Five-Fold Ordering in High-Pressure Perovskites RMn3O6 (R = Gd-Tm and Y).

Abstract

Cation and anion ordering plays an important role in the properties of materials, in particular, in the properties of perovskite materials. Here we report on unusual 5-fold cation/charge ordering in high-pressure-synthesized (at 6 GPa and ∼1670 K) RMn3O6 perovskites with R = Gd-Tm and Y. R3+, Mn2+, and Mn3+ cations are ordered at the A site in two separate chains consisting of R3+ and alternating Mn2+ (in tetrahedral coordination) and Mn3+ (in square-planar coordination), while Mn3+ and mixed-valent Mn3+/Mn4+ are ordered at the B site in layers. The ordering can be represented as [R3+Mn2+0.5Mn3+0.5]A[Mn3+Mn3.5+]BO6. The triple cation ordering observed at the A site is very rare, and the layered double-B-site ordering is also scarce. RMn3O6 compounds crystallize in space group Pmmn with a = 7.2479(2) Å, b = 7.4525(3) Å, and c = 7.8022(2) Å for DyMn3O6 at 213 K, and they are structurally related to CaFeTi2O6. They are prone to nonstoichiometry, R1-δMn3O6-1.5δ, where δ = -0.071 to -0.059 for R = Gd, δ = 0 for R = Dy, δ = 0.05-0.1 for R = Ho and Y, and δ = 0.12 for R = Er and Tm. They show complex magnetic behaviors with several transition temperatures, and their magnetic properties are highly sensitive to the δ values.