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 Bi_0.63Ca_0.37MnO₃, Bi_0.67Ca_0.33MnO₃ and Bi_0.71Ca_0.24MnO_3−δ. The anomalies in the χ⁻¹(T) curves of Bi_0.71Ca_0.24MnO_3−δ and Bi_0.63Ca_0.37MnO₃ appear to be associated to two different mechanisms, which are not observed in the intermediate Bi_0.67Ca_0.33MnO₃. 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. Bi_0.71Ca_0.24MnO_3−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 Bi_0.63Ca_0.37MnO₃.