Evolution of two bulk-superconducting phases in Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, Sm) by external hydrostatic pressure effect

Polycrystalline samples of Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transition temperatures (Tc) within the range of 2.1–2.7 K under ambient pressure, the estimated superconducting volume fraction was small, which indicates non-bulk nature of superconductivity in those samples under ambient pressure. A dramatic increase in shielding fraction, which indicates the emergence of the bulk superconductivity was achieved by applying external hydrostatic pressures. We found that two phases, low-P phases with Tc = 2.5–2.8 K and high-P phases with Tc = 10.0–10.8 K, were induced by the pressure effect for samples with RE = La, Ce, Pr, and Nd. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions, which was explained by the correlation between external and chemical pressure effects. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for understanding the effects of both external and chemical pressures to the emergence of bulk superconductivity and pairing mechanisms in BiCh2-based superconductors.

Scientific RepoRtS | (2020) 10:12880 | https://doi.org/10.1038/s41598-020-69889-w www.nature.com/scientificreports/ BiCh 2 -based (Ch: S, Se) systems 18,19 . By applying in-plane chemical pressure, the local structural disorder in the tetragonal structure is removed, and bulk superconductivity is induced 20,21 . To induce bulk superconductivity, external pressure effects are also effective [22][23][24][25][26][27][28][29][30][31][32] . For REO 0.5 F 0.5 BiS 2 and EuFBiS 2 , the T c at ambient condition (~ 2.5 K and ~ 0.5 K) dramatically increase to ~ 10 K under high pressure [23][24][25][26] , and the origin of the increase in T c was explained by a structural transition from tetragonal (P4/nmm) to monoclinic (P2 1 /m) at around 1 GPa for LaO 0.5 F 0.5 BiS 2 and EuFBiS 2 23,24 . In spite of the interesting phenomena under high pressure in those BiS 2 -based compounds, the pressure studies have been performed by electrical resistivity measurements. To the best of our knowledge, only two works have used magnetization as a probe for the pressure studies, which confirmed bulk superconductivity in LaO 0.5 F 0.5 BiS 2 and EuFBiS 2 under high pressure 23,24 . Therefore, further experiments on how the bulk characteristics of superconductivity could be achieved under pressures are needed. In addition, as summarized above, there are two ways, chemical and external pressure effects, to induce bulk superconductivity. Note that those two bulk superconducting phases have a different crystal structure system of tetragonal (under chemical pressure) and monoclinic (under external pressure). Therefore, in order to further investigate the interplay among superconducting characteristics (T c and bulk nature), external pressure, and chemical pressure, it is essential to establish a new system where discussion about the relationships among those factors is possible. In this study, to search for a system which enables us to study such relationships, we have studied the chemical and external pressure effects for the Sr 0.5 RE 0.5 FBiS 2 (RE: La, Ce, Pr, Nd, and Sm) system. SrFBiS 2 is a parent phase of those target materials and a semiconductor with a band gap. The substitution of Sr 2+ with RE 3+ induces electron carriers in the BiS 2 layer, and filamentary superconductivity appears at approximately 2.8 K in La-, Ce-, and Pr-doped compounds [12][13][14][15][29][30][31][32] . Based on electrical resistivity measurements conducted under high pressures, a considerable increase in T c was observed in previous pressure experiments 22,[29][30][31][32] . The highest T c in Sr 0.5 RE 0.5 FBiS 2 is approximately 10 K, and the pressure dependences of T c exhibits a sharp increase at the critical pressure (~ 1 GPa). Since there has been no further report on the pressure-induced superconductivity in Sr 0.5 RE 0.5 FBiS 2 , we have studied the superconducting phase diagrams for Sr 0.5 RE 0.5 FBiS 2 from magnetization experiments under high pressure and discussed the interplay among superconducting characteristics, chemical pressure, and external pressure. Two bulk superconducting phases with a lower T c (low-P phase) and a higher T c (high-P phase) were confirmed.

