Crystal structure of the pyrochlore oxide superconductor KOs$_2$O$_6$

TL;DR

This study reveals the crystal structure of the superconductor KOs₂O₆, showing a unique cation position and significant atomic rattling, which may influence its superconducting properties.

Contribution

The paper provides the first detailed single-crystal X-ray structure of KOs₂O₆, highlighting its $eta$-pyrochlore structure and unusual K cation rattling behavior.

Findings

K atom located at 8b site, not 16d

Large atomic displacement parameter for K

K cation exhibits intensive rattling within the cage

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 $\beta$-pyrochlore structure with space group $Fd\bar{3}m$ and lattice constant $a$ = 10.089(2)~\AA at 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 $U_\mathrm{iso}$ = 0.0735(8)~\AA$^2$ 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 $\beta$-pyrochlore oxide superconductors AOs$_2$O$_6$ (A = Cs, Rb and K), and is greatest for the smallest K cation.