High‐pressure synthesis, crystal structure, and properties of a novel quadruple perovskite CeMn 3 Rh 4 O 12

A novel transition metal oxide CeMn 3 Rh 4 O 12 has been synthesized using high‐pressure and high‐temperature conditions of 12 GPa and 1573 K. CeMn 3 Rh 4 O 12 crystallizes in the AA' 3 B 4 O 12 ‐type quadruple perovskite structure with A ‐site ordering in a cubic space group of Im3 ¯ $\bar 3$ in the high‐temperature phase at above 310 K, whereas CeMn 3 Rh 4 O 12 undergoes a structural phase transition with symmetry lowering down to probable triclinic symmetry on cooling below this temperature. X‐ray absorption spectroscopy reveals that all the constituent ions are trivalent at room temperature, suggesting the valence state of Ce 3+ Mn 3+ 3 Rh 3+ 4 O 12 . Magnetization study demonstrates an antiferromagnetic transition attributed to A' ‐site Mn 3+ ions at T N  = 35 K, whereas a Curie–Weiss analysis finds an additional paramagnetic component attributed to the Ce 3+ 4f 1 ions. We discuss the differences in structural and magnetic properties between CeMn 3 Rh 4 O 12 and LaMn 3 Rh 4 O 12 , eventually proposing a possible stereochemical effect of Ce 3+ on crystal structure in the quadruple perovskite.