Raman scattering inKOs2O6

Abstract

$\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ with a $\ensuremath{\beta}$-pyrochlore structure undergoes a superconducting transition at ${T}_{c}=9.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is the highest temperature among a series of compounds, $A{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ ($A=\mathrm{K}$, Rb, and Cs). In this structure, it is expected that vibrations of the atoms have a large amplitude and a large anharmonicity because an alkali atom is weakly bounded in a large space. On the other hand, $\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ shows another transition at ${T}_{p}=7.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is considered as a structural transition. Moreover, two structures have been reported at room temperature. To investigate structures and anharmonicity of vibrations, Raman scattering spectra of $\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ have been measured from $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to room temperature. Six Raman-active modes are successfully assigned. The assignment concludes that the symmetry at room temperature is $Fd\overline{3}m$, not $F\overline{4}3m$. A vibration of potassium at $70\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ with a ${T}_{2g}$ symmetry clearly shows a large anharmonicity. Moreover, a similar large anharmonicity has also been observed for the oxygen cage modes with the symmetries of ${E}_{g}$ at $260\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and ${T}_{2g}$ at $240\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The electron-lattice interaction is estimated by the linewidth. The strongest interaction is that of the ${T}_{2g}$ mode at $710\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which modifies the bond length between osmium and oxygen, and the next one is ${E}_{g}$. The lack of phonon anomaly at ${T}_{c}$ and ${T}_{p}$ suggests that the ${E}_{u}$ mode seems to be important for ${T}_{c}$ and ${T}_{p}$ since this ${E}_{u}$ mode has the low-energy and strong electron-lattice interactions, judging from the similar displacement with the ${E}_{g}$ mode.