Charge and orbital orders and structural instability in high-pressure quadruple perovskite CeCuMn6O12

High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb)

A-site-ordered quadruple perovskites RMn₇O₁₂ with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at T_N1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below T_N2 ≈ 12 K were observed in SmMn₇O₁₂ and EuMn₇O₁₂ at high magnetic fields. Very weak kinklike dielectric anomalies were observed at T_N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn₇O₁₂ shows signs of nonstoichiometry expressed as (Tb_{1- x}Mn _x)Mn₇O₁₂, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn₇O₁₂ and EuMn₇O₁₂ were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn₇O₁₂ and (Tb_0.88Mn_0.12)Mn₇O₁₂ were studied by synchrotron X-ray powder diffraction at 295 K.