Overlapping growth windows to build complex oxide superlattices

Perovskite oxide superlattices are of particular interest due to novel phenomena emerging at interfaces which are beyond the bulk properties of the constituent layers. However, building perovskite superlattices comprised of stoichiometric layers with sharp interfaces has proven challenging. Here, the synthesis of a series of high quality (SrTiO3)n/(CaTiO3)n superlattice structures grown on LSAT substrates is demonstrated by employing hybrid molecular beam epitaxy, where Ti was supplied using metal-organic titanium tetraisopropoxide (TTIP), and Sr and Ca were supplied using conventional effusion cells. By careful adjustment of the cation fluxes of Sr and Ca with respect to the TTIP flux, the growth windows of SrTiO3 and CaTiO3 were overlapped, allowing us to grow the individual superlattice layers with self-regulated stoichiometry. Stable and repeatable reflection high-energy electron diffraction oscillations during the entire ∼2.5 h growth period indicated good source flux stability. The structural quality of the superlattice films were determined by scanning transmission electron microscopy and synchrotron-based X-ray diffraction, revealing periodic, phase pure, homogenous superlattice structures with abrupt interfaces. Utilization of perovskite stoichiometric growth windows offers great potential for accessing and realizing interface driven phenomena in versatile perovskite superlattice materials with chemistries beyond titanates

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 CaTiO3, 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 TC = 350 K in these powders. The same polar phase is also seen as a surface layer in bulk CaTiO3 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 perovskites