Sn substitution effect on structural and physical properties in La 2 O 2 Bi 3 Ag 1−x Sn x S 6

Figure 1a displays the room temperature XRD patterns for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5). All the La 2 O 2 Bi 3 Ag 1−x Sn x S 6 samples are crystallized in the tetragonal structure with the space group of P4/nmm. An impurity phase of La 2 Sn 2 O 7 was observed for x = 0.2–0.5. Figure 1b shows the shift in the 103 peak position, which slightly shifts towards the low angle side for x = 0.1. As the Sn concentration increases from x = 0.2 to 0.5, the 103 peak shifts towards the higher angle side. The schematic image of the crystal structure of La 2 O 2 M 4 S 6 is shown in Fig. 1c. For the La 2 O 2 Bi 3 Ag 1−x Sn x S 6 phase, Bi selectively occupies the M1 site, and Sn is expected to substituted for Ag at the M2 site, which was qualitatively confirmed by Rietveld refinements. The evolutions of the lattice parameters by the Sn substitution are shown in Fig. 1d,e. The lattice parameters are a = 4.061(1) Å and c = 19.445(1) Å for x = 0 and a = 4.0648(1) Å and c = 19.48(1) Å for x = 0.1. The lattice parameter c tends to decrease by Sn substitution for x = 0–0.2, and then, it continuously increases with increasing x for x = 0.2–0.5. The lattice parameter a increases for the x = 0.1, but it tends to decrease with increasing x for higher x. However, the changes in those lattice parameters are small. On the basis of the lattice parameter evolutions, we consider that the Sn substitution does not largely affect the lattice volume, which may be due to smaller ionic radius of Sn2+ (93 pm) than that of Ag+ (113 pm). The La 2 O 2 Bi 3 AgS 6 -type structure can be considered as the stacking of a La 2 O 2 layer, two BiS 2 layers, and an NaCl-type AgBiS 2 layer. In such a layered structure, the unit cell is almost determined by the hardest layer due to the different ionic bonding nature. In this structure, La 2 O 2 layer structure is the hardest.

The actual ratio of the metals in the conducting layer (Bi, Ag, and Sn) in the La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5) samples were examined by EDX. The nominal and analyzed values of x are plotted in the Fig. 1f. Although there is slight deviation from the nominal values, the analyzed values of x EDX linearly increases with increasing nominal x. We found that Bi is slightly excess for x = 0.1–0.3. Therefore, y parameter with a formula of La 2 O 2 Bi 3+y (Ag 1−x Sn x ) 1−y S 6 was introduced to analyze the Bi concentration. The analyzed values of y EDX are plotted in Fig. 1g. y EDX is higher for x = 0.1–0.3 but almost zero for x = 0, 0.4, and 0.5. Therefore, we consider that the Bi excess can be ignored in the discussion on the Sn substitution (and Se substitution) effect, and we use the formula La 2 O 2 Bi 3 Ag 1−x Sn x S 6 in this paper.

Figure 2a–c show the temperature dependences of magnetic susceptibility (4πχ-T) under an applied magnetic field of 10 Oe for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0.3–0.5). The diamagnetic signals in the 4πχ curve were observed below 2.2, 2.8, and 2.6 K for x = 0.3, 0.4, and 0.5, respectively. A large diamagnetic signal was observed below 2.8 K in the ZFC curve for x = 0.4. The shielding value fractions estimated from 4πχ (ZFC) at 1.9 K is nearly 20% [See Fig. 2(d).] while it is still not saturated. From the susceptibility results, we consider that Sn substitution is effective to improve the superconducting properties of La 2 O 2 Bi 3 AgS 6 but not sufficient to induce bulk superconductivity.

Figure 2 Superconducting properties examined from magnetic susceptibility for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0.3–0.5). (a–c) Temperature (T) dependences of magnetic susceptibility (4πχ) for x = 0.3–0.5 measured in the ZFC and FC modes with an applied magnetic field of 10 Oe. (d) Sn concentration dependence of the shielding volume fraction estimated using the ZFC data at 1.9 K. Full size image

Figure 3 shows the temperature dependences of electrical resistivity from 300 to 0.1 K for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5). The electrical resistivity at 300 K decreases with increasing Sn concentration up to x = 0.3 and increases again for x = 0.4 and 0.5. The normal-state resistivity of the Sn-doped samples changes remarkably. For example, the pure sample (x = 0) shows a linear decrease in resistivity on cooling below the anomaly temperature T* = 180 K. A similar behavior was observed up to x = 0.2. The resistivity anomaly at T* appears for x ≤ 0.2, and the T* shifts towards the lower temperature side with increasing x. In contrast, the normal-state ρ(T) for x = 0.3–0.5 shows an upturn below ~50 K. The anomaly disappears for x ≥ 0.3. Figure 3g shows the zoomed view of the Figs. 3a–f near the superconducting transition. The T c clearly increases with increasing Sn concentration in La 2 O 2 Bi 3 Ag 1−x Sn x S 6 . The highest T c was achieved for x = 0.4, and T c decreases for a higher substitution with x = 0.5.

