Observation of persistent centrosymmetricity in the hexagonal manganite family

The controversy regarding the ferroelectric behavior of hexagonal InMnO3 is resolved by using a combination of x-ray diffraction (XRD), piezoresponse force microscopy (PFM), second harmonic generation (SHG), and density functional theory (DFT). While XRD data show a symmetry-lowering unit-cell tripling, which is also found in the multiferroic hexagonal manganites of P6(3)cm symmetry, PFM and SHG do not detect ferroelectricity at ambient or low temperature, in striking contrast to the behavior in the multiferroic counterparts. We propose instead a centrosymmetric P (3) over barc phase as the ground-state structure. Our DFT calculations reveal that the relative energy of the ferroelectric and nonferroelectric structures is determined by a competition between electrostatics and oxygen-rare-earth covalency, with an absence of covalency favoring the ferroelectric phase.