2. Heterogeneous materials

A material is inherently heterogeneous, and many of its properties derive from this fact:

• the existence of surfaces, external or internal (porosity): the layer of surface atoms reacts and balances with its environment, primarily during synthesis, and this disturbance extends over several atomic layers depending on the nature of the chemical bonds, gradients and external parameters (temperature, partial pressure);

• the existence of interfaces, such as grain boundaries in polycrystals, macles in single crystals, nanodomain walls in solid solutions, particularly ferro/piezoelectric compounds, etc. Even in reputedly "homogeneous" compounds, such as glasses and amorphous/polymers, inorganic or organic, strong local heterogeneities exist but on a much smaller scale, that of structural units, typically a few (tens...