Figure 3 Electrical transport properties for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5). (a–f) Temperature dependences of electrical resistivity from 300 to 0.1 K for x = 0–0.5. The anomaly temperature in the ρ(T) curves is indicated by T*. (g) The ρ(T) curves in the temperature range of 0.1–4.0 K. Full size image

The room-temperature Seebeck coefficient (S) for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5) are shown in Fig. 4. The Seebeck coefficient is a good scale for the carrier concentration in BiS 2 -based compounds30. We observed a slight change in S by Sn substitution. The S in x = 0.2–0.4 are almost the same, but that for x = 0 and 0.5 are slightly large. This suggests that the carrier concentrations for x = 0.2–0.4 are higher than those for x = 0 and 0.5. This seems to be related to the evolution of T c . However, the large change in T c from 0.6 to 2.3 K between x = 0.1 and 0.4 cannot be simply understood by the carrier concentration only.

Figure 4 Seebeck coefficient for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5). The room-temperature Seebeck coefficient (S) is plotted as a function of nominal Sn concentration (x). Full size image

Figure 5 shows the superconductivity phase diagram of La 2 O 2 Bi 3 Ag 1−x Sn x S 6 , which shows the interplay between the resistivity anomaly temperature (T*) and the superconducting transition temperature (T c zero). The T* is suppressed by the Sn substitution, and it disappears at x = 0.3. The T c gradually increases with increasing x in La 2 O 2 Bi 3 Ag 1−x Sn x S 6 . The highest T c zero = 2.3 K is achieved for x = 0.4. A lower T c zero = 1.9 K is observed for the highest (solubility-limit) Sn concentration of x = 0.5.

Figure 5 Phase diagram for La 2 O 2 Bi 3 Ag 1−x Sn x S 6 (x = 0–0.5). The x (Sn concentration estimated from EDX) dependences of T c zero and T* are plotted as a function of Sn concentration (x). SC denotes superconductivity. Full size image

Here, we discuss about the possible influence of the presence of the La 2 Sn 2 O 7 impurity to the composition. Due to the change in the impurity amount for x = 0–0.5, the actual compositions may deviate from the nominal compositions. Although the Sn concentration to that of Ag was checked by EDX (Fig. 1f), oxygen deficiency in the blocking layer was not checked in this study. However, we consider that oxygen deficiency was not introduced because it is expected to make the structure unstable even if it has been introduced in the La 2 O 2 Bi 3 AgS 6 -type structure. We have tried to dope electrons by oxygen deficiency or fluorine substitution for the La 2 O 2 Bi 3 AgS 6 -type structure. However, in the La 2 O 2 Bi 3 AgS 6 system, such trials of fluorine substitutions (or oxygen deficiency) resulted in decomposing of the La 2 O 2 Bi 3 AgS 6 -type structure into LaO 1−x F x BiS 2 . This indicates that an electron-doped composition with the La 2 O 2 Bi 3 AgS 6 -type structure cannot be obtained easily due to the competition to the high stability of the REO 1−x F x BiS 2 -type phase. In addition, oxygen deficiency has not been observed in the REOBiS 2 systems.

Superconducting properties of Se-doped La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 5.7 Se 0.3

As shown above, the Sn substitution improved the superconducting properties in La 2 O 2 Bi 3 Ag 1−x Sn x S 6 , and the highest T c and shielding volume fraction were obtained for x = 0.4. In the BiS 2 -based compounds, partial Se substitutions for the S site of the superconducting BiS 2 layers have significantly improved the superconducting properties and the bulk characteristics of superconductivity. Therefore, we tried to substitute the S site by Se for the x = 0.4 sample. The 5%-Se sample La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 5.7 Se 0.3 was successfully synthesized, but samples with higher Se concentration contained selenide impurity phases. The solubility limit of Se for the S site is around 5%. The composition estimated from the EDX analyses for Bi, Ag, Sn, S, Se elements was La 2 O 2 Bi 3.09 Ag 0.65 Sn 0.26 S 5.73 Se 0.27 . Since the obtained composition is close to the nominal formula, we call the sample with the nominal value below.

Figure 6 shows the XRD pattern and the Rietveld refinement result for La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 5.7 Se 0.3 . Although two peaks related to the La 2 Sn 2 O 7 impurity phase were observed, other peaks could be refined using the tetragonal (P4/nmm) model with a reliability factor R wp of 13.4%. In the refinement, Se was assumed to be substituted for the S1 site. The lattice parameters were a = 4.0759(2) Å and 19.4824(11) Å, which are clearly larger than those of La 2 O 2 Bi 3 Ag 1−x Sn x S 6 due to the presence of Se.

Figure 6 X-ray diffraction analysis for La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 5.7 Se 0.3 . XRD pattern and the Rietveld refinement result are shown. The arrows indicate the peaks for the impurity phase La 2 Sn 2 O 7 . The inset image shows the crystal structure depicted using the structural parameters obtained from the Rietveld refinement. Full size image

Figure 7 displays the superconducting properties of La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 5.7 Se 0.3 . As shown in Fig. 7a, a large shielding volume fraction close to 100% was observed. From the resistivity measurements (Fig. 7b), zero resistivity was observed at 3.0 K, and the onset temperature (T c onset) was 3.5 K; we estimated the temperature where the resistivity becomes almost 90% of normal-state resistivity. Although superconductivity was observed, the ρ(T) curve still shows a semiconducting-like localization at low temperatures. We have measured ρ(T) under magnetic fields up to 9 T. The obtained T c onset and T c zero were plotted in Fig. 7d to evaluate the upper critical field H c2 and the irreversible field H irr . The H c2 (0) was estimated as 2.15 T using the WHH model (Werthamer-Helfand-Hohenberg model)31. In addition, from rough estimation with a linear fitting of H irr , the H irr (0) was estimated as 1.0 T.