Bulk Amorphous Alumina: The Density-Driven Interplay of Pentahedral Pyramids and Octahedra for High Dielectric Permittivity

The synthesis of single-component oxide glasses, represented by Al2O3 (alumina), is an unexplored area because they have no glass-forming ability. We synthesized a transparent bulk amorphous alumina with excellent thermal conductivity, hardness, and dielectric permittivity by applying high pressure to an amorphous alumina porous thin film. We found that a pyramidal 5-fold polyhedron (AlO5), which resembles a deformed octahedron without one oxygen vertex, is a principal structural unit in the amorphous alumina. The application of high pressure induces the formation of an edge-sharing matrix composed of further-deformed AlO5 pyramids and increased AlO6 octahedra, resulting in the improvement of the dielectric permittivity exceeding α-Al2O3. Our newly proposed synthesis method, which achieves an anomalous matrix formed by the density-driven interplay of pyramids and octahedra, has the potential to pave the way for the creation of a wide range of novel bulk amorphous materials without glass-forming ability, exhibiting excellent physicochemical properties.