Most bertolides among crystalline compounds of non-molecular structure: oxides, chalcogenides and other binary compounds of ionic structure; ternary compounds such as oxide bronzes.
Bertolides are usually considered as solid solutions of the excess component (atoms, ions) in the basic, stoichiometric, substance. The values of their maximum solubility limit the region of homogeneity - the region of stable existence of the nonstoichiometric phase. The solubility of excess components in various compounds ranges from thousandths of a percent to several percent. Examples:
nonstoichiometric phase TiO is stable in the range of TiO1,25 — TiO0,65,
Oxygen deficiency in ZrO2 at 1200 ° C can reach a molar 14% (corresponding to the formula ZrO2 – x with 0 ≤ x <0.28),
near the melting point of PbO, the solubility of Pb in it does not exceed 10−2%, Oxygen - 10−3% (which corresponds to the formula PbO1-x from -0.001 <x <0.01).
Often non-stoichiometric are compounds in which the metal is in the intermediate stage of oxidation. Here, the formation of a solid solution is facilitated by the fact that part of the metal ions changes the degree of oxidation. For example, in the case of an excess of oxygen in titanium (II) oxide TiO1 + x part of the titanium ions increases the degree of oxidation.
Cationic defect in FeO crystal
The formation of bertolides is associated with the violation of the periodicity of the crystal lattice and the appearance of defects that determine the most important properties of bertolides - electrophysical, optical, magnetic and so on. Such defects are often the centers of color, the intensity of which is associated with the magnitude of the deviation from the stoichiometric composition. For example, violation of stoichiometry in the direction of excess metal turns colorless BaO crystals into blue, NaCl - into yellow, KCl - into purple. Sodium tungsten bronze as it decreases in sodium changes color from golden yellow (NaWO3) to dark blue-green (Na0.3WO3), passing through red and purple.
