Intrinsic electronic phase separation and competition between $G$-type, $C$-type and $CE$-type charge and orbital ordering modes in Hg${}_{1-x}$Na${}_x$Mn${}_3$Mn${}_4$O${}_{12}$

Abstract

The novel series of hole-doped quadruple manganite perovskites Hg${}_{1-x}$Na${}_x$Mn${}_3$Mn${}_4$O${}_{12}$ (HNMO) has been synthesized and its charge and orbital order behavior investigated through high-resolution synchrotron powder x-ray diffraction techniques. Through careful Rietveld refinements of structural models $via$ symmetry-motivated approaches, we show that the ground state of HNMO compositions adopts a polar $G$-type charge and orbital ordered state, which is rare in manganite perovskites, and is robust as a sole phase up to a critical doping level. Upon this critical doping, coincident with that in which colossal magnetoresistance (CMR) is maximal in canonical manganite perovskites, electronic phase separation occurs between $G$-type and orbital order with charge disorder-type states. The latter state has recently been identified in Ca${}_{1-x}$Na${}_x$Mn${}_3$Mn${}_4$O${}_{12}$ perovskites, and proposed to be the competing insulating state from which CMR phenomena emerges. We show the mechanism for the formation of the $G$-type state is due to charge transfer processes which may occur through a coupling of distortions involving structural and charge and orbital degrees of freedom, ultimately driving the polar ground state through an improper-like ferroelectric polarization mechanism. These results will act as an important recipe for designing novel ferroelectric-active materials, in addition to expanding the richness of charge and orbital ordered states in manganite perovskites.

11 pages, 8 figures