Structural phase transition at low temperature, corresponding to charge ordering in the CMR perovskites LN0.5A0.5MNO3

The magneto resistive perovskites Nd0.5Sr0.5MnO3 and Pr0.5Sr0.41Ca0.09MnO3 undergo a transition from anti ferromagnetic insulator to ferromagnetic metal as function of temperature. The room temperature phase is orthorhombic with the space group Imma and the cell parameters a ≈ √2ap, b ≈ 2ap, and c ≈ √2ap. A structural phase transition related to charge ordering accompanying the transition from ferromagnetic state to antiferromagnetic state has been evidenced by low temperature electron diffraction. This transition is reversible and a new superstructure, with a P-type orthorhombic cell and lattice parameters parameters a ≈ 2√2ap, b ≈ 2ap, and c ≈ √2ap, is formed.