Divalent element oxides

Putile Ceramic Pigments. StmcturaHy, aH mtile pigments are derived from the most stable titanium dioxide stmcture, ie, mtile. The crystal stmcture of mtile is very common for AX2-type compounds such as the oxides of four valent metals, eg, Ti, V, Nb, Mo, W, Mn, Ru, Ge, Sn, Pb, and Te as weH as haHdes of divalent elements, eg, fluorides of Mg, Mn, Fe, Co, Ni, and Zn. 

The rare earth metals. The rare earth metals are extremely reactive elements especially with respect to the normal atmospheric gases. The light trivalent lanthanides oxidize with air at room temperature they should be stored (and handled) in vacuum or under He or Ar. Divalent Eu oxidizes much more readily than any of... 

Retention in the Hostrock. For granite, apparent surface to mass ratios of about 3 m /kg for Cs, 2 m /kg for divalent elements and <10 m /kg for tri- and tetravalent elements have been estimated, based on Kd/Ka-measurements on Cs, Sr and Am on granitic macro surfaces (1,31). Retention factors are given in Table IX, as generated from measured K.-values (from Table V), assuming long contact time, non-oxidizing conditions and representative fracture and permeability data for the rock (p. 

General Properties of Compounds of Cobalt.—Cobalt, in its salts, behaves both as a trivalent and a divalent element, although the trivalent salts are unstable except in the case of double salts. The colour of cobalt salts varies greatly according to the degree of hydration, but it is usually pink or blue. Cobalt salts absorb nitric oxide,1 and with ammonia readily yield complex ammino derivatives. 

General Properties of Compounds of Nickel.—Nickel usually behaves as a divalent element. Its salts when anhydrous are yellow in colour. They can usually be obtained in various stages of hydration, in which condition they axe usually green, and frequently yield series of double salts, analogous in physical properties and in chemical constitution to many double salts obtainable with divalent iron. Like copper salts, those of nickel readily unite with ammonia. Like ferrous salts they absorb nitric oxide, but without change of colour.1... 

In the A,M02 layer oxides M is a transition metal with two oxidation states or a mixture of tetravalent and trivalent (or divalent) elements. A wide range of composition, as observed in the case of chalcogenides (0 < x < 1) has not been found. The higher ionic character of the bonds requires x > 0.5. 

Figure 10.6 schematically shows idealized lattice structures for continuous n-type and p-type oxide semiconductors containing point defects, such as interstitial cations, cation vacancies, and electron holes. For convenience, the metal is treated as a divalent element. The electron hole is a positive mobile electronic carrier in the valence band in an oxide crystal structure. 

Lead cations occur in both the divalent (+2) oxidation state, which is the most stable of the group IVB elements, and in the tetravalent (H-4) oxidation state. The divalent oxidation state usually dominates the inorganic chemistry of lead, while the tetravalent state dominates its organic chemistry. The coordination numbers of its divalent compounds range from 2 to 7, while those of its tetrahedral compounds range from 4 to 8. Its stereochemistry is usually octahedral or tetrahedral. 

Coprecipitation of ferric ions and divalent ions such as Mg, Cd, Zn and Pb does not lead to the spinel structure, but to M(0H)2 and the oxyhydroxide FeOOH [98]. The synthesis of the spinel requires heat treatment of the corresponding hydroxides or carbonates. Spinel feirites partially replaced with divalent elements are prepared indirectly by precipitation of Fe and as hydroxides, followed by oxidation of the suspension at 65 "C in air [99-103]. The stoichiometric phases are never obtained, however, except in the case of zinc. Precise conditions of acidity and composition (M /Fe ) must be chosen in order to prevent precipitation of hydroxides or basic salts of the divalent element and formation of the oxyhydroxide a-FeOOH. 

The difficulties in synthesizing the mixed spinel seem related to both thermodynamic and kinetic issues. Hydroxides (and/or basic salts) of divalent elements are more stable than the ferrite at low temperatures. The large reactivity difference between iron and chromium explains the rapid crystallization of iron oxides or oxyhydroxides compared with the chromium compounds, as well as the segregation of both elements. 

The inorganic chemistry of lead is dominated by the divalent (2" ) oxidation state rather than the tetravalent (4 ) oxidation state. The divalent state is more dominant in Group IV B elements as the atomic number increases. Dominance of the divalent state occurs because, within Group IV B, there is a decrease in single bond strength with increasing atomic number [13]. 

In our design, divalent Ca was chosen to partially substitute the trivalent atoms, and La and Ce were selected for a trivalent element because their ionic size (rLas+ = l.SOA rce3+=l-48A) was close to that of Ca (rca2+ = T48A) [21]. Like La, the Ce element also generally shows a formal -i-3 oxidation state in in-termetallics. Erom the reactions of the elements, we have identified as major phases the electron-precise/deficient alloys, Ln5.xCaxGe4 (Ln=La, Ce x=3.37,... 

---