Two Types of Charge Ordering in the Half-Doped Manganites of Bi0.5(Ca,Sr)0.5MnO3

From our study of the evolution of charge ordering in Bi0.5Ca0.5-xSrxMnO3 (x = 0-0.5) with chemical pressure, we Found the detailed nature of two distinct char-c ordering schemes, associated with the structural change from Pnma- to Imma-type symmetry at x = 0.2-0.25. Charge ordering for x = 0-0.2 (Pnma) is Consistent with the checkerboard-type pattern. commonly observed in half-doped manganites. On the other hand, a large tolerance factor for x = 0.25-0.5 (Imma) stabilizes a double-stripe charge ordering scheme. These findings indicate the bicritical nature of two competing charge ordered states and provide further insight towards understanding the dominant charge ordering mechanism in half-doped manganites.