  Using first-principles calculations, we study theoretically the stable 2H
hexagonal structure of BaMnO3. We show that from the stable high temperature
P63/mmc structure, the compound should exhibit an improper ferroelectric
structural phase transition to a P63cm ground state. Combined with its
antiferromagnetic properties, 2H-BaMnO3 is therefore expected to be
multiferroic at low temperature. The phase transition mechanism in BaMnO3
appears similar to what was reported in YMnO3 in spite of totally different
atomic arrangement, cation sizes and Mn valence state.
