Lattice-form-dependent orbital shape and charge disproportionation in charge- and orbital-ordered manganites

Abstract

The orbital shapes and charge disproportionations at nominal Mn 3+ and Mn 4+ sites for the chargeand orbital-ordered phases have been studied on half-doped manganites Pr(Sr0.1Ca0.9)2Mn2O7 and Eu0.5Ca1.5MnO4 with double-layer and single-layer Mn-O networks, respectively, by means of xray structural analyses, in comparison with Pr0.5Ca0.5MnO3 with the pseudo cubic network. In a single-layer Eu0.5Ca1.5MnO4 system, the (y 2 −z 2 )/(z 2 −x 2 )-type orbital shape is observed, while the (3y 2 −r 2 )/(3x 2 −r 2 )-type orbital shape in a pseudo cubic Pr0.5Ca0.5MnO3 system. In a double-layer Pr(Sr0.1Ca0.9)2Mn2O7 system, the orbital shape is found to undergo a large change upon thermally induced rotation of orbital stripe. Furthermore, clear charge disproportionation is observed for the pseudo cubic and double-layer systems, while not in the single-layer system. These results indicate that the orbital shape and charge disproportionation are sensitive to the dimension of Mn-O network.

Microstructure of bilayer manganite PrCa2Mn2O7 showing charge/orbital ordering

“…The simulated HAADF-STEM image based on this model (as insetted in the right of Fig. 5(b)) shows a good agreement with the experimental results, in contrast to the disordered phase of Pr(Ca 0.9 Sr 0.1 ) 2 Mn 2 O 7 , 40 where both Ca and Pr atoms locate at the cages of the MnO 6 octahedra and also in the AO layers.…”

“…5(b) is characterized by whiter spots arranged in line (emphasized by arrows in the right) and located in the middle of the two adjacent dark lines (rock salt layers), implying that the heavy Pr atoms (the whiter spots) locate inbetween the bilayer MnO 6 layers, which is not concordant with the results by X-ray diffraction. 13,40 A structural model of PrCa 2 Mn 2 O 7 with Pr atoms only located in-between the bilayer MnO 6 layers is therefore proposed in Fig. 5(c).…”

Structural changes at the semiconductor-insulator phase transition in the single-layered perovskiteLa0.5Sr1.5MnO4

“…48 This description shares the theoretical results of Ferrari et al, 53 who proposed that the CO-OO ordering in La 0.5 Ca 0.5 MnO 3 is better described by a charge-density wave of oxygen holes. Not far from this vision, Okuyama et al 54 more recently reported on a possible correlation between the dimensionality of the lattice and the charge segregation in manganites, which they found to be far from ionic in all studied cases by an x-ray structural study. They conclude that single-layered (n = 1) systems should generally show a negligible charge disproportionation between Mn sites, notably smaller than for the nonlayered case (n = ∞).…”

Theory for charge and orbital density-wave states in manganite La0.5Sr1.5MnO4Yao1, Chen2, Gao3 et al. 2013Phys. Rev. B4010View full textAdd to dashboardCite

“…The ordered orbitals have been suggested to be d 3x 2 −z 2 and d 3y 2 −z 2 12,27,28 . Meanwhile, some x-ray scattering experiments 29,30 combined with local-density approximation including on-site Coulomb interactions (LDA+U) calculations 30 indicate that the orbital ordering is dominated by d x 2 −z 2 and d y 2 −z 2 , which is also supported by means of x-ray structural analyses 31 . To further examine this issue, we have performed the mean field calculations to examine the ordering between a general linear combination of d 3z 2 −r 2 and d x 2 −y 2 , and have found that the ordering between + and − has the lowest energy, which is also consistent with our RPA analysis.…”