Abstract
The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the {frac{3}{8}}^{th} doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn{}_{3}^{A^{prime} }Mn{}_{4}^{B}O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.
Highlights
The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR)
Insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level
A further key issue that has complicated the study of the phase diagram of the manganites is that the control parameter x does not act to change the hole (Mn4+) concentration, and leads to band narrowing due significant changes in MnO6 octahedral rotation and tilt angles caused by both varying ionic radii of dopants (e.g. Ca2+ and La3+) and that of Mn3+ and Mn4+ 8
Summary
The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR). Through the detailed crystallographic analysis of the phase diagram of a prototype system (AMnA3 0 MnB4 O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. The apparently intrinsic phase segregation in this regime of the OO phase diagram[5], make it hard to ascertain the microscopic structure of ttfoohrebxien=essus0lea.3ntitn4i,ag6l)l,OycOsoimnpsiihlsaatrisnetgoaorthof auatncadhtexxq==uer3812,b(owea.hgri.dcshMeehnma3s+ob:dMeeelnsn4p+arsoscpuhomasreegdde ordering and CE-type orbital order[7]. This basic pattern of ordering is inconsistent with average valence state (Mn3.375+). A further key issue that has complicated the study of the phase diagram of the manganites is that the control parameter x does not act to change the hole (Mn4+) concentration, and leads to band narrowing due significant changes in MnO6 octahedral rotation and tilt angles caused by both varying ionic radii of dopants (e.g. Ca2+ and La3+) and that of Mn3+ and Mn4+ 8
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