Body centered cubic

At 31OC, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865C it again transforms into a body-centered cubic structure. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235oG, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure. 

Mole fraction basis 7x Body-centered cubic bcc... 

Molybdenum hexafluoride [7783-77-9] MoF, is a volatile liquid at room temperature. It is very moisture sensitive, hydrolysing immediately upon contact with water to produce HF and molybdenum oxyfluorides. MoF should therefore be handled in a closed system or in a vacuum line located in a chemical hood. The crystals possess a body-centered cubic stmcture that changes to orthorhombic below —96° C (1,2). The known physical properties are Hsted in Table 1. 

Eig. 1. SoHd—Hquid phase diagram for ( ndashrule ) He and (—) He where bcc = body-centered cubic, fee = face-centered cubic, and hep = hexagonal close-packed (53). To convert MPa to psi, multiply by 145. 

Fig. 1. Iron—carbon phase diagram, where a is the body-centered cubic (bcc) a-iron, y is the face-centered cubic y-iron, and Fe C is iron carbide(3 l)...

A similar effect occurs in highly chiral nematic Hquid crystals. In a narrow temperature range (seldom wider than 1°C) between the chiral nematic phase and the isotropic Hquid phase, up to three phases are stable in which a cubic lattice of defects (where the director is not defined) exist in a compHcated, orientationaHy ordered twisted stmcture (11). Again, the introduction of these defects allows the bulk of the Hquid crystal to adopt a chiral stmcture which is energetically more favorable than both the chiral nematic and isotropic phases. The distance between defects is hundreds of nanometers, so these phases reflect light just as crystals reflect x-rays. They are called the blue phases because the first phases of this type observed reflected light in the blue part of the spectmm. The arrangement of defects possesses body-centered cubic symmetry for one blue phase, simple cubic symmetry for another blue phase, and seems to be amorphous for a third blue phase. 

Lithium magnesium alloys, developed during World War 11, have found uses in aerospace appHcations. Lithium alters the crystallization of the host magnesium from the normal hexagonal stmcture to the body-centered cubic stmcture, with resultant significant decreases in density and increases in ductibiHty. 

Iron (qv) exists in three aHotropic modifications, each of which is stable over a certain range of temperatures. When pure iron free2es at 1538°C, the body-centered cubic (bcc) 5-modification forms, and is stable to 1394°C. Between 1394 and 912°C, the face-centered cubic (fee) y-modification exists. At 912°C, bcc a-iron forms and prevails at all lower temperatures. These various aHotropic forms of iron have different capacities for dissolving carbon. y-Iron can contain up to 2% carbon, whereas a-iron can contain a maximum of only about 0.02% C. This difference in solubHity of carbon in iron is responsible for the unique heat-treating capabilities of steel The soHd solutions of carbon and other elements in y-iron and a-iron are caHed austenite and ferrite, respectively. 

Fig. 1. Schematic of the hysteresis loop associated with a shape-memory alloy transformation, where M. and Afp correspond to the martensite start and finish temperatures, respectively, and and correspond to the start and finish of the reverse transformation of martensite, respectively. The physical property can be volume, length, electrical resistance, etc. On cooling the body-centered cubic (bcc) austenite (parent) transforms to an ordered B2 or E)02...

Only body-centered cubic crystals, lattice constant 428.2 pm at 20°C, are reported for sodium (4). The atomic radius is 185 pm, the ionic radius 97 pm, and electronic configuration is lE2E2 3T (5). Physical properties of sodium are given ia Table 2. Greater detail and other properties are also available... 

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

Vanadium [7440-62-2] V, (at. no. 23, at. wt 50.942) is a member of Group 5 (VB) of the Periodic Table. It is a gray body-centered-cubic metal in the first transition series (electronic configuration When highly pure, it is very soft and dutile. Because of its high melting point, vanadium is referred to as a... 

Calcium has a face-centered cubic crystal stmcture (a = 0.5582 nm) at room temperature but transforms into a body-centered cubic (a = 0.4477 nm) form at 428 2° C (3). Some of the more important physical properties of calcium are given in Table 1. For additional physical properties, see references 7—12. Measurements of the physical properties of calcium are usually somewhat uncertain owing to the effects that small levels of impurities can exert. 

Solid carbon monoxide exists in one of two aUotropes, a body-centered cubic or a hexagonal stmcture. The body-centered stmcture converts into the hexagonal stmcture at 62 K with a heat of transition of 0.632 kj/mol (0.151 kcal/mol) (5). The melting point at atmospheric pressure is 68.1 K and... 

Fig. 2. Structures for the solid (a) fee Cco, (b) fee MCco, (c) fee M2C60 (d) fee MsCeo, (e) hypothetical bee Ceo, (0 bet M4C60, and two structures for MeCeo (g) bee MeCeo for (M= K, Rb, Cs), and (h) fee MeCeo which is appropriate for M = Na, using the notation of Ref [42]. The notation fee, bee, and bet refer, respectively, to face centered cubic, body centered cubic, and body centered tetragonal structures. The large spheres denote Ceo molecules and the small spheres denote alkali metal ions. For fee M3C60, which has four Ceo molecules per cubic unit cell, the M atoms can either be on octahedral or tetrahedral symmetry sites. Undoped solid Ceo also exhibits the fee crystal structure, but in this case all tetrahedral and octahedral sites are unoccupied. For (g) bcc MeCeo all the M atoms are on distorted tetrahedral sites. For (f) bet M4Ceo, the dopant is also found on distorted tetrahedral sites. For (c) pertaining to small alkali metal ions such as Na, only the tetrahedral sites are occupied. For (h) we see that four Na ions can occupy an octahedral site of this fee lattice.

Stainless and heat-resisting steels containing at least 18% by weight chromium and 8% nickel are in widespread use in industry. The structure of these steels is changed from magnetic body centered cubic or ferritic crystal structure to a nonmagnetic, face-centered cubic or austenitic crystal structure. 

FIG. 5 A crystalline structure. The particles are located near preferred lattice sites. A body-centered cubic (bcc) structure is shown. 

Three types of unit cells. In each case, there is an atom at each of the eight corners of the cube. In the body-centered cubic unit cell, there is an additional atom in the center of the cube. In the face-centered cubic unit cell, there is an atom in the center of each of the six faces. 

Body-centered cubic cell (BCC). This is a cube with atoms at each comer and one in the center of the cube. Here again, comer atoms do not touch each other. Instead, contact occurs along the body diagonal the atom at the center of the cube touches atoms at opposite comers. 

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