Critical point of the competition between different orbital- and Bi lone pair-orderings in the Bi-rich part of the BiCaMnO system

The present paper deals with the evidence of an unprecedented ordering in the Bi-rich part of the BiCaMnO diagram, where the spin glass state is observed at low temperature. The χ−1(T) curves of three compounds, corresponding to different Ca/Bi+Ca ratio varying between 0.37 and 0.25, have been characterized and discussed in correlation with the structural transitions as a function of T. Three different behaviours have been observed for Bi0.63Ca0.37MnO3, Bi0.67Ca0.33MnO3 and Bi0.71Ca0.24MnO3−δ. The anomalies in the χ−1(T) curves of Bi0.71Ca0.24MnO3−δ and Bi0.63Ca0.37MnO3 appear to be associated to two different mechanisms, which are not observed in the intermediate Bi0.67Ca0.33MnO3. For the Bi-rich compound, an unprecedented [111]p-type ordering of the perovskite cell is reported, which is associated to a change of the slope in the magnetic curve. Bi0.71Ca0.24MnO3−δ appears at the convergence point of a complex competition between the complementary lone pair ordering/orbital ordering of the ferromagnetic BiMnO3 and the charge ordering/orbital ordering of the pseudo CE antiferro-magnetic Bi0.63Ca0.37MnO3.