  We report on TmMnO3 far infrared emissivity and reflectivity spectra from
1910 K to 4 K. At the highest temperature the number of infrared bands is lower
than that predicted for centrosymmetric P63/mmc (D6h4) (Z=2) space group due
high temperature anharmonicity and possible defect induced bitetrahedra
misalignments. On cooling, at ~1600 K TmMnO3 goes from non-polar to an
antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at
the ~700 K. The 300 K reflectivity is fitted using 19 oscillators and this
number of phonons is maintained down to 4 K. A weak phonon anomaly in the band
profile at 217 cm-1 (4 K) suggests subtle Rare Earth magnetoelectric couplings
at ~TN and below. A low energy collective excitation is identified as a THz
instability associated with room temperature eg electrons in a d-orbital
fluctuating environment. It condenses into two modes that emerge pinned to the
E-type antiferromagmetic order hardening simultaneously down to 4 K. They obey
power laws with TN as the critical temperature and match known zone center
magnons. The one peaking at 26 cm-1, with critical exponent \b{eta}=0.42 as for
antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth.
The band at ~50 cm-1, with \b{eta}=0.25, splits at ~TN into two peaks. The
weaker band of the two is assimilated to the upper branch of gap opening in the
transverse acoustical (TA) phonon branch crossing the magnetic dispersion found
in YMnO3. (Petit et al, 2007 PRL 99, 266604). The stronger second at ~36 cm-1
corresponds to the lower branch of the TA gap. We assign both excitations as
zone center magnetoelectric hybrid quasiparticles concluding that in NdMnO3
perovskite the equivalent picture corresponds to an instability which may be
driven by an external field to transform NdMnO3 into a multiferroic compound by
perturbation enhancing the TA phonon-magnetic correlation.

Strongly Correlated Electrons;Materials Science