Hydrothermal Synthesis, Structure, and Superconductivity of Simple

Feb 24, 2017 - Department of Materials Science and Engineering, University of Rajshahi, Rajshahi 6205, Bangladesh. ‡ Center for Crystal Science and ...
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Hydrothermal Synthesis, Structure, and Superconductivity of Simple Cubic Perovskite (Ba0.62K0.38)(Bi0.92Mg0.08)O3 with Tc ∼ 30 K Mirza H. K. Rubel,†,‡ Takahiro Takei,‡ Nobuhiro Kumada,*,‡,§ M. Mozahar Ali,‡ Akira Miura,∥ Kiyoharu Tadanaga,∥ Kengo Oka,⊥ Masaki Azuma,¶ Eisuke Magome,# Chikako Moriyoshi,# and Yoshihiro Kuroiwa# †

Department of Materials Science and Engineering, University of Rajshahi, Rajshahi 6205, Bangladesh Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae-cho, Kofu 400-8511, Japan ∥ Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan ⊥ Faculty of Science, Chuo University, 112-8551, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 192-0393, Japan ¶ Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Kanagawa 226-8503, Japan # Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8526, Japan ‡

S Supporting Information *

ABSTRACT: We have synthesized a new superconducting perovskite bismuth oxide by a facile hydrothermal route at 220 °C. The choice of starting materials, their mixing ratios, and the hydrothermal reaction temperature was crucial for obtaining products with superior superconducting properties. The structure of the powder sample was investigated using laboratory X-ray diffraction, high-resolution synchrotron X-ray diffraction (SXRD) data, and electron diffraction (ED) patterns [transmission electron microscopy (TEM) analysis]. The refinement of SXRD data confirmed a simple perovskitetype structure with a cubic cell of a = 4.27864(2) Å [space group Pm3̅m (No. 221)]. Elemental analysis detected magnesium in the final products, and a refinement based on SXRD and inductively coupled plasma data yielded an ideal undistorted simple cubic perovskite-type structure, with the chemical composition (Ba0.62K0.38)(Bi0.92Mg0.08)O3. ED patterns also confirmed the simple cubic perovskite structure; the cube-shaped microstructures and compositional homogeneity on the nanoscale were verified by scanning electron microscopy and TEM analyses, respectively. The fabricated compound exhibited a large shielding volume fraction of about 98% with a maximum Tcmag of ∼30 K, which was supported by the measured bismuth valence as well. Its electrical resistivity dropped at ∼21 K, and zero resistivity was observed below 7 K. The compound underwent thermal decomposition above 400 °C. Finally, the calculated band structure showed a metallic behavior for this hydrothermally synthesized bismuth oxide.

1. INTRODUCTION Among all oxide materials, those with the perovskite and related structures are of great importance because their properties can be tailored by the substitution of different elements into their crystal lattices. Perovskite materials containing bismuth oxides are of special significance1−6 as a consequence of the wide range of interesting properties such as superconductivity, ferroelectricity, and ferromagnetism that they exhibit. Recently, novel A-site-ordered double perovskite7−9 and simple perovskite10,11 superconductors have been discovered by a facile low-temperature hydrothermal route. In these perovskites, the partial alternation of cations in the A and B sites significantly influenced the structure and superconducting properties. The stability of unusual or mixedvalence states of bismuth in the crystal structures also seems to play a significant role in the appearance and control of high-Tc superconductivity.7,8,12−17 In general, cubic perovskites with © 2017 American Chemical Society

tilted BO6 octahedra were viewed as the best hosts for superconductivity in these busmuth oxide materials. Bi atoms usually adopt trivalent and pentavalent states, with the trivalent state being more common. It is quite difficult to obtain pentavalent bismuthates by high-temperature reactions, except in systems containing barium oxide. In fact, some superconducting perovskite bismuth oxides, for instance, (Sr,A)BiO3 (A = K, Rb),18 K1−xBi1+xO3 (x = 0.0−0.1),19 and (K,Bi)BiO3,20 including AgBiO321 and BiNiO322 oxides, have been synthesized only under high pressures. However, the well-known noncuprate superconducting system Ba1−xKxBiO3 (BKBO)4 was prepared by a solid-state reaction that resulted in a simple perovskite with cubic (Pm3̅m) symmetry, which is the most likely system to exhibit the highest Tc. Kumada et al. and other Received: July 30, 2016 Published: February 24, 2017 3174

