Polar Oxides without Inversion Symmetry through Vacancy and Chemical Order.
One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegard's principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca2Fe2O5and Sr2Fe2O5. We then realize epitaxial (SrFeO2.5)1/(CaFeO2.5)1thin film superlattices possessing both anion-vacancy order and Sr and Ca chemical order at the subnanometer scale, confirmed through synchrotron-based diffraction and aberration corrected electron microscopy. Through a detailed symmetry analysis and density functional theory calculations, we show that A-site cation ordering lifts inversion symmetry in the superlattice and produces a polar compound. Our results demonstrate how control of anion and cation order at the nanoscale can be utilized to produce acentric structures markedly different than their constituents and open a path toward novel structure-based property design.
Department of Materials Science and Engineering, Drexel University , Philadelphia, Pennsylvania 19104, United States.|Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
Joshua
J
Young
Department of Materials Science and Engineering, Drexel University , Philadelphia, Pennsylvania 19104, United States.
Eun Ju
EJ
Moon
Department of Materials Science and Engineering, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Debangshu
D
Mukherjee
Department of Materials Science and Engineering, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Greg
G
Stone
Department of Materials Science and Engineering, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Venkatraman
V
Gopalan
Department of Materials Science and Engineering, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Nasim
N
Alem
Department of Materials Science and Engineering, Drexel University , Philadelphia, Pennsylvania 19104, United States.
Steven J
SJ
May
Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.|Materials Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
James M
0000-0003-0508-2175
JM
Rondinelli