Charge- and Orbital-Order Transitions in the A-Site-Ordered Quadruple Perovskite NdCuMn6O12

AMn7O12 perovskites (with A = divalent elements) show complex structural and magnetic transitions including incommensurate orbital density waves and coupled/decoupled modulated spin helicity originating from charge-ordered Mn3+/Mn4+ cations with the 3:1 ratio at the B perovskite sites and unusual apically compressed Jahn–Teller distortions of MnO6 octahedra. The same Mn3+:Mn4+ ratio can be achieved in RCuMn6O12 compositions, where R is a trivalent rare-earth cation. Therefore, the comparison in behavior of AMn7O12 and RCuMn6O12 is of interest. In this work, the A-site-ordered quadruple perovskite NdCuMn6O12 was prepared by a high-pressure high-temperature method. Its structural properties were investigated by synchrotron powder X-ray diffraction between 100 K and 350 K and laboratory powder X-ray diffraction between 5 K and 300 K. It shows a first-order structural phase transition from Im-3 symmetry (at high temperatures) to R-3 symmetry near 292 K. The structural transition is accompanied by charge (Mn3+/Mn4+) and unusual orbital (on the Jahn–Teller active Mn3+ cations located in MnO6 octahedra) orders. However, no additional structural/orbital modulations were found at lower temperatures in comparison with AMn7O12. Magnetic properties were investigated by temperature- and field-dependent magnetization and specific heat measurements, where a ferrimagnetic transition was found near 120 K. In addition, low-temperature magnetic anomalies were observed near 20 K, probably originating from the Nd sublattice.