Results
Sample characterization and physical properties at ambient pressure. Figure 1 depicts the powder synchrotron X-ray diffraction (XRD) patterns for Sr 0.5 La 0.5 FBiS 2 . For Sr 1-x RE x FBiS 2 (RE = Ce, Pr, Nd, and Sm), see Supplemental Fig. S1a-d. The crystal structure of the obtained samples was well refined using the Rietveld method. They were well refined using a tetragonal structure with the P4/nmm space group. Small impurity peaks due to REF 3 (RE: La, Ce) and Bi 2 S 3 were also detected in the case of the Pr-, Nd-, and Sm-based samples. As shown in Fig. 2, we observed that the lattice constant a decreased with decreasing RE ionic radius; however, the lattice constant c increased under these conditions. The obtained values were in agreement with previous reports 12,30,31 . The chemical composition ratios of the samples were determined using energy dispersive X-ray spectroscopy (EDX). These results showed that the chemical compositions of the obtained samples were in reasonable agreement with the nominal compositions ( Table 1). The electrical resistivity of the samples at ambient pressure was measured down to 1.6 K (Fig. 3). In all the samples, semiconducting behaviour was observed;   www.nature.com/scientificreports/ moreover, the resistivity-temperature (ρ-T) curve indicated a slight increase in ρ on cooling, which implied that the conduction electrons were weakly localized due to the in-plane local disorder in the BiS 2 layer [18][19][20][21] . A superconducting transition was observed at T c = 2.7, 2.7, 2.6, 2.6, and 2.1 K for RE = La, Ce, Pr, Nd, and Sm, respectively (see the inset of Fig. 3). In addition, a superconducting transition was also observed in the temperature dependence of magnetization, as plotted in Fig. 4. The T c estimated based on magnetization was in agreement with that obtained based on the resistivity measurements. This is the first study to report on the observation of superconductivity in Sr 0.5 Nd 0.5 FBiS 2 and Sr 0.5 Sm 0.5 FBiS 2 under ambient pressure. However, in these samples, the estimated shielding volume fraction was less than 6%. This indicates that chemical pressure effects, which is expected to be generated by the substitution of smaller RE such as Nd and Sm, are insufficient to induce bulk superconductivity in the Sr 1−x RE x FBiS 2 system. To obtain bulk superconductivity, which is essentially important for describing intrinsic superconducting properties, we applied external pressure on the samples.
External pressure effect. Figure 5a shows the temperature dependences of the magnetization of Sr 0.5 La 0.5 FBiS 2 when increasing the applied pressure to 1.15 GPa. The T c of approximately 2.7 K (low-P phase) remained almost unchanged up to 0.84 GPa; alternatively, there was an evident increase in the shielding volume fraction. This result indicates that the external pressure effectively enhances the shielding volume fraction, which corresponds to the emergence of bulk nature of the superconducting states in the Sr 0.5 La 0.5 FBiS 2 samples at a low-P regime. A remarkable increase in T c up to T c max = 10.8 K (high-P phase) was observed at P > 0.95 GPa. The enhancement in the shielding volume fraction was also observed in the high-P phase, which was achieved by increasing applied pressure to the maximum pressure, without a noticeable change in the T c . The drastic increase in T c is explained by a tetragonal to a monoclinic phase. Figure 6a presents the laboratory X-ray diffraction patterns of Sr 0.5 La 0.5 FBiS 2 at room temperature under various applied pressures of up to 3.4 GPa. Shifts of the (001) and (004) peaks to higher angles clearly indicate the shrinkage of the lattice along the c-axis due to the pressure. In contrast, a relatively smaller shift of the (110) peak was detected, which indicates that the in-plane size remains almost unchanged. Strong peak broadening was observed for the (200) peak above 1.1 GPa, indicating peak splitting due to the lowering of in-plane structural symmetry. Similar peak splitting on the (200) peak was observed for isostructural LaO 0.5 F 0.5 BiS 2 and EuFBiS 2 samples under high pressure 23,24 , and a resultant structural transition from a tetragonal to monoclinic phase was detected. We noticed that the (200) peak asymmetrically split into two or more peaks, as depicted in Fig. 6b. This unexpected evolutions of the XRD pattern may be due to the inhomogeneity of applied pressure and the flexible nature of the in-plane structure of BiS 2 -based compounds. The critical pressure of 1.1 GPa estimated from the XRD corresponds satisfactorily with the P c estimated from the magnetization measurements. To further analyse crystal structure of this phase under pressure, synchrotron XRD experiments under homogeneous pressure conditions are needed.
