Coexistence of `Charge Order' Modulation, Tweed Microstructure and Needle Twins in La1-xCaxMnO3 (x=0.67 and 0.71)

Using transmission electron microscopy, we have observed the coexistence of two non-ferromagnetic phases in La$_{1-x}$Ca$_x$MnO$_3$ ($x = 0.67$ and $0.71$) at 90 K: a modulated `charge ordered' phase and a modulation-free region composed of needle twins. Micron-sized regions containing these needle twins formed below 170~K over 30 seconds in La$_{0.33}$Ca$_{0.67}$MnO$_3$ when cooled at a rate of $\sim 0.2$~K/minute. A tweed microstructure was observed in the interface region between these two phases. We estimate that the needle twins occupied less than 10\% of the volume of the manganites investigated here.