Charge/Orbital Ordering Structure ofPr1−xCaxMnO3(x=3/8) Examined by Low-Temperature Transmission Electron Microscopy

The structural phase transition of ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ ( $x=3/8\mathrm{}$) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector ${\mathit{q}}_1=(0,1/2,0)$ appeared below 230 K, indicating formation of the $d_{{3x}^2{-r}^2}{/d}_{{3y}^2{-r}^2}$ type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector ${\mathit{q}}_2=(1/4,1/4,1/2)$ appeared, suggesting an additional ordering of excess $1/8{\mathrm{Mn}}^{3+}$, necessary due to the deviation of $x$ from $1/2$, with the occupation of the $d_{{3z}^2{-r}^2}$ type of $e_g$ orbital.