The pressure dependence of T c is summarized in Fig. 5f; the light blue, blue, and pink regions indicate the filamentary superconductivity, bulk superconductivity in the low-P phase, and bulk superconductivity in the high-P phase, respectively. The low-P phase shifts to the high-P phase when a pressure slightly exceeding the critical pressure (P c ) for the high-P phase is applied. In regard to this, a phase diagram was created using the shielding volume fraction of 20% or more as a bulk state in order to discuss the phase transition. In the pressure range of 0-0.84 GPa, T c (P) remains almost constant (~ 2.7 K). The bulk superconductivity of the sample was induced by  Fig. 5a-j and respectively. The high-P phase of Sr 0.5 Sm 0.5 FBiS 2 was not observed up to 1.28 GPa, which is nearly the upper limit of the pressure measurement apparatus used in this experiment. This is the first report offering evidence of the bulk nature of two different (low-P and high-P) phases of the Sr 1-x RE x FBiS 2 (RE: La, Ce, Pr, and Nd) superconductors when subjected to pressure.

Discussion
In this section, we discuss the relationship between external pressure effects, chemical pressure effects, and evolution of superconductivity in Sr 0.5 RE 0.5 FBiS 2 . On comparing the evolutions of superconductivity for RE = Ce-Sm and that for RE = La, a slight decrease in T c for the low-P phase was observed with increasing pressure for the RE = Ce, Pr, Nd and, Sm samples. Similar trend was observed in a phase diagram of tetragonal phase of BiCh 2based systems examined as a function of chemical pressure 18,19,33 . Moreover, we observed that the T c of the high-P phase showed a trend of decreasing with decreasing RE ionic radius. This trend is also common to that observed in high-pressure studies for REO 0.5 F 0.5 BiS 2 with different RE 22 . The P c increased with decreasing RE ionic radius. Specifically, P c was roughly estimated as 0.95, 1.11, 1.17, and 1.33 GPa for La 3+ (with an ionic radius of 1.16 Å), Ce 3+ (with an ionic radius of 1.14 Å), Pr 3+ (with an ionic radius of 1.13 Å), and Nd 3+ (with an ionic radius of 1.11 Å), respectively, where those ionic radii are values for a coordination number of 8. We noticed that the shift in P c by replacing RE in Sr 0.5 RE 0.5 FBiS 2 is clearly small as compared to the case of REO 0.5 F 0.5 BiS 2 ; P c was ~ 2 GPa for NdO 0.5 F 0.5 BiS 2 28 . These trends, the increase in P c and the decrease in T c with a decrease in RE ionic radius, were also observed for REO 0.5 F 0.5 BiS 2 compounds 22,26,28 ; specifically, the T c varied from approximately 10 to 6 K. The different pressure evolutions of superconducting phases in between the Sr 0.5 RE 0.5 FBiS 2 and REO 0.5 F 0.5 BiS 2 systems would be understood by the difference in the substitution sites. For Sr 0.5 RE 0.5 FBiS 2 , the Sr site is partly (50%) substituted by RE, and hence the Sr-F bonds are partly remained. In the case of REO 0.5 F 0.5 BiS 2 , all the RE site is replaced by different RE, and hence the RE-(O,F) bond length should systematically decrease according to the RE ionic radius. Therefore, the different sensitivity of the crystal structure and superconductivity to external pressures were observed between Sr 0.5 RE 0.5 FBiS 2 and REO 0.5 F 0.5 BiS 2 . The difference should be caused by the chemical bonding states of the SrF-based and REO-based layers and the interlayer interaction between the blocking layers and BiS 2 conducting layers. Our results suggest that the structure of the blocking layer largely affects the evolution of superconducting phases under high pressures. The results obtained in this study will be useful for material design of BiCh 2 -based superconductors with a higher T c .

