X‐Ray Diffraction and Magnetic Studies of Solid Solutions Bi 1− x Ca x MnO 3

Solid solutions with the general formula Bi 1– x Ca x MnO 3 (0.2 ≦ x ≦ 1) are found to possess the perovskite structure. The phase diagram based on X‐ray diffraction studies involving cubic, orthorhombic and monoclinic modifications is constructed. It is concluded from conductivity and X‐ray data that the transition into the monoclinic II modification is connected with some type of electron ordering on manganese ions. The changes in the paramagnetic Curie temperature resulting from these structural transitions are discussed. The magnetic ordering occurring in the system at about 110 °K can be explained if Zener's double exchange interaction Mn 3+ ‐O 2− ‐Mn 4+ is assumed to occur only in modifications in which electron ordering does not exist. The antiferromagnetic exchange interaction Mn 4+ ‐O 2− ‐Mn 4+ and the weak exchange interaction Mn 3+ ‐O 2− ‐Mn 3+ (the latter being ferromagnetic at a > 3.8 Å or antiferromagnetic at a < 3.8 Å) are proved to be responsible for ferromagnetism in the range 0.2 ≦ x < 0.4, antiferromagnetism in the range 0.4 ≦ x ≦ 0.8 and weak ferromagnetism in the range 0.8 < x ≦ 1.