DOI: 10.1021/acs.inorgchem.6b01853 Inorg. Chem. 2017, 56, 3174−3181

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Inorganic Chemistry researchers have synthesized a large number of pentavalent bismuthates from hydrated sodium bismuthate (NaBiO3· nH2O)6−10,23−32 by low-temperature hydrothermal reactions. Some of these exhibit considerably attractive properties such as superconductivity, photocatalytic activity, and magnetic frustration. Examples of materials obtained by this method include Bi2O423 (which is the first instance of distinct Bi3+ and Bi5+ lattice sites), trirutile-type ABi2O6 (A = Mg, Zn),24 ilmenitetype AgBiO325 and LiBiO3,26and Bi3.33(VO4)2O2 single crystals.27 Besides, NaBiO3·nH2O yielded additional pentavalent bismuth oxides such as SrBi2O6,28 (Li,Na)BiO3,29 BaBi2O6,30 CaBi2O6, and PbBi2O631 through ion-exchange reactions. The crystal structure of BaBi2O6,29 CaBi2O6, and PbBi2O5.9·nH2O31 was determined by X-ray diffraction (XRD) studies, and the structural refinement of MgBi2O6,24 SrBi2O6, and (Pb1/3Bi2/3)O1.432 was performed and reported elsewhere. The successful synthesis of these pentavalent bismuthates led to the preparation of mixed-valent bismuth oxide based simple10,11 and double perovskite7−9 superconductors by hydrothermal reactions. Several authors have performed first-principle studies33 of these superconductors to demonstrate their metallicity and other properties. Later, the hydrothermal synthesis of a new Bi/Pb-based (Ba0.82K0.18)(Bi0.53Pb0.47)O3 simple perovskite superconductor10 inspired the search for other new bismuth oxide perovskite materials with improved superconducting properties by this method. So far, to the best of our knowledge, the hydrothermal synthesis of a Ba1−yKyBi1−xMgxO3 superconducting system has never been reported. Herein, the main starting material was MgBi2O6 with a trirutile structure,24 which was also obtained by a hydrothermal reaction. In this study, we report a new compositional (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 , Bi/Mg-based, environment friendly (Pb-free), simple perovskite material with improved superconducting properties (Tc ∼ 30 K), using the starting materials MgBi2O6, Ba(OH)2·8H2O, and KOH by optimizing hydrothermal conditions. This superconductor possessed an ideal undistorted simple cubic phase analogous to BKBO4,12−14,34−38 and our previously reported (Ba0.82K0.18)(Bi0.53Pb0.47)O3 system.10 The refinement based on synchrotron X-ray diffraction (SXRD) data provides a structural model for the synthesized product, as shown in Figure 1. Herein, the A site is occupied by Ba and K atoms, whereas Bi is partially substituted by Mg in the B site, forming a network of edge-sharing (Bi/Mg)O6 octahedra. In this paper, we describe the hydrothermal synthesis, magnetic and electronic transport, crystal structure, thermal behavior, and electronic band structure of this new compositional bismuth oxide perovskite superconductor.

Figure 1. Crystal structure of an undistorted (Ba 0.62 K 0.38 )(Bi0.92Mg0.08)O3 simple cubic (Pm3m ̅ ) perovskite, depicting edgesharing (Bi/Mg)O6 octahedra. The undistorted octahedral angle (180°) and interatomic distance are also shown in the unit cell.

