Interstitial impurity

Figure 9.1 Schematic representation of magnetic defects in a nonmagnetic matrix (a) impurity substituent and (b) impurity interstitial.

Ce enters the CaF2 lattice substitutionally for the host Ca ions. Several site symmetries are possible depending upon the charge compensation. Non-locally compensated Ce + produces a center of 0 symmetry, whereas C4v symmetry results when F" ions in interstitial positions take up nn positions to the impurities along <110> directions. In addition many higher-order cluster configurations of these impurity-interstitial pairs exist (23). 

The high temperatures used for sintering also lead to a significant purification of the metal by volatilization of impurities. Interstitials, such as oxygen and carbon, cannot be removed completely and have to be held at a low level otherwise, they significantly affect... 

Refractory metals and alloys are generally dissolution-resistant in liquid metals and corrosion is often controlled by reactions with impurity/interstitial elements [13,14,27]. In the case of refractory metal alloys based on niobium or tantalum, the concentration of oxygen in the alloy is an important parameter with respect to corrosion in alkali metals, particularly lithium [34-38]. As little as 300 wppm of oxygen in niobium will induce catastrophic penetration of the niobium by lithium. Interstitial oxygen will also cause penetration of niobium tmd tantalum by sodium or potassium, but the threshold of oxygen concentration is higher. 

Figure 9.8 shows a two-dimensional representation of a crystal lattice with some common types of atomic point defects. A vacancy occurs when an atom is absent from a lattice site that is normally occupied. An interstitial occurs when an atom sits in a place in the crystal that is not a distinct lattice site, but rather in between lattice sites. Figure 9.8 shows two types of interstitials. A self-interstitial contains an atom of the same type that makes up the host crystal, while an impurity interstitial consists of a foreign atom. A substitutional impurity occurs when a foreign atom occupies a lattice site normally housed by a host atom. In compound solids, such as AB, we can have misplaced atoms, where species A sits in a B site or vice versa. 

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

The presence of defects and impurities is unavoidable. They are created during tire growtli or penetrate into tlie material during tlie processing. For example, in a crystal grown from tire melt, impurities come from tire cmcible and tire ambient, and are present in tire source material. Depending on factors such as tire pressure, tire pull rate and temperature gradients, tire crystal may be rich in vacancies or self-interstitials (and tlieir precipitates). 

A point defect refers to a localized defect (such as a monovacancy) or impurity (such as interstitial O). This includes any relaxation and/or distortion of the crystal around it. Many point defects are now ratlier well understood, especially in Si, tlranks to a combination of experiments providing infonnation of microscopic nature... 

In addition to the configuration, electronic stmcture and thennal stability of point defects, it is essential to know how they diffuse. A variety of mechanisms have been identified. The simplest one involves the diffusion of an impurity tlirough the interstitial sites. For example, copper in Si diffuses by hopping from one tetrahedral interstitial site to the next via a saddle point at the hexagonal interstitial site. 

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

The vacancy is very mobile in many semiconductors. In Si, its activation energy for diffusion ranges from 0.18 to 0.45 eV depending on its charge state, that is, on the position of the Fenni level. Wlrile the equilibrium concentration of vacancies is rather low, many processing steps inject vacancies into the bulk ion implantation, electron irradiation, etching, the deposition of some thin films on the surface, such as Al contacts or nitride layers etc. Such non-equilibrium situations can greatly affect the mobility of impurities as vacancies flood the sample and trap interstitials. 

Pig-iron or cast iron contains impurities, chiefly carbon (up to 5 ). free or combined as iron carbides. These impurities, some of which form interstitial compounds (p. I I3i with the iron, make it hard and brittle, and it melts fairly sharply at temperatures between 1400 and 1500 K pure iron becomes soft before it melts (at 1812 K). Hence cast iron cannot be forged or welded. 

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

Theoretical studies of diffusion aim to predict the distribution profile of an exposed substrate given the known process parameters of concentration, temperature, crystal orientation, dopant properties, etc. On an atomic level, diffusion of a dopant in a siUcon crystal is caused by the movement of the introduced element that is allowed by the available vacancies or defects in the crystal. Both host atoms and impurity atoms can enter vacancies. Movement of a host atom from one lattice site to a vacancy is called self-diffusion. The same movement by a dopant is called impurity diffusion. If an atom does not form a covalent bond with siUcon, the atom can occupy in interstitial site and then subsequently displace a lattice-site atom. This latter movement is beheved to be the dominant mechanism for diffusion of the common dopant atoms, P, B, As, and Sb (26). 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

Cr C Cr C chromium iton(l l) [12052-89-0] CrFe (c phase), and chromium iron molybdenum(12 36 10) [12053-58-6] Cr 2F 36 o Q phase), are found as constituents in many alloy steels Ct2Al23 and CoCr ate found in aluminum and cobalt-based alloys, respectively. The chromium-rich interstitial compounds, Ci2H, chromium nitrogen(2 l) [12053-27-9] Ct2N, and important role in the effect of trace impurities on the... 

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