So far, it was believed that hexagonal (h-) InMnO3 exhibit
the same type of multiferroic order as the other compounds from the
h-RMnO3 family (R = Sc, Y, Dy - Lu), including, in
particular, a unit-cell-tripling improper ferroelectric order. Here we
present experimental evidence for the absence of ferroelectricity in
hexagonal InMnO3 based on three different techniques: x-ray
diffraction (XRD),
... [Show full abstract] piezoresponse force microscopy (PFM) and optical
second harmonic generation (SHG). XRD data are ambiguous because they
can be described likewise by the non-ferroelectric P3c structure and by
the ferroelectric P63cm structure present in the other
h-RMnO3 compounds. However, PFM at room temperature and SHG
measurements at low temperature uniquely reveal the absence of
ferroelectric order in InMnO3. We therefore propose that
InMnO3 exhibits antiferrodistortive, but non-ferroelectric
order according to the P3c symmetry. Density functional calculations
show that the relative energy between the P3c and P63cm
structures is determined by a competition between electrostatic and
covalency effects, with an absence of covalency favoring the
ferroelectric structure.