Characterization. The XRD patterns of the products were obtained on a Rigaku X-ray diffractometer (RINT 2000 V) with Cu Kα radiation (λ = 1.54056 Å) at the secondary beam. Electron diffraction (ED) patterns and elemental mapping were analyzed by scanning transmission electron microscopy (STEM; Technical Orisis FEI). Scanning electron microscopy (SEM; JEOL JEM-6500F) was used to observe the real microstructure and morphology of the samples. Elemental compositions were confirmed by inductively coupled plasma (ICP; SPS 3500 DD, Hitachi) and iodometric titrations using an automatic titrator (TOA DKK AUT-701). Highresolution SXRD data of powder samples were recorded in a Debye− Scherrer camera installed at the beamline BL02B2 at the SPring-8 facility, Hyogo, Japan. The powder products were loaded into quartz glass capillary tubes (⌀ = 0.2 mm) and rotated during the measurements. The wavelength was calibrated to 0.413653 or 0.413829(1) Å using CeO2 as the standard. The program RIETANFP39 was used for the structural refinement. During the refinement, parameters such as the scale factor and background parameters were refined, and the peak shape was approximated by a split pseudo-Voigt function. The crystal structure was drawn by the VESTA40 software. The thermal stability was studied by thermogravimetric analysis (TGA) from 30 to 800 °C in a helium gas flow. The temperature dependence of the direct-current (dc) magnetic susceptibility measurement was carried out in both zero-field-cooling (ZFC) and field-cooling (FC) modes over the temperature range 2.0−40.0 K, in an external magnetic field of 10 Oe using a vibrating sample magnetometer (PPMS, Quantum Design). The electrical resistivity of the pressed pellet was measured between 2.0 and 250.0 K using a standard four-probe method (PPMS, Quantum Design). The band structure and atom-projected density of states (DOS) were calculated using the program CASTEP41 based on density functional theory. The generalized gradient approximation with the Perdew−Burke− Ernzerhof exchange-correlation functional42 was implemented. The Broyden−Fletcher−Goldfarb−Shanno minimization algorithm was utilized for the geometry optimization of the structures with minimum energy. Geometry optimization was achieved using convergence thresholds of 5 × 10−6 eV/atom for the total energy, 0.01 eV/Å for the maximum force, 0.02 GPa for the maximum stress, and 5 × 10−4 Å for the maximum displacement in the lattice parameter. The sampling of the irreducible Brillouin zone was gridded using 14 × 14 × 14 Monkhorst−Pack k-point mesh, and the plane-wave energy cutoff (Ecut) of 800 eV was used for the calculations.

2. EXPERIMENTAL AND COMPUTATIONAL SECTION Sample Preparation. The starting material MgBi2O6 was obtained as a single phase from the hydrothermal reaction of NaBiO3·nH2O and MgCl2·6H2O (atomic ratio of Mg:Bi = 4:1) at 130 °C. Powder XRD patterns and refinement based on neutron diffraction data of MgBi2O6 confirmed its trirutile-type structure.24 Afterward, samples were synthesized using the starting materials in the molar ratio MgBi2O6:Ba(OH)2·8H2O:KOH = 1:1:20, with 7.5 mL of distilled water in a Teflon-lined steel autoclave under hydrothermal conditions. The tightly closed autoclaves were heated at 180−240 °C for 48 h to ensure that the reactions were complete. After the autoclave was cooled and depressurized, the solid black powders were separated by filtration, repeatedly washed with deionized water, and dried in air at 70 °C for 6−8 h. 3175

