Using first-principles calculations, we have investigated 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.

Improper ferroelectricity and multiferroism in 2H-BaMnO3. Julien Varignon and Philippe Ghosez, Phys. Rev. B 87, 140403(R) (2013).

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO3 and LaAlO3 have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO3 and LaAlO3, and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO3 layer is diluted with SrTiO3, and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO3 in this solid solution, and thereby also with the layer’s formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism. 

Tunable conductivity threshold at polar oxide interfaces, M.L. Reinle-Schmitt, C. Cancellieri, D. Li, D. Fontaine, M. Medarde, E. Pomjakushina, C.W. Schneider, S. Gariglio, Ph. Ghosez, J.-M. Triscone and P.R. Willmott, Nature Communications  3, 932 (2012) 

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