Bi0.5Sr0.5-xCaxMnO3 phase diagram:: a discontinuous evolution

The structural phase diagram of the system Bi0.5Sr0.5-xCaxMnO3 has been established using magnetic and transport measurements and electron diffraction versus temperature. Two domains separated by a biphasic region are evidenced. Both monophasic are antiferromagnetic below T-N approximate to 140 K and paramagnetic above this temperature, and exhibit a charge-ordered (CO) state, with T-CO > T-N, characterized by the supercell "a approximate to 2a(proot)2, b approximate to 2a(p), c approximate to a(p)root2" (a(p) is the ideal perovskite parameter). But. in spite of their close relationships, these two types of manganites are different. The Sr-rich manganites (0 less than or equal to x less than or equal to 0.20) belong to the Imma subgroup, exhibit high T-CO values ranging from 520 to 460 K, T-CO decreasing with the size of the A cations. In contrast, the Ca-rich manganites (0.30 less than or equal to x less than or equal to 0.50) belong to the Puma subgroup, with significantly smaller T-CO values (300 to 340 K), T-CO increasing as the size of the A-site cation decreases. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.