A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga₂O₃ epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they exhibited hexagonal P6₃mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga₂O₃ layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna2₁ space group symmetry, referred to as κ-Ga₂O₃. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P6₃mc space group symmetry usually referred to as ε-Ga₂O₃ in the current literature. The crystal structure of these Ga₂O₃ layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna2₁ space group and previous observations.

We report on the high-pressure synthesis of ${\text{BiMn}}_7{\text{O}}_{12}$, a manganite displaying a “quadruple-perovskite” structure. Structural characterization of single-crystal samples shows a distorted and asymmetrical coordination around the Bi atom due to presence of the $6{\text{s}}^2$ lone pair, resulting in noncentrosymmetric space group $Im$, leading to a permanent electrical dipole moment and ferroelectric properties. On the other hand, magnetic characterization reveals antiferromagnetic transitions in agreement with the isostructural compounds, and the dielectric constant shows anomalies matching the magnetic transition temperatures. ${\text{BiMn}}_7{\text{O}}_{12}$ is therefore a promising multiferroic material, with evidence of coupling between magnetic and dielectric properties.