The crystal structure of the prototypic ceramic superconductor BaPbO3: An X-ray and neutron diffraction study
Tóm tắt
The structure of the distorted perovskite BaPbO3 was studied with high-resolution X-ray diffraction at 300 K and 26 K and with neutron diffraction at 300 K. Simultaneous refinement of the neutron and X-ray data sets (300 K) using the Rietveld method yields a monoclinic structure with the space groupI 2/m and lattice parametersa=6.0278 (1) Å,b=6.0664(1) Å,c=8.5109(1) Å, and γ=90.083 (2)o. The tilting of the oxygen octahedra is given asa
−
a
−
c
0 in Glazer's notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110]
c
and
$$[1\bar 10]$$
c
. This is in contrast to the observations in BaBiO3. The structure of BaBiO3 has the same space groupI 2/m, the similar dimensions of the unit cell and the same tilt system, but β=∢ ([110]
c
, [001]
c
) as the monoclinic angle. As a consequence there is only one type and size of, PbO6 octahedra but two types of octahedra in BaBiO3. This fact may influence the occurrence of superconductivity in solid solutions (Ba(Pb1−x
Bi
x
)O3 containing a large fraction of lead by enhancing valence fluctuations.
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