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3. RESULTS AND DISCUSSION 3.1. Effect of the Synthesis Temperature on the Obtained Phase and Structure. The hydrothermal reaction temperature significantly affected the phases of the final products. The laboratory XRD pattern of all of the synthesized samples is shown in Figure S1 in the Supporting Information (SI). Among these, the sample prepared at 220 °C was indexed as a single perovskite-like phase with the cubic cell of a = 4.2791(3) Å (Figure S1c in the SI). The systematic absences and lack of extinction conditions suggest the ideal space group Pm3̅m (No. 221), which is consistent with our result10 for (Ba0.82K0.18)(Bi0.53Pb0.47)O3 and other studies of the BKBO system.4,11,12,17 The structure of the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 sample prepared at 220 °C was also confirmed by ED (TEM analysis). The characteristic 100 and 110 diffraction spots explicitly reveal a simple cubic perovskite cell (Figure S2 in the SI), consistent with that observed in the XRD pattern. The chemical analysis results obtained from ICP detected the elements Bi, Ba, K, and Mg for all of the samples. The amount of each element detected was converted into a weight percent, from which the molar ratios were obtained. The same starting materials yielded different molar concentrations of the elements in the final products (see Table S1 in the SI) with a change of the reaction temperature from 180 to 240 °C. This scenario is similar to our previous report.8 The molar ratios of elements obtained for the sample prepared at 220 °C was as follows: Ba:K:Bi:Mg = 0.62:0.38:0.90:0.09. This result was taken into account during the subsequent refinement using SXRD data. Iodometric titrations confirmed the presence of both Bi5+ and Bi3+ in the products. The oxidation state of Bi in the final products obtained from ICP data (mole percent of Bi) and iodometric titration results (mole percent of Bi5+ and Bi3+) are summarized in the SI (Table S1). It is noted that the average Bi valence decreased from 4.45 to 4.24 (Table S1 in the SI) with an increase of the reaction temperature from 180 to 240 °C. This reduction in the valence state might be related to the thermal transformation of Bi5+, which has been observed in other pentavalent bismuth oxides.8,32 The valences of Bi for the samples synthesized at 200 and 220 °C were 4.37+ and 4.36+, respectively. The measured Bi valences are in agreement with those reported for simple perovskites16,17 and our published double perovskite superconductors,7,8 where the highest Tc appeared at a Bi valence of 4.3−4.4+. Therefore, the observed Bi valence and related carrier concentrations are crucial for the occurrence and control of superconducting properties in bismuth oxide perovskite superconductors. However, the average valence of Bi calculated from the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 composition is 4.52+, which is a little higher than the observed values of the chemical analysis results. This difference might be the result of either small impurities and/or an error in the estimation from ICP analysis data. Figure S3a in the SI shows an irregular flowerlike SEM micrograph for MgBi2O6. On the other hand, the cube-shaped microstructure was clearly visualized with a 3−20 μm particle size for all of the hydrothermally prepared products (Figure S3b−e in the SI). Further, the elemental homogeneity of Bi, Ba, K, Mg, and O in the nanoscale region for the synthesized sample at 220 °C was confirmed by STEM imaging and EDS-based elemental mapping (Figure S4 in the SI). In this section, we focus on a detailed structural analysis of the phase obtained by hydrothermal synthesis at 220 °C from

high-resolution SXRD data because it exhibited the best superconducting properties (Tc ∼ 30 K) of all our samples. Because no extinction conditions were observed from either the ED or XRD patterns, the structural refinement of the Ba−K− Bi−Mg−O system was performed assuming a single perovskitetype phase with a highly symmetric space group (No. 221) as before.10 In the initial structural model, we incorporated the chemical analysis results (Ba, K, Bi, and Mg in the nominal molar ratio 0.62:0.38:0.90:0.09) while considering probable cation distributions in the A and B sites of the unit cell. For the refinement, it was assumed that Ba/K and Bi/Mg occupy the A and B sites, respectively, and the oxygen occupancy was fixed at unity. The atomic positions at the crystallographic sites A (1a; 0, 0, 0), B (1b; 0.5, 0.5, 0.5), and O (3d; 0.5, 0, 0) were fixed for the structural model. Next, the occupancies were optimized and converged to Ba/K (0.62/0.38) and Bi/Mg (0.92/0.08) at the A and B sites, according to chemical analysis data, to improve the refinement results (Table 1). The diffraction peaks are not Table 1. Crystallographic Site, Occupancy Ratio, Atomic Coordinates, and Debye−Waller Temperature Factor, Biso, for (Ba0.62K0.38)(Bi0.92Mg0.08)O3 Obtained from the SXRD Patterna atom

site

occupancy factor (g)

x

y

z

Biso (Å2)

