Definition of a “material”
We generally define a “material” as a single-phase chemical compound, that is, a material that can be made homogeneously beginning from the atomic scale.
Composite materials with separate phases at a scale larger than the scale of a few nanometers would thus be defined as two separate materials. However, in hybrid organic-inorganic semiconductors, structural units can be large and the difference between a single-phase material and two separate materials in a composite can be somewhat undefined. Where necessary (e.g., for so called quasi-twodimensional hybrid perovskites or “2.5D” hybrid perovskites), well-defined “materials” that are nanostructured will be included in the database pragmatically.
Some materials may form in more than a single atomic structure (e.g., different crystalline polymorphs), depending on synthesis conditions and/or thermodynamic conditions. In these cases, structurally different and distinct phases may be identified as different compounds in the database.