Doped alkali halide crystals

The interpretation used to explain the absorption spectra of Tl(l) and Pb(ll) solid-state complexes can also be extended to aqueous solution spectra of Pb(ll) complexes (Fig. 5) (54—60, 62). Although aqueous lead halide absorption spectra reported by Fromherz et al. (54, 55) were not originally interpreted in the context of CT spectroscopy (56, 125), in retrospect it is clear that the bands reported are the same as those observed in doped alkali halide crystals, as are later spectra reported by Bendiab et al. (59, 60). For example, in solution a spectral shift to longer wavelengths is observed for increasing atomic number of the halogen ligand (54, 55), which is consistent with a CT process (126). 

It is supposed to originate from the dissociation of 84 . This dissociation is perhaps significant at high temperatures, like those of the synthesis of ultramarine pigments or those of doping of alkali halide crystals. 83 and 82 radicals have also been observed in the alkali halides doped with sulfur [30, 31]. Another radical anion polysulfide, 84 , has been identified, by EPR experiments, in solution in DMF, originating from the dissociation of 8g , which is the least reduced polysulfide in this solvent [32]. 

F -centers (i.e., two electrons trapped in the same anion vacancy) and M-centers (two electrons occupying two adjacent anion vacancies, i.e., two adjacent F-centers) can also be present. Symons [78] observed by electron spin resonance spectroscopy that in solid alkali halide crystals doped with metal the F-centers are the most stable, while F -centers and M-centers may be formed at higher concentrations of trapped electrons. Durham and Greenwood [79] proposed that the dissolved metal dissociated into metal cations and nearly free electrons scattered in the conduction band. 

A number of studies have involved the doping of simple inorganic systems, followed by bromine NMR to determine the gradient-elastic tensor of various alkali halide crystals, which provides another way for determining the EFG tensor at the halogen site. Pulsed double-resonance experiments have also been used to establish the presence of impurity ions in alkali halide crystals. ... 

Two groups have recently claimed to have prepared the radical H2S", which has two electrons more than NH2 or PH2 (14, 56). One group prepared their radical by photolysis of alkali halide crystals doped with HS" ions. At 20°K. only trapped hydrogen atoms were detected, but after annealing at 110°K. for a few seconds and recooling, spectra assigned to S and H2S" were obtained. The results for the latter species, summarized in Table VI, show indeed that one sulfur and two equivalent protons are present. Since the protons remain magnetically equivalent for all orientations, the molecule was taken to be linear. 

Figure 21 Potential energy diagram of the ground and the first excited electronic states of [Ag(CN)32 (eclipsed configuration) as plotted from extended Huckel calculations. The excimer [Ag(CN)32 corresponds to the potential minimum of the excited state. The optical transitions shown are (a) excimer emission, (b) solid state excitation and (c) dilute solution absorption. (Reproduced with permission from Omary MA and Patterson HH (1998) Luminescent homoatomic exciplexes in dicyanoargentate 0) ions doped in alkali halide crystals 1. Exciplex tuning by site-selective excitation. Journal of the American Chemical Society 120 7606-7706.

Treadwell and Huber reported the electrolytic reduction of aqueous K4[Fe(CN)s] in the presence of excess KCN yielding a colourless solution which reduces one mole of [Fe(CN)6] . This was interpreted as indicating the formation of an iron(I) cyano complex, although the possible formation of a hydrido species such as [Fe(CN)5H] has not been ruled out by subsequent chemical studies. The complex ions [Fe(CN)6H] " and [Fe(CN)5] have been observed in pulse radiolysis studies of aqueous hexacyanoferrate(II) solutions. y-Irradiation of single crystals of alkali halides doped with hexacyanoferrate(II) ions have yielded ESR spectra characteristic of a number of iron(I) species. - These include [Fe(CN)5r -, [Fe(CN)5(H20)] -, [Fe(CN)5Cl] -, [Fe(CN)4Cl2] ", [Fe(CN)5Br] and [Fe(CN)4Br2] . The spectra of the pentacyano species contain some evidence for a bent cyanide ligand. ... 

Two of the alkali halides have been used as a scintillator material, viz. Nal and Csl, both doped with Tl. Table 9.5 summarizes some of their properties. Also included are Csl Na and undoped Csl. The emission spectra of the TI+-doped crystals ate given in Fig. 9.9. 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

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