Sodium trimer ordering on a NaxCoO2 surface.

摘要: Sodium ion ordering on an in situ cleaved ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$ ($x=0.84$) surface has been studied by ultrahigh vacuum scanning tunneling microscopy at room temperature. Three main phases, with $p(3\ifmmode\times\else\texttimes\fi{}3)$, ($\ensuremath{\surd}7\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}7$), and ($2\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}2\ensuremath{\surd}3$) hexagonal unit cells a Na concentration of $1/3$, $3/7$, $1/2$, respectively, were identified. One surprising finding is that trimers act as the basic building blocks order long range. The stability attributed to increased coordination oxygen indicated ab initio calculations, possibly finite temperature configuration entropy.