The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12

High-pressure synthesis of A-site ordered perovskite PbMn3(CrMn3)O12 with long-range antiferromagnetic ordering and a spin glass transition

An AA'3B4O12-type perovskite oxide PbMn3(CrMn3)O12 was synthesized by high-pressure solid-state reactions at 8 GPa and 1373 K. Synchrotron X-ray diffraction shows a cubic crystal structure with the space group Im3̄. The charge states are verified by X-ray photoelectron spectroscopy to be PbMn3+3(Cr3+Mn3+2Mn4+)O12, where the Pb2+ and Mn3+ are 1 : 3 ordered respectively at A and A' sites, while the Cr3+, Mn3+ and Mn4+ are disorderly distributed at the B site. PbMn3(CrMn3)O12 features a long-range antiferromagnetic order of A'-site Mn3+ spins at about 66 K and a subsequent spin glass transition around 36 K due to the randomly distributed Cr3+, Mn3+, and Mn4+ cations at the B site. This unique stepwise order of A' and B-site spins indicates weak A'-B site spin interactions, which are dominated by the difference in the B-site Mn3+/Ni2+ and Mn4+ number in the quadruple perovskites AMn3B4O12.

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

Dielectric and Spin-Glass Magnetic Properties of the A-Site Columnar-Ordered Quadruple Perovskite Sm2CuMn(MnTi3)O12

Perovskite-type ABO₃ oxides show a number of cation-ordered structures, which have significant effects on their properties. The rock-salt-type order is dominant for B cations, and the layered order for A cations. In this work, we prepared a new perovskite-type oxide, Sm₂CuMn(MnTi₃)O₁₂, with a rare columnar A-site order using a high-pressure, high-temperature method at about 6 GPa and about 1700 K. Its crystal structure was studied with synchrotron powder X-ray diffraction. The compound crystallizes in space group P4₂/nmc (No. 137) at room temperature with a = 7.53477 Å and c = 7.69788 Å. The magnetic properties of the compound were studied with dc and ac magnetic susceptibility measurements and specific heat. Spin-glass (SG) magnetic properties were found with T_SG = 7 K, while specific heat, in the form of C_p/T, showed a strong, very broad anomaly developing below 20 K and peaking at 4 K. The dielectric constant of Sm₂CuMn(MnTi₃)O₁₂ was nearly frequency and temperature independent between 8 K and 200 K, with a value of about 50. Cu²⁺ doping drastically modified the magnetic and dielectric properties of Sm₂CuMn(MnTi₃)O₁₂ in comparison with the parent compound Sm₂MnMn(MnTi₃)O₁₂, which showed a long-range ferrimagnetic order at 34-40 K. The antisite disorder of Cu²⁺ and Mn²⁺ cations between square-planar and octahedral sites was responsible for the SG magnetic properties of Sm₂CuMn(MnTi₃)O₁₂.

Triple A-Site Cation Ordering in the Ferrimagnetic Y2CuGaMn4O12 Perovskite

A new member of A-site columnar-ordered A₂A′A″B₄O₁₂ quadruple perovskites with the composition of Y₂CuGaMn₄O₁₂ was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y₂]_A[Cu_0.8Mn_0.2]_A′[Ga_0.8Mn_0.2]_A″[Mn_3.6Cu_0.2Ga_0.2]_BO₁₂. It has the space group P4₂/nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at T_C = 115 K with the ordered moment of 2.19 μ_B at each B site and 0.89 μ_B at the A′ or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A′ or A″ site are ordered in the opposite direction. Cu²⁺ doping drastically changes magnetic properties as “parent” Y₂MnGaMn₄O₁₂ just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y₂CuGaMn₄O₁₂: the lattice parameter a almost linearly decreases from 1.5 K to T_C and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

A new member of the A-site columnar-ordered quadruple perovskite A₂A′A″B₄O₁₂ family, Y₂CuGaMn₄O₁₂, was prepared at high pressure and high temperature, with triple A-site ordering and ferrimagnetic properties.