Bi Mg Ba K O

1a 1a 1b 1b 3d

0.92 0.08 0.62 0.38 1.0

0 0 1 /2 1 /2 1 /2

0 0 1 /2 1 /2 0

0 0 1 /2 1 /2 0

0.05(2) =B(Bi) 0.64(3) =B(Ba) 0.88(2)

a

Numbers in parentheses are the assumed standard deviations of the last significant digit.

symmetric and have broad shoulders, which are possibly attributed to lower symmetry from cubic or the coexistence of another cubic phase with slightly different lattice parameters. The shapes of the 200, 220, and 222 diffraction peaks are similar (Figure S6 in the SI); thus, multiple cubic phases are more likely. The final refined chemical composition of this cubic perovskite was formulated as (Ba0.62K0.38)(Bi0.92Mg0.08)O3. The total chemical composition calculated in this simple cubic perovskite structure is very close to that obtained from the elemental analysis data described earlier. Figure 1 presents the crystal structure of this new compositional simple cubic perovskite superconductor. In this structure, the Ba/K atoms occupy the A site in the center of the unit cell, and the Bi/Mg atoms form a network of perfectly regular, identical, nontilting, corner-sharing (Bi/Mg)O6 octahedra at the corners (B site). The Bi/Mg−O bond distance of 2.139(3) Å (shown in Figure 1) is in good agreement with 2.144(4) and 2.1336(3) Å in the Bi/Pb−O (reproduced data) and Bi/Na−O simple and double perovskite superconductors,8,10 respectively. The shortest Bi− O bond lengths, 2.11 and 2.1098(4) Å, respectively, in those perovskite superconductors,43,44 however, are somewhat smaller than those in this report. The octahedral framework in the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 structure remains undistorted (∠Bi/Mg−O−Bi/Mg = 180°) and similar to the ideal one40 in BKBO and differs from the strongly distorted Sr0.4K0.6BiO3 structure [∠Bi−O−Bi = 152.6(2)°] and the slightly tilted octahedra [∠Bi/Na−O−Bi/Na = 176.0(3)°] in the (K1.00)(Ba1.00)3(Bi0.89Na0.11)4O12 double perovskite.8 The SXRD Rietveld refinement profile (Figure 2) indicates that the 3176

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(Meissner−Ochsenfeld effect) below a certain critical superconducting temperature, Tc. With respect to the perfect diamagnetism of 4πM/H = −1, the superconducting volume fractions were estimated from the magnetization unit (emu) using calculated densities (7.25 g/cm3) from the SXRD refinement data. Figure 3 shows the volume susceptibility

Figure 2. Rietveld refinement profile for the SXRD (λ = 0.413653 Å) data of a (Ba0.62K0.38)(Bi0.92Mg0.08)O3 cubic (Pm3̅m) perovskite at room temperature. The markers denote observed data, solid lines are for the calculated profiles, and their differences are plotted at the bottom. The short vertical lines indicate the positions of allowed Bragg peaks. The inset shows an expansion between 7.50° and 8.83°, demonstrating no splitting of the characteristic 110 reflection.

compound peaks could be indexed as an ABO3 simple perovskite-type structure with lattice parameter a = 4.27864(2) Å and volume (V) = 78.33(4) Å3; these values of a and V are slightly smaller in comparison to those reported in our previous work.10 This decrease in the lattice parameter and volume could be due to the similar ionic radii of Mg2+ (89 pm) and Bi5+ (90 pm)45 at the B site and the increase of K doping at the A site, respectively. The successful structure refinement yielded good R factors (Rwp = 6.11%, RB = 1.16%, Rp = 4.47%, and RF = 0.65%) compared to those reported.10 Detailed crystal data and structural parameters, including selected interatomic distances, are summarized in Tables 1 and 2, respectively.

