In solid-state physics, foreign atoms (also foreign admixtures ) are individual atoms of another chemical element in the crystal lattice of the host crystal. Impurities belonging to the group of the point defects , and may, depending on their size and the thermal equilibrium either as substitutional impurity atoms ( substitution atom ) on the regular lattice sites, or as interstitial impurities ( interstitial atoms occupy interstitial sites).
Foreign atoms can strongly influence and change the chemical and physical properties of the original crystal. They are therefore often added to a host element or a host compound with this intention, but can also get into it unintentionally, such as, for example , sulfur , which is considered a reduction pest when iron is smelted .
Well-known examples of the targeted use of foreign atoms for technical applications are the changes in the electrical properties of semiconductor crystals , the so-called doping or the refinement of metals during alloying .
Many well-known examples of the accidental storage of foreign atoms can be found in the case of minerals , where these admixtures often ensure a wide variety of colors in otherwise colorless minerals such as B. fluorite (CaF 2 ) or corundum (Al 2 O 3 ), whose precious stone variants ruby and sapphire get their coveted red or bluish color through the addition of chromium or iron, titanium and vanadium.