Ions forming solid solutions

Several isovalent ions form solid solutions with KTP (Table II), showing that this structure is relatively tolerant, with respect to isovalent impurities, as are the traditional nonlinear optical oxide crystal structures. But due to the relatively limited range of nonstoichiometry in KTP, aliovalent impurities, such as divalent Ba, Sr and Ca introduced through ion exchange in nitrate melts, which substitute on the K site, are incorporated at concentrations less than one mole percent.(36) Typical impurity concentrations present in flux and hydrothermally grown KTP are shown in Table ID. 

Table n. Ions Forming Solid Solutions in Several Nonlinear Optical Oxide Crystals... 

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates ate Hsted in Table 15. The most important apphcations of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba2Ti04, barium orthotitanate, titanates do not contain discrete TiO ions but ate mixed oxides. Ba2Ti04 has the P-K SO stmcture in which distorted tetrahedral TiO ions occur. 

Solid solutions will only form between ions with similar radii ( 15 %). Table 3.15 shows the radii in crystal lattices of divalent cations that might form solid solutions in soils. Hence, for example Mn +,Fe + and Cd + might be expected to form solid solutions in CaCOs, but Cu + and Zn + would not. However, soils do not necessarily behave the same as pure systems. Thus there is little evidence for strong association of Cd + or Pb + with calcite (CaCOs) in soil systems, despite having similar radii to Ca + (McBride, 1994). However Cd + and Pb + are both commonly associated with hydroxyapatite (Caio(P04)6(OH)2),... 

The thermodynamic incompatibility of many of the solid phases present with each other as well as their local environment, results in formation of secondary minerals. Although the secondary materials may comprise only a small volume fraction of the waste, they (1) tend to increase in amount with time, as weathering processes proceed, (2) typically form at grain surfaces and are thus physically liable to react with percolating gas or liquids, and (3) may exhibit sites suitable for sorption or crystallo-chemical incorporation of trace elements (see Donahoe, 2004). Frequently observed secondary minerals include jarosite and ettringite the former is known to sorb ions such as Mn and As, whereas ettringite can form solid solutions, in which SO4 is replaced by Cr04 (Kumarathasan et al. 1990). 

Ionic crystals can form solid solutions, too. KBr, in the presence of KG1, will form crystals, in which some of the Br ions are substituted by Cl ions. If KC1 is in excess, a KC1 crystal is formed with some of the chlorine ions replaced by bromine ions. Smaller ions, like F , or larger, like I"", do not replace Br ions in KBr in considerable quantities KF and KBr do not form solid solutions, unless at high temperature or between very narrow limits. In chloride bromide systems the composition of the solid solution may range from pure chloride to pure bromide. 

In Section 24 it was shown that, under favorable conditions, two oxides of the same metal, in different states of valency, may form solid solutions which have been described as compounds with variable composition. The stabilizing factor in this case is the increase in entropy, due to the random distribution of the two positive ions these systems, strictly speaking, are stable only at elevated temperature. The conditions may be such that two oxides form a real compound, because this process is connected with a decrease in energy. The compounds formed in this way have a stoichiometric composition, with two kinds of positive ions in fixed positions, so arranged that the energy of the system is minimal. A good example of a compound of this type is Fe304. 

According to V. M. Goldschmidt, two substances with the same basic formula and crystal structure form solid solutions in a concentration range that depends on the degree of similarity of their ionic radii. A large range of solid solutions may be expected if the radius of the larger ion does not exceed that of the smaller by more than 15% [3.73a],... 

Solid solutions can also be formed to a limited extent with substances that have a similar formula but a different crystal structure. One of the substances imposes its structure upon the other. In lattices which are stable even if some sites in the unit cell are unoccupied, substances with different formulae can form solid solutions (anomalous mixed crystals). The incorporated ions must be of similar size, but do not need to have the same valency because electroneutrality is ensured by the presence of a compensating valency elsewhere in the lattice. Changes in the relative concentrations in the solid solutions lead to continuous variation of their properties. 

The corundum structure is assumed by three oxides of primary importance in catalysis, namely, o -C.r2(b, w-AbCF, and a-FeiOy (which, inter alia, form solid solutions in the complete compositional range) (21, 27). The three-dimensional arrangement of the ions in this hexagonal structure consists of a hexagonal close-packed array of anions with the cations occupying two-thirds of the octahedral interstices. 

Although the first result is in line with those obtained for MgO-NiO, MgO-CoO, and ZnO-CoO solid solution, the second suggests that the chemistry of silica-supported ions has some unique characteristics that merit further consideration. The ability to form multicarbonyl species suggests a state of coordinative unsaturation higher than that observed for the same or similar ions in solid solutions and represents a strong indication that high... 

Alkali halides which contain ions whose sizes are not too different, can form solid solutions. McKinstry (26) prepared some of these solid solutions in the systems KBr-KCl, KBr-RbBr, RbBr-RbCl, and RbCl-KCl and derived the lattice parameters as functions of the temperature from their x-ray diffraction patterns. In all cases where he found substantial deviations from additivity the solid solutions had the higher expansivities. 

Isomorphous substances often crystallize together from a mixed solution to form solid solutions, single crystals containing both substances. In a solid solution the different ions (such as Mn++ and Ca+ +) are arranged at random in the positions occupied by one kind of ion alone in a pure substance. For example, solid solu tions of chrome alum, KCr(S04)2 12HoO, and ordinary alum, KA1(S04) 12H20,... 

Another type of solid-solution formation is encountered when two ions as a pair can replace two other ions in a crystal lattice. Grimm and Wagner pointed out that such a twofold replacement can occur if the two salts have the same type of chemical structure and crystallize in the same type of crystal with lattice dimensions not too dissimilar. An apt example is barium sulfate, which can form solid solutions... 

The highest order in macroscopic electrode design is represented by insertion electrodes, according to Fig. 3(b). Redox reactions occur in the solid phase. The ions as redox partners form solid solutions. This does not exclude the possibility that in porous insertion electrodes both mechanisms are operative (cf. [28]). 

Dolomites found in nature seldom have exact stoichiometric composition and are frequently structurally rich in calcium (protodolomite). Dolomite, as well as calcite, has a tendency to form solid solutions with many metal ions. Calcite has a tendency to accommodate Mg in its structure to form magnesian calcite. Kineticall>, the deposition of magnesian calcite may be more favorable than the deposition of dolomite. 

Properties Dense, silvery solid. D 19.0, mp 1132C, bp3818C, heat of fusion 4.7 kcal/mole, heat capacity 6.6 cal/mole/C. Strongly electropositive, ductile and malleable, poor conductor of electricity. Forms solid solutions (for nuclear reactors) with molybdenum, niobium, titanium, and zirconium. The metal reacts with nearly all nonmetals. It is attacked by water, acids, and peroxides, but is inert toward alkalies. Green tetravalent uranium and yellow uranyl ion (U()2") are the only species that are stable in solution. 

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