Figure 3. dc magnetic susceptibility curves for the samples synthesized at various temperatures (from 180 to 240 °C) in an applied magnetic field of 10 Oe in the ZFC mode. The inset shows the highest Tcmag of ∼30 K for the sample prepared at 220 °C.

versus transition temperature (K) curves for the products synthesized at various temperatures from 180 to 240 °C in the ZFC mode with H = 10 Oe. All of the samples showed a superconducting diamagnetic signal, and the onset Tcmag was observed between 24 and 30 K with a shielding volume fraction of 40−105% at 2 K (Table S1 in the SI). Among these, the sample synthesized at 220 °C exhibited a very clear diamagnetic (shielding volume fraction ∼98%) superconducting behavior with a relatively sharp onset Tcmag of ∼30 K (Figure 3, inset). Notably, the values of Tcmag and shielding volume fraction obtained are about 7.2 K and 34%, respectively, higher than those of our published Ba−K−Bi−Pb system.10 This large shielding volume fraction with higher Tc ensures the bulk superconducting nature of the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 compound. However, the FC data, which correspond to a Meissner volume fraction of approximately 15% at 2 K, is much lower than the ZFC data. This degradation might be a result of the vortex pinning in the FC mode measurement, which was also reported in other perovskite superconductors.7,8,10 The higher observed superconductivity (Tc and volume fraction) in this study is predicted owing to the closer ionic radii of Bi and Mg atoms at the B site than those of the reported10 Bi and Pb atoms, as well as the high threedimensional cubic symmetry of the superconducting system.14,18 Figure 4 shows the temperature dependence of the electrical resistivity ρ(T) for the highest Tcmag (∼30 K) sample, measured by fabricating a pellet from the sample powder using a cubic-anvil-type high-pressure system (6 GPa/RT). The resistivity curve reveals a semiconducting behavior in the temperature range 250−21 K, which dropped at ∼21 K (Tconset), attaining zero resistivity (Tc0) at a relatively low temperature of ∼7 K. The negative gradient of the resistivity and the broad transition between Tcmag and Tconset was attributed to grain boundaries and/or high pressure pressing and has been observed not only in our previous studies but also

Table 2. Refined Structural Parameters and Crystal Data for Cubic (Ba0.62K0.38)(Bi0.92Mg0.08)O3 Prepared Hydrothermally at 220 °Ca phase name and number scale factor chemical formula fw (g/mol) cryst syst space group (No. 221) sample color volume (Å3) calcd density (g/cm3) Z value lattice param (Å) bond distance Bi/Mg−O (Å) reliability factors Rwp (%) Rp (%) RB (%) RF (%) Sfit

structure and 1 2.98279 × 10−5 (Ba0.62K0.38)(Bi0.92Mg0.08)O3 342.21 cubic Pm3̅m black 78.33(4) 7.25 1 4.27864(2) 2.139(3) 6.11 4.47 1.16 0.65 4.61

a

Numbers in parentheses are the assumed standard deviations of the last significant digit.

3.2. Effect of the Reaction Temperature on the Superconductivity. In this study, we have investigated the influence of the hydrothermal reaction temperature on the superconducting properties of the perovskite bismuthates. It is established that when a superconductor is cooled gradually under a magnetic field, it behaves as an ideal diamagnet 3177

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exhibited slightly inferior diamagnetic behavior in comparison to the as-prepared sample. Upon heating to 600 °C, the sample showed a significant decrease in the diamagnetic signal and Tcmag and no longer exhibited superconductivity after heating at 700 °C. Thus, these results suggest that the simple cubic perovskite bismuthate with superconducting properties can be obtained only by a low-temperature hydrothermal method. However, this new compositional bismuth oxide superconductor is different from the reported simple perovskite compounds,56,57 which were prepared by solid-state reactions at approximately 800 °C and do not undergo phase transition and loss of superconductivity. 3.4. Electronic Band Structure. Figure 5 shows the calculated electronic band structure and the atom-projected

Figure 4. Temperature dependence of the electrical resistivity ρ(T) of the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 sample pellet. The inset shows a Tconset of ∼21 K and a Tczero of ∼7 K on an expanded scale.

