Infrared absorption study of charge ordered $La{}_{0.5}Ca{}_{0.5-x}S{r}_{x}Mn{O}_3$ $(0.1\le x\le 0.5$

Infrared absorption study has been carried out on a series of half doped manganites, $L{a}_{0.5}C{a}_{0.5-x}S{r}_{x}Mn{O}_3$ $(0.1\le x\le 0.5)$, with varying magnetic ground state. The charge ordering transition observed in samples with {\normalsize $x\le 0.3$ is accompanied by a mode at $\sim 525c{m}^{-1}$ in addition to the stretching mode at $615c{m}^{-1}$ and bending mode at $400c{m}^{-1}$. Phonon hardening is found to occur below the CE - type antiferromagnetic ordering temperature. The value of the insulating gap decreases on doping with Sr from $727c{m}^{-1}$ to $615c{m}^{-1}\mathrm{.}$}