Masahiko Tanaka

ABO₃ perovskite materials with small cations at the A site, especially with ordered cation arrangements, have attracted a lot of interest because they show unusual physical properties and deviations from general perovskite tendencies. In this work, A-site-ordered quadruple perovskites, RMn₃Ni₂Mn₂O₁₂ with R = Nd, Sm, Gd, and Dy, were synthesized by a high-pressure, high-temperature method at about 6 GPa. Annealing at about 1500 K produced samples with additional (partial) B-site ordering of Ni²⁺ and Mn⁴⁺ cations, crystallizing in space group Pn–3. Annealing at about 1700 K produced samples with disordering of Ni²⁺ and Mn⁴⁺ cations, crystallizing in space group Im–3. However, magnetic properties were nearly identical for the Pn–3 and Im–3 modifications in comparison with ferromagnetic double perovskites R₂NiMnO₆, where the degree of Ni²⁺ and Mn⁴⁺ ordering has significant effects on magnetic properties. In RMn₃Ni₂Mn₂O₁₂, one magnetic transition was found at 26 K (for R = Nd), 23 K (for R = Sm), and 22 K (for R = Gd), and two transitions were found at 10 K and 36 K for R = Dy. Curie–Weiss temperatures were close to zero in all compounds, suggesting that antiferromagnetic and ferromagnetic interactions are of the same magnitude.

ABO₃ perovskite materials with small cations at the A site, especially those with ordered cation arrangements, have attracted a great deal of interest because they show unusual physical properties and deviations from the general characteristics of perovskites. In this work, perovskite solid solutions (Lu_0.5Mn_0.5)(Mn_1−xTi_x)O₃ with x = 0.25, 0.50, and 0.75 were synthesized by means of a high-pressure, high-temperature method at approximately 6 GPa and approximately 1550 K. All the samples crystallize in the GdFeO₃-type perovskite structure (space group Pnma) and have random distributions of the small Lu³⁺ and Mn²⁺ cations at the A site and Mn^4+/3+/2+ and Ti⁴⁺ cations at the B site, as determined by Rietveld analysis of high-quality synchrotron X-ray powder diffraction data. Lattice parameters are a = 5.4431 Å, b = 7.4358 Å, c = 5.1872 Å (for x = 0.25); a = 5.4872 Å, b = 7.4863 Å, c = 5.2027 Å (for x = 0.50); and a = 5.4772 Å, b = 7.6027 Å, c = 5.2340 Å (for x = 0.75). Despite a significant dilution of the A and B sublattices by non-magnetic Ti⁴⁺ cations, the x = 0.25 and 0.50 samples show long-range ferrimagnetic order below T_C = 89 K and 36 K, respectively. Mn cations at both A and B sublattices are involved in the long-range magnetic order. The x = 0.75 sample shows a spin-glass transition at T_SG = 6 K and a large frustration index of approximately 22. A temperature-independent dielectric constant was observed for x = 0.50 (approximately 32 between 5 and 150 K) and for x = 0.75 (approximately 50 between 5 and 250 K).

There is an emerging topic in the science of perovskite materials: A‐site columnar‐ordered A₂A′A′′B₄O₁₂ quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A′ and A′′ cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy₂CuMnMn₄O₁₂ (1) and Ho₂MnGaMn₄O₁₂ (2) by neutron and X‐ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho₂]_A[Mn]_A′[Ga_0.66Mn_0.34]_A′′[Mn_3.66Ga_0.34]_BO₁₂ and [Dy₂]_A[Cu_0.73Mn_0.27]_A′[Mn_0.80Dy_0.20]_A′′[Mn_1.89Cu_0.11]_B1[Mn₂]_B2O₁₂. There are clear signatures of Jahn‐Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar‐ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long‐range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K.

The structural phase transitions of Rb₂MoO₄ and Rb₂WO₄ have been reinvestigated with the use of heat capacity measurement and synchrotron X-ray and neutron scattering techniques. In Rb₂MoO₄, the existence of polymorphous structures has been confirmed. These crystal structures at 290 K are a monoclinic β-K₂MoO₄ type one and an orthorhombic β-K₂SO₄ type one. In an annealed sample, two typical first-order-type anomalies were observed at 783 and 752 K. Furthermore, new λ-type anomaly equivalent to the second normal-incommensurate phase transition in A₂BO₄-type ferroelectrics was observed at 223 K. On the other hand, the crystal structure at 290 K of Rb₂WO₄ was a monoclinic β-K₂MoO₄ type one and no phase transition was detected down to 8 K.