Crystal structure of the pyrochlore oxide superconductor KOs 2 O 6
Jun-Ichi Yamaura, Shigeki Yonezawa, Yuji Muraoka, Zenji Hiroi
Abstract
We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs
2
O
6
. The structure was identified as the
β
-pyrochlore structure with space group
Fd
3
¯
m
and lattice constant
a
= 10.089(2)~Åat 300 K: the K atom is located at the 8
b
site, not at the 16
d
site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter
U
iso
= 0.0735(8)~Å
2
at 300 K for the K cation, which suggests that the K cation weakly bound to an oversized Os
12
O
18
cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of
β
-pyrochlore oxide superconductors AOs
2
O
6
(A = Cs, Rb and K), and is greatest for the smallest K cation.