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1 Hydrothermal Synthesis, Structure, and Superconductivity of a simple cubic perovskite (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 with T c ~30 K Mirza H. K. Rubel 1,2, Takahiro Takei 2, Nobuhiro Kumada 2 *, M. Mozahar Ali 2, Akira Miura 3, Kiyoharu Tadanaga 3, Kengo Oka 4, Masaki Azuma 5, Eisuke Magome 6, Chikako Moriyoshi 6 and Yoshihiro Kuroiwa 6 1 Department of Materials Science and Engineering, University of Rajshahi, Rajshahi-6205, Bangladesh. 2 Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae-cho, Kofu , Japan. 3 Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo , Japan. 4 Faculty of Science, Chuo University, , , Kasuga, Bunkyo-ku, Tokyo, Japan. 5 Materials and Structural laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Kanagawa , Japan. 6 Department of Physical Science, Hiroshima University, Kagamiyama, Higashihiroshima, Hiroshima Japan. *Corresponding author: Nobuhiro Kumada. Fax: ; Tel: ; kumada@yamanashi.ac.jp S1

2 Figure S1. X-ray powder diffraction patterns of perovskite bismuthates hydrothermally prepared at various temperatures from 180 to 240 C using Ba/MgBi = 1.00 molar ratio. Asterisks denote the presence of impurities for the samples. Inset shows an enlarged view of 200 reflection for the sample synthesized at 220 C. S2

3 Figure S2. Electron diffraction pattern of crystalline sample (a) for simple cubic (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 along [001] zone axis and (b) bright-field image of the sample synthesized hydrothermally at 220 C. S3

4 Figure S3. SEM images of hydrothermally synthesized samples at (a) 130 C (MgBi 2 O 6 starting material)(b) 180 C (c) 200 C (d) 220 C and (e) 240 C. S4

5 Figure S4. (a) STEM image of (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 simple perovskite and (b f) EDS element maps correspond to Bi, Ba, K, Mg and O elements, respectively. S5

6 Figure S5. TG-curve of the samples prepared hydrothermally at 220 C showing corresponding mass loss under He flow. S6

7 Figure S6. Expanded high-temperature SXRD patterns for 200, 220 and 222 peaks of the sample prepared at 220 C. The measuring temperature range is from room temperature to 780 K. The peaks of the (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 sample from room temperature to 679 K are indexed as the cubic perovskite structure. S7

8 Figure S7. DC magnetic susceptibility (χ) curves for the sample synthesized at 220 C heated at various temperatures from room to 700 C measured under an external field of 10 Oe in ZFC mode. The inset shows the expanded view of the highest T c mag ~30 K for the asprepared sample. S8

9 Figure S8. The calculated partial DOS for (Ba 0.62 K 0.38 )(Bi 0.92 Mg 0.08 )O 3 cubic ( Pm3m) simple perovskite at ambient conditions. S9

10 Elemental analysis The chemical analysis results obtained from ICP detected the elements Bi, Ba, K, and Mg for all the samples. First, the amount of detected elements by ICP analysis was converted in weight percent. Later, the concentration of each element in molar ratio was calculated using the simple math from these weight percent data. In addition, iodometric titration measured the amount of Bi 5+ in the synthesized products. The oxidation state of Bi in the final products was calculated from ICP data (mol% of Bi) and iodometric titration results (mol% of Bi 5+ ) as described follows: Measured mol% of Bi 5+ was divided by total mol% of Bi. We assumed the residual for Bi 3+. Total average Bi valence in the products was determined by taking sum of average valence from Bi 5+ and Bi 3+. Table S1. Synthesis conditions, average bismuth valence and superconductivity of samples prepared at various hydrothermal reaction temperatures using same starting materials. Hydrothermal reaction temperatures Ba:K:Bi:Mg molar ratio in the final products Measured average bismuth valence Transition temperature, T c (K) Shielding volume fraction at 2 K (%) 180 C 0.59:0.42:0.85: C 0.61:0.40:0.89: C 0.62:0.38:0.90: C 0.63:0.41:0.92: S10