Binary oxides and hydroxides

The first reported Mossbauer spectrum of a-Fe203 was by Kistner and Sunyar [1], who thereby recorded the first chemical isomer shift and electric quadrupole hyperfine interactions to be observed by this technique. With a single-line source the room-temperature spectrum comprises six lines from a hyperfine field of 515 kG the chemical isomer shift (Table 10.1) is 

Tabk 10.1 Mossbauer parameters for binary iron oxides and hydroxides 

Experimentally, it is found that the zhi - i transitions do remain stationary while the other four do not, thereby confirming model (b) as the correct one. 

There is a small apparent drop in the quadrupole interaction above the Morin temperature which suggests a small departure from axial symmetry and an asymmetry parameter of rj 0 03 [e(r Tt /e T Tm) = 1-94, not 2 00 as predicted]. The quadrupole splitting increases above the Neel temperature due to a small contraction along the trigonal axis. The chemical isomer shift does not show any irregularities at the Morin transition (Fig. 10.3). 

A field of 9 kG for example lowers the transition temperature by 17°, the general pattern of behaviour being consistent with Dzyaloshinsky s thermodynamic theory of weak ferromagnetism. 

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The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

Clay minerals and their substituent oxides will be emphasized here. Gays are among the most important natural catalysts (63-64, 84-86). Zeolites, clays and a few substituent binary oxides of clays, namely Si02, Al CL, MgO, CaO, TiO, and Fe-oxides and hydroxides are among the best characterized mineral catalysts, in spite of... 

In addition to the estimated properties, we measured the thermochemistry of several important vapor species. These measurements were conducted in a Knudsen effusion cell using special line-of-sight vaporization under subambient pressures with flowing O2 and H2O vapor mixtures [4]. The gaseous species over silica [5], manganese oxide [6], lanthana, alumina, and palladium metal were detected and relative partial pressures measured as a function of temperature. These vapor pressure measurements were calibrated by using the known metal atom or binary metal oxide volatility as a calibration source. Oxide species concentrations were measured relative to that of a reference compound, e.g., metal atom. The identification of oxide and hydroxide compounds was facilitated by Ae technique of threshold electron ionization [7]. These data were then evaluated using estimated entropy functions and the third law temperatures. 

Table 10. Enthalpy change of the dissociation reactions for some molecules of binary alkali and alkali earth oxides and hydroxides...

Salts have particular hazards, depending on which salt family they belong to. Salts generally do not bum, but can be oxidizers and snpport combustion. Some salts are toxic and some may be water-reactive. Salt compounds can also be divided into families. There are six salt families that will be presented in this book binary, binary oxide, peroxide, hydroxide, oxysalts, and cyanide salts. 

Binary salts have varying hazards, one of which is toxicity. Some of the binary salts are highly toxic, such as sodium fluoride, calcium phosphide, and mercuric chloride. Cyanide salts are also highly toxic, such as sodium cyanide and potassium cyanide. The remaining salts, binary oxides, peroxide, hydroxides, and oxysalts are generally not considered toxic. 

Our binary oxide aerogels regardless of approach to preparation consisted of very highly dispersed amorphous aluminate and zirconate, gels with larger separated phases of counter-ion oxides and hydroxides after calcination. 

Alkali-metal-based oxides, such as alkali metal oxides, binary oxides and hydrotalcite-like compoimds (layered double hydroxides), are promising adsorbents for CO2 capture, owing to their relatively high adsorption capacity, low cost and wide availability. The microstructure of these oxides, such as their surface area, porosity, particle size and dispersion, determines the CO2 adsorption capacity and multi-cycle stability [28]. 

The next five chapters deal with deposition of specific groups of semiconductors. In Chapter 4, II-VI Semiconductors, all the sulphides, selenides, and (what little there is on) tellurides of cadmium (most of the chapter), zinc (a substantial part), and mercury (a small part). (Oxides are left to a later chapter.) This chapter is, understandably, a large one, due mainly to the large amount of work carried out on CdS and to a lesser extent on CdSe. Chapter 5, PbS and PbSe, provides a separate forum for PbS and PbSe, which provided much of the focus for CD in earlier years. The remaining sulphides and selenides are covered in Chapter 6, Other Sulphides and Selenides. There are many of these compounds, thus, this is a correspondingly large chapter. Chapter 7, Oxides and Other Semiconductors, is devoted mainly to oxides and some hydroxides, as well as to miscellaneous semiconductors that have only been scantily studied (elemental selenium and silver halides). These previous chapters have been limited to binary semiconductors, made up of two elements (with the exception of elemental Se). Chapter 8, Ternary Semiconductors, extends this list to semiconductors composed of three elements, whether two different metals (most of the studies) or two different chalcogens. 

The remaining studies reviewed in this section deal with binary oxides that contain titania. Shibata and Kiyoura (140) measured surface acidities by the n-butylamine titration method of the Ti02-Zr02 system as a function of composition and method of preparation. Samples were prepared by calcination of coprecipitated mixtures of titanium and zirconium hydroxides that were made by ammonia or urea addition. The products had... 

Alouche et al. studied the selective hydrogenation of rapeseed oil over reduced Ni-Ce oxides and the effects of aluminum incorporation to them. The binary Ce-Ni oxide presented a good selectivity in the partial hydrogenation, as studied in a flow system at temperatures of 190-250°C, but with a large 7JE isomerization. On the other hand, use of ternary Ce-Ni-Al oxides [e.g., Ce/Al = 1, Ni/(Ce + Al) = 5], prepared from the nitrates of cerium, nickel, and aluminum by coprecipitation using potassium hydroxide, allowed a decrease in the extent of the 7JE isomerization.100... 

This chapter is concerned with the thermal decompositions of oxides and peroxides. There are obviously very important connections with the reactions of hydroxides (Chapter 8) and so-called peroxysalts, which contain hydrogen peroxide of crystallization (included in Chapter 7 on hydrates). Hydrated oxides vary from compounds accurately represented by the stoichiometric formula M(OH) , to phases which contain discrete HjO molecules. The chemistry of oxides should also be considered in the context of the other binary compounds (e.g. hydrides, nitrides, carbides, sulphides etc.) dealt with in Chapter 10. 

Of particular interest was the way in which detailed information could be derived from voltammetric studies of clay minerals treated aerobically with iron (11) solutions. This caused the precipitation of a thin, active layer of iron (HI) oxi(hydroxides) which could later be used for the sorption of arsenate(V) ions [69]. By employing the voltammetry of immobilized microparticles, it was possible to distinguish different iron species, namely (i) ion-exchangeable, labile, or sorbed iron (HI) ions (ii) ferrihydrite or lepidocrocite and (iii) crystalline hematite or goethite. Cepria et al. subsequently employed the voltammetry of immobilized microparticles in the phase analysis of iron (III) oxides and oxi(hydroxides) in binary mixtures, as well as in cosmetic formulations [70]. 

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