Halides Mossbauer effect

Apart fiom ESCA which addresses some of these problems [18,19] Mossbauer effect spectroscopy (ME) is the method which may provide the most detailed information concerning the dopant and its interactions with the polymer matrix. Mossbauer spectroscopy has, however, one important limitation—the dopant species must contain a Mossbauer active nucleus. Although the Mossbauer effect can be observed in more than 40 elements, only in a few of them are ME studies carried out routinely. These are Fe, Sn, Sb and Fortunately metal halides and iodine are the most popular dopants for conjugated polymers. 

The preparation of polypyrrole doped with sulpho-nated metallocyanine anions and other complex anions gave rise to detailed Mossbauer effect studies in these systems. It should be stressed that Mossbauer spectroscopy of the above mentioned compounds is more complex than in the case of polypyrrole doped with metal halides or metal cyanides. 

Several authors have investigated the Mossbauer effect for Co-doped cobalt halides and sulphides.Charge states are usually attributed to Auger after-effects, which may or may not be extinguished in a time shorter than the nuclear lifetime. 

Raman and Mossbauer studies.97 Thus the compounds are partially oxidized and should be more correctly expressed as [M(DPG)2KIs)o.2 with the nickel in a formal oxidation state of 2.20.97 The electrical conductivity in the Ni atom chain direction is 10-2 fi 1 cm-1, 105 times that of the unoxidized parent compound.97 98 The temperature dependence of the conductivity indicates that the compound is a semiconductor with AE = 0.19+0.01 eV. Table 3 indicates that changing the halide has little effect on the conductivity but that the Ni complex is more conducting than the Pd analogue. 

Tin compounds have been most studied,173 especially those of the SnCl3 ion, by 119Sn nmr and Mossbauer spectra. The SnClj ligand has a high trans effect and trans influence but has a rather low nucleophilicity. Thus in contrast to N, P, and As donors, SnCl2 will not cleave halide bridges in compounds with, for example, Pt(ju-Cl)2Pt bridges. 

The tris-(2-aminomethylpyridine)iron(II) halides, [Fe(2-NH2pic)3]X2 (X=C1, Br, I), also show a spin crossover [5]. In this instance the halide anions have a very strong effect on the equilibrium, as shown by the magnetic moment versus temperature plot in Fig. 8.2. No interpretation can be formulated using a simple thermal excitation model. The chloride gives Mossbauer spectra indicating T2 at room temperature, Ai at 4-2 K, and both states in coexistence at 77 K, with no detectable interconversion within the observation time-scale. 

The effects of various organic solvents on the Mossbauer parameters are well illustrated by the studies by Vertes and Burger [Ve 72] on rapidly frozen non-aqueous solutions of antimony pentachloride and tin(IV) halides (Table 5.6). 

The Mossbauer parameters of the tin halides show that in these systems the correlation between the isomer shifts and the donicity values is by no means as clear as for antimony pentachloride. The isomer shifts of tin tetrachloride agree, within experimental error, in frozen dimethyl sulphoxide, dimethylformamide and tributyl phosphate solutions, and those of tin tetraiodide agree in dimethyl sulphoxide, dimethylformamide and ethanol solutions. The Gutmann donicities of these solvents lie in the range 20-30. For both tin compounds, a further decrease in the donicity of the solvent causes an increase in the isomer shift, which indicates an increase in the electron density at the tin nucleus. This effect appeared in acetonitrile solution with tin tetrachloride, and in tributyl phosphate and carbon tetrachloride solutions with tin tetraiodide. (It must be emphasized that the last two solvents have low relative permittivities, whereas that of acetonitrile is high.)... 

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