Reentrant Structural Transitions and Collapse of Charge and Orbital Orders in Quadruple Perovskites

Abstract

Charge and orbital degrees of freedom determine properties of many materials, and are central to many important phenomena. At high temperatures, thermal fluctuations overcome them, and high‐symmetry structures are realized. On decreasing temperature, different charge‐ and orbital‐order transitions take place accompanied by symmetry lowering. Remarkable exceptions to this general tendency, realized in Cu‐doped BiMn7O12 quadruple perovskites, are presented. Introduction of Cu2+ produces mixtures of Mn3+ and Mn4+ and charge degree of freedom. BiCuMn6O12 (and compositions in the vicinity) exhibits well‐defined 1:3 charge order of Mn4+ and Mn3+ and orbital order of Mn3+ near room temperature, but both charge and orbital orders collapse below about 115 K with the reentrance of the high‐temperature cubic Imtrue3‾ phase. What is interesting the collapse can be controlled by a magnetic field even without long‐range magnetic order, and the collapsed phase shows nearly zero thermal expansion.

Complex Structural Behavior of BiMn7O12 Quadruple Perovskite

“…A-site ordered quadruple perovskites, AA′ 3 B 4 O 12 , show many interesting physical and chemical properties: 1,2 for example, reentrant structural transitions, 3 intersite charge transfer and disproportionation, 2 giant dielectric constant, 4 multiferroic properties, 5 and high catalytic activity. 6 AA′ 3 B 4 O 12 has a 12fold-coordinated A site and a square-planar-coordinated A′ site, while B sites have the usual octahedral coordination for perovskites.…”

“…a Crystal data: space group Im (No. 8, unique axis b, cell choice 3), Z = 2; a = 7.5168(10) Å, b = 7.3707(6) Å, c = 7.5254(6) Å, β = 91.293(2)°, and V = 416.83(7) Å 3 ; R1(all) = 3.84%, wR2(all) = 8.80%, and goodness of fit 1.203; ρ cal = 6.259 g/cm3 .…”

Spin-Glass Magnetic Properties of A-Site Columnar-Ordered Quadruple Perovskites Y2MnGa(Mn4–xGax)O12 with 0 ≤ x ≤ 3

“…21 Doped AMn 7 O 12 manganites show re-entrant structural transitions and other interesting structural properties. 22 We recently found perovskite manganites with the A-site columnar-ordered quadruple perovskite structure having a general formula of A 2 A′A″B 4 O 12 23 and a 1:3 composition (or c l o s e t o 1 : 3 ) . T h e i r f o r m u l a i s R M n 3 O 6 (=A 2 Mn 3+ Mn 2+ Mn 4 O 12 ) with A = R = Gd−Tm and Y.…”

Emergent helical texture of electric dipoles

“…As is the case for other manganites (16,17), a small amount of hole doping achieved, for example, by the replacement of A site Mn 3+ with Cu 2+ , is expected to tune both instabilities because it affects both the JT ordering (doped holes are localized on B sites (18,19) such that Mn 3+ tends towards non JT active Mn 4+ ) and the immediate coordination of Bi 3+ . Similar to the undoped compound, BiCu0.1Mn6.9O12 is cubic at high temperatures (structural parameters for this phase as well as for the phases discussed below are summarized in tables S1 to S5 and figures S1 to S8 of (20)), and undergoes a JT order phase transition to the non-polar monoclinic structure with I2/m SG at a slightly lower TJT ~ 560 K (Fig.…”