in some other superconductors.7,8,10,46−53 Notably, in this paper, the magnitudes of both Tconset (∼21 K) and Tc0 (∼7 K) of (Ba0.62K0.38)(Bi0.92Mg0.08)O3 cubic perovskite are higher than those in our previously reported (Ba0.82K0.18)(Bi0.53Pb0.47)O3 sample.10 However, the observed zero resistivity at very low temperature (∼7 K) compared to that at Tconset (∼21 K) might be interpreted in terms of the fraction of impending superconducting clusters in the synthesized product. This similar phenomenon was observed in our previous (Ba0.82K0.18)(Bi 0.53 Pb 0.47 )O 3 compound 1 0 and reported for the Bi2Sr2Ca1−xPrxCu2O (Bi2212 + Pr) polycrystalline material54 whose zero-resistance superconducting states relied on the fraction of superconducting clusters. It is also claimed that Tc0 near Tconset can only be obtained for samples exhibiting a large enough superconducting fraction with a very sharp transition. 54,55 Unfortunately, the compound (Ba 0.62 K 0.38 )(Bi0.92Mg0.08)O3 in this work showed a smeared transition and may have a small fraction of superconducting clusters that might be responsible for the relatively low value of Tczero. 3.3. Thermal Behavior. Figure S5 in the SI depicts the TGA curve of the sample prepared hydrothermally at 220 °C, showing a total mass loss of 2.6% from 300 to 800 °C. On the other hand, the reported sample10 synthesized at 240 °C reveals somewhat greater mass loss at ∼3.8% from 450 to 800 °C. This deviation found in the weight loss profiles may be a result of either oxygen deficiency or different chemical compositions in the structure. However, no significant mass loss was visualized below 300 °C. Therefore, it was assumed that the observed loss of mass was caused by the evolution of oxygen, accompanying the transformation of Bi5+ to Bi3+, which agrees with previous reports.7,8 Figure S6 in the SI shows the high-temperature SXRD patterns at ca. 100 K intervals for the (Ba0.62K0.38)(Bi0.92Mg0.08)O3 solid upon heating from room temperature to 1064 K. The top of the peaks linearly shift toward lower angle up to 679 K, indicating no phase transition. Nonetheless, the shift of the shoulder is more significant above 475 K, implying the possibility that other phase(s) with slightly different lattice parameters start(s) to decompose. Above 679 K, the clear decomposition of the simple perovskite structure was found. Figure S7 in the SI reveals the superconducting behavior of the solid related to this structural change due to decomposition at high temperatures. It is seen that the sample heated at 400 °C

Figure 5. Band structure and atom-projected DOS for (Ba0.62K0.38)(Bi0.92Mg0.08)O3with flat Bi/Mg 6s,6p/3s and O 2p bands at EF(dashed horizontal line) at point X. The total DOS is shown on the right side of the figure.

total DOS of (Ba0.62K0.38)(Bi0.92Mg0.08)O3. The energy band (red) with large dispersion crossing the Fermi level (EF) indicates the metallicity of the synthesized compound. The partial DOS (PDOS) confirms the strong hybridization between the s and p orbitals in the band crossing the Fermi level, as shown in Figure S8 in the SI. The O 2p and Bi/Mg 6s,6p/3s orbital electrons mainly contribute to the DOS at EF, and the O 2p orbital contribution is higher in comparison to other orbitals, in agreement with the characteristics of the reported PDOSs of simple and double perovskites.7,8,53,56 Thus, the conduction band crossing the Fermi level is formed from the O 2p and Bi/Mg 6s,6p/3s electrons. Importantly, the conduction band is very flat with the formation of a saddle point at X in the vicinity of EF, which is regarded as a favorable condition for enhanced pairing of electrons. A similar phenomenon was also reported for the BKBO cubic perovskite superconductor.58 However, in this study, the value of the total DOS and the number of bands crossing EF is very low compared to our reported Bi/Pb-based sample.10 This could be due to the substitution of Mg for Bi in the place of Pb. The higher Tc in this study could be expressed using the relationship derived from the BCS theory, Tc = 1.14θ exp[−1/D(EF) U], where θ is the Debye temperature, D(EF) is the DOS at the Fermi level, and U is an electron−phonon attractive interaction parameter. The criterion for Tc of either an element or a compound involves the electron density of orbitals D(EF) of one spin at EF and the electron lattice interaction U. Therefore, the calculated result for Tc based on D(EF) of the product supports the experimental data (Tc) of 3178

