Emergent room temperature polar phase in CaTiO3 nanoparticles and single crystals

Polar instabilities are well known to be suppressed on scaling materials down to the nanoscale, when the electrostatic energy increase at surfaces exceeds lowering of the bulk polarization energy. Surprisingly, here we report an emergent low symmetry polar phase arising in nanoscale powders of CaTiO 3 , the original mineral named perovskite discovered in 1839 and considered nominally nonpolar at any finite temperature in the bulk. Using nonlinear optics and spectroscopy, X-ray diffraction, and microscopy studies, we discover a well-defined polar to non-polar transition at a T C = 350 K in these powders. The same polar phase is also seen as a surface layer in bulk CaTiO 3 single crystals, forming striking domains with in-plane polarization orientations. Density functional theory reveals that oxygen octahedral distortions in the surface layer lead to the stabilization of the observed monoclinic polar phase. These results reveal new ways of overcoming the scaling limits to polarization in perovskitesT.T.A.L., E.B., H.A., A.S.B., R.C.H., and V.G. gratefully Penn State MRSEC Center for Nanoscale Science Grant Nos. DMR-1420620 and DMR-1807768. V.G. acknowledges support from the Department of Energy Grant No. DE-SC0012375 for the X-ray diffraction microscopy work with Argonne National Laboratory. We gratefully thank Dr. A. N. Morozovska, Dr. E. A. Eliseev, and Professor J. M. Rondinelli for extensive theory discussions. M.O.R., C.H., P.M., and L.E.B. acknowledges funding from the Spanish Ministry of Economy and Competitiveness (MINECO) under Project No. MAT2016-76106-R and the Comunidad de Madrid Grant No. S2013/MIT-2740. L.E.B. and M.O.R. also acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through The “María de Maeztu” Programme for Units of Excellence in R&D (Grant No. MDM-2014-0377). U.A. and N.A.S. were financially supported by the ETH Zürich and by the ERC Advanced Grant program, No. 291151. Computer resources were provided by the ETH Zürich (Euler cluster). Assistance from SAMBA beamline staff at SOLEIL is acknowledge