Destabilization of the cooperative Jahn-Teller effect in Sm 0.2Ca 0.8MnO 3 by Ru-doping

The structural study of Sm _0.2Ca _0.8MnO ₃ and Sm _0.2Ca _0.8Mn _0.9Ru _0.1O ₃ versus temperature has been carried out by neutron diffraction and electron microscopy in connection with the magnetic and transport properties. A structural transition from a Pnma paramagnetic insulating structure to a P2 ₁/m C-type antiferromagnetic insulating state at T_N≈150 K is observed for the undoped phase. This low temperature monoclinic structure results from a cooperative Jahn-Teller (JT) distortion of manganese cations, hindering the appearance of magnetoresistance in Sm _0.2Ca _0.8MnO ₃. In contrast, the Ru-doped phase, which exhibits two magnetic transitions at T_C≈200 and T_N≈110 K, keeps the Pnma structure in the whole temperature range, from 4 to 300 K, showing that Ru-doping destabilizes the JT effect. The influence of ruthenium doping upon the structure and the appearance of ferromagnetism and metallicity in Sm _0.2Ca _0.8MnO ₃ is discussed by comparing its behavior with those of the x=0.90, 0.85, and 0.80 samples in the Sm _{1- x}Ca _xMnO ₃ series.