Hydrothermal Synthesis, Crystal Structure, and Superconductivity of a Double-Perovskite Bi Oxide

Double-perovskite
Bi oxides are a new series of superconducting
materials, and their crystal structure and superconducting properties
are under investigation. In this paper, we describe the synthesis
and characterization of a new double-perovskite material that has
an increased superconductive transition temperature of 31.5 K. The
structure of the material was examined using powder neutron diffraction
(ND), synchrotron X-ray diffraction (SXRD), and transmission electron
microscopy (TEM). Rietveld refinement of the sample based on ND and
SXRD data confirmed an A-site-ordered (K1.00)­(Ba1.00)3­(Bi0.89Na0.11)4O12 double-perovskite-type structure with the space
group Im3̅m (No. 229). This
structural analysis revealed the incorporation of Na with Bi in the
structure and a bent bond between (Na, Bi)–O–(Na, Bi).
TEM analyses also confirmed a cubic double-perovskite structure. This
hydrothermally synthesized compound exhibited a large shielding volume
fraction, exceeding 100%, with onset of superconductivity at ∼31.5
K. Its electrical resistivity dropped near onset at ∼28 K,
and zero resistivity was confirmed below 13 K. The calculated band
structure revealed that the metallicity of the compound and the flatness
of the conduction bands near the Fermi level (EF) are important for the appearance of superconductivity.