Two types of charge ordering in the half-doped manganites of Bi _0.5(Ca,Sr) _0.5MnO ₃

From our study of the evolution of charge ordering in Bi _0.>5Ca _0.5-xSr _xMnO ₃(x = 0-0.5) with chemical pressure, we found the detailed nature of two distinct charge ordering schemes, associated with the structural change from Pnma- to Imma-type symmetry at × = 0.2-0.25. Charge ordering for × = 0-0.2 (Prima) is consistent with the checkerboard-type pattern, commonly observed in half-doped manganites. On the other hand, a large tolerance factor for × = 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.