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Inorganic Chemistry Present Address

the present research. However, in this study, the superconductivity is also related to the Bi valence of the hydrothermally synthesized samples from 180 to 240 °C. Investigations revealed that the less than half-filled Bi 6s band at Bi4.39+, Bi4.41+, and Bi4.4+ confirmed the metallicity and generated maximum carriers for the previously reported superconductors.7,8,59,60 The measured Bi valence was 4.36+ in (Ba0.62K0.38)(Bi0.92Mg0.08)O3 for the generation of maximum carriers and confirmation of metallic characteristics that led to the highest Tcmag of about 30 K.

§

N.K.: Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae-cho, Kofu 400-8511, Japan. Author Contributions

N.K. conducted this research work in collaboration with M.A. and Y.K. M.H.K.R. prepared powder samples and performed laboratory XRD measurements. T.T., E.M., C.M., and Y.K. performed SXRD experiments. M.H.K.R., A.M., T.T., and N.K. performed chemical analysis. M.H.K.R., A.M., and N.K. performed structural refinements with help from E.M., C.M., and Y.K. Magnetic measurements were performed by M.H.K.R., A.K., and N.K. All of the authors discussed the results; M.H.K.R. wrote the manuscript with comments from the coauthors.

4. CONCLUSIONS In conclusion, a new compositional (Ba0.62K0.38)(Bi0.92Mg0.08)O3 simple perovskite bismuthate has been successfully synthesized under facile hydrothermal conditions at 220 °C. The influence of the reaction temperatures on the structure and properties was analyzed. The crystal structure of the compound prepared at 220 °C was investigated by Rietveld refinement of high-quality SXRD data and TEM analysis. Refinement of (Ba0.62K0.38)(Bi0.92Mg0.08)O3 based on the SXRD data revealed it to be an undistorted ideal simple cubic perovskite with mixed occupancy at both A and B sites that yielded a good fit and satisfactory structural parameters. The refined structure was also confirmed by the ED pattern. ICP analysis determined the compositional homogeneity at the nanoscale, and TEM and SEM analysis revealed the cube-shaped microstructures of the solids. The susceptibility and electrical resistivity measurements exhibited a large superconducting volume fraction with a maximum onset of Tcmag at ∼30 K and Tconset at ∼21 K, respectively. Zero resistivity was observed below ∼7 K. The thermal behavior related to mass loss and superconductivity was also revealed. In addition, the highest Tcmag was supported by the experimental bismuth valence ∼4.36, which is considered crucial for the appearance and control of the superconductivity. Furthermore, the calculated band structure confirmed the metallic behavior of the (Ba 0.62 K 0.38 )(Bi0.92Mg0.08)O3 simple cubic perovskite. In summary, to the best of our knowledge, this is the first example of hydrothermally synthesized high-Tc simple perovskite superconductor incorporating Mg in the Bi site. Therefore, on the basis of this work, new high-Tc superconductors can be envisaged and realized using other pentavalent bismuthates in the starting materials via hydrothermal reaction.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron experiments were performed at SPring-8 with approval of the Japan Synchrotron Radiation Research Institute (Proposals 2014A1008 and 2016B1163). This research work was partly supported by JSPS KAKENHI Grant 26420678.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01853. Laboratory XRD patterns, SXRD patterns, magnetic susceptibility curves, TGA curves, SEM and STEM images, and calculated PDOSs (PDF) X-ray crystallographic data of (Ba0.62K0.38)(Bi0.92Mg0.08)O3 (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-55-220-8615. Fax: +81-55-220-8270. ORCID

Nobuhiro Kumada: 0000-0002-0402-5809 Akira Miura: 0000-0003-0388-9696 3179

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