conclusion
We showed the results of the synthesis, crystal structure analysis, resistivity, and magnetic susceptibility measurements investigated under ambient and high pressures for Sr 0.5 RE 0.5 FBiS 2 (RE: La, Ce, Pr, Nd, and Sm). The effects of external pressure on magnetization resulted in abrupt increments in T c up to 10-10.8 K for the samples with RE = La, Ce, Pr, and Nd. Based on the analyses of the shielding volume fraction estimated via magnetic susceptibility measurements, we found that two bulk superconducting phases (low-P and high-P phases) can be induced by external pressure for Sr 0.5 RE 0.5 FBiS 2 . For RE = La, we have confirmed a structural transition from laboratory XRD under high pressure, which is a common trend with those observed for LaO 0.5 F 0.5 BiS 2 and EuFBiS 2 . The critical pressure, where T c sharply increased to the high-P phase, shifted to a higher pressure with decreasing RE ionic radius. This implied that both the external and chemical pressures were affecting T c . In addition, we have compared the obtained phase diagrams for Sr 0.5 RE 0.5 FBiS 2 and REO 0.5 F 0.5 BiS 2 . We found differences in the sensitivity of the crystal structure and superconducting characteristics to external pressure effects between Sr 0.5 RE 0.5 FBiS 2 and REO 0.5 F 0.5 BiS 2 , which should be caused by the different chemical bonding states in the blocking layers and the interlayer interaction between the blocking layers and BiS 2 conducting layers.  The phase purity and the crystal structure of the Sr 0.5 RE 0.5 FBiS 2 (RE: La, Ce, Pr, Nd, and Sm) samples were examined by powder synchrotron XRD (ambient pressure XRD) with an energy of 25 keV (λ = 0.49657 Å) at the beamline BL02B2 of SPring-8 under a proposal No. 2019A1101. The synchrotron XRD experiments were performed at room temperature with a sample rotator system, and the diffraction data were collected using a high-resolution one-dimensional semiconductor detector MYTHEN [Multiple mythen system] with a step of 2θ = 0.006°. To investigate the evolution of crystal structure of Sr 0.5 La 0.5 FBiS 2 , laboratory XRD experiments under high pressure up to 3.4 GPa were performed at room temperature using a Mo-K α radiation on a Rigaku (MicroMax-007HF) rotating anode generator equipped with a 100 µm collimator. Daphne 7474 was used as a pressure medium.
The crystal structure parameters were refined using the Rietveld method with a RIETAN-FP software 34 . The actual compositions of the obtained samples were analysed using an energy dispersive X-ray spectroscopy (EDX) on TM-3030 (Hitachi).
The temperature dependence of magnetic susceptibility at ambient pressure and under high pressures were measured using a superconducting quantum interference devise (SQUID) with MPMS-3 (Quantum Design). Hydrostatic pressures were generated by the MPMS high pressure Capsule Cell. The sample was immersed in a pressure transmitting medium (Daphene 7373) covered with a Teflon cell. The pressure at low temperature was calibrated from the superconducting transition temperature of Pb manometer. The electrical resistivity was measured on a GM refrigerator system (Made by Axis) using a conventional four-probe method. For the resistivity measurements, gold wires were connected to the samples with a silver paste.