Rise of A-site columnar-ordered A₂A′A′′B₄O₁₂ quadruple perovskites with intrinsic triple order

Abstract

A-site-ordered AA'BO quadruple perovskites (with twelve-fold coordinated A and square-planar coordinated A' sites) were discovered in 1967. Since then, there have been considerable research efforts to synthesize and characterize new members of such perovskites. These efforts have led to the discoveries of many interesting physical and chemical properties, such as inter-site charge transfer and disproportionation, giant dielectric constant, multiferroic properties, reentrant structural transitions and high catalytic activity. The first member of A-site columnar-ordered AA'A''BO quadruple perovskites (with ten-fold coordinated A, square-planar coordinated A' and tetrahedrally coordinated A'' sites), CaFeTiO, was discovered in 1995, and for 19 years it was the only representative of this family. In the last few years, AA'A''BO perovskites have experienced rapid growth. Herein, we present a brief overview of the recent developments in this field and highlight an under-investigated status and great potential of AA'A''BO, which can be prepared mainly at high pressure and high temperature. The presence of the A'' site gives an additional degree of freedom in designing such perovskites. The AA'A''BO perovskites are discussed in comparison with well-known AA'BO perovskites.

Intrinsic Triple Order in A‐site Columnar‐Ordered Quadruple Perovskites: Proof of Concept

“…As it was expected from the crystallographic considerations, Ga 3+ was not found at the square‐planar A′ site. Distribution of Ga 3+ occurs between the tetrahedral A′′ site and octahedral B site with the preferred occupation of the A′′ site (with about 2 : 1 (A′′ : B) ratio).…”

“…On the other hand, specific heat and magnetic measurements showed that Dy 2 CuMnMn 4 O 12 shows three long‐range magnetic order transitions at T N3 =21 K, T N2 =125 K, and T N1 =160 K (Figure ). The intrinsic nature of the three phase transitions is further supported by the fact that all R 2 CuMnMn 4 O 12 (R=Dy−Lu and Y) exhibit similar magnetic properties and systematic changes of phase transition temperatures …”

“…Synthesis conditions (such as, pressure, annealing temperature, and cooling rates) should, of course, have effects on the degree of cation ordering as was found in Ca 2 CuMnFe 2 Re 2 O 12 . However, the general tendency should be primarily determined by the cation nature, size, and coordination preferences …”

“…The a + a + a + (1 : 3) tilt system results in the A‐site‐ordered AA′ 3 B 4 O 12 quadruple perovskites with a twelve‐fold coordinated A site and a square‐planar coordinated A′ site ,. The a + a + c − (1 : 1 : 2) tilt system results in the so‐called A‐site columnar‐ordered A 2 A′A′′B 4 O 12 quadruple perovskites with a ten‐fold coordinated A site, a square‐planar coordinated A′ site, and a tetrahedrally coordinated A′′ site (Figures and S1) . However, in the majority of examples reported so far, A′ and A′′ cations are the same (A′=A′′), and the intrinsic triple order is hidden, for example, in CaFeTi 2 O 6 , CaMnTi 2 O 6 , RMnMnSbO 6 (R=La, Pr, Nd and Sm), RMnGaTiO 6 (R=Sm and Gd), RMnMn 2 O 6 ≡RMn 3 O 6 (R=Gd‐Tm and Y), CaMnFeReO 6 and CaMnMnReO 6 , where short formula look like double perovskites.…”

“…Dy 2 CuMnMn 4 O 12 and Ho 2 MnGaMn 4 O 12 were prepared by a high‐pressure high‐temperature method at 6 GPa and about 1550 K for 2 h in Au capsules (Ho 2 MnGaMn 4 O 12 ) and about 1670 K for 2 h in Pt capsules (Dy 2 CuMnMn 4 O 12 ) in a belt‐type high‐pressure apparatus. We emphasize that the high‐pressure high‐temperature method is an essential tool for the preparation of most A‐site‐ordered quadruple perovskites ,. Ho 2 MnGaMn 4 O 12 (space group P 4 2 / nmc , a =7.3630(1) Å and c =7.7517(1) Å at room temperature (RT)) contained about 2 wt.…”