Crystal Structure Phase Transitions in the Perovskites A(Cu 1− x Mn x )Mn 4 O 12 (A = Na + , Ca 2+ , Y 3+ , La 3+ )

X‐Ray diffraction and neutronographical studies on the crystal structure of the perovskitelike compounds A(Cu 1− x Mn x )Mn 4 O 12 have been undertaken. It has been shown that the samples with 50% of Mn 3+ in the octahedral sites have a cubic unit cell, while the samples with higher Mn 3+ composition may have a monoclinic or rhombohedral unit cell. The temperature increase raises the symmetry to the cubic one. The concentration ranges with different symmetry type have been determined. The lowering of the symmetry of the unit cell is due to the cooperative Jahn‐Teller effect.