB/fu. Hence we observe that in the Eu2SrBi2S4−xSexF4 series population of Eu2+ states attains a maximum for x = 1.5. With a further increase in Se concentration (x = 2) the population of the Eu2+ state decreases. In our present high-field studies, we find that the Eu2+ component has increased in both series of compounds EuSr2Bi2S2Se2F4 and Eu2SrBi2S2.5Se1.5F4 as a consequence of addition of Se. Some of the Se-containing compounds exhibit
superconductivity, as we have shown above. However, enhancement of the Eu2+ component in high magnetic fields has no bearing on their superconducting properties, as their Hc2 values (
