alloying with an element which either donates extra electrons into the d band or causes a lattice expansion narrowing the d band. At the surface, we expect the 5d-band structure to be perturbed by virtue of the change in local symmetry and the reduction of the coordination number, z. It is not clear without detailed calculations which of these mechanisms is dominant. It is easy to see, however, that a narrowing of the d band by virtue of the reduced coordination number (bandwidth is assumed proportional to Jz) would raise the Fermi energy and begin to populate the 4/ 6 state* This in turn would cause an expansion of the lattice, resulting in further band narrowing. In this way a relatively small perturbation could result in an instability leading to the intermediatevalence state which is experimentally observed. More detailed theoretical treatment is clearly needed to provide an understanding of this surface-valence transition.It is natural to inquire whether this phenomenon is likely to occur in other elements. Among the rare-earth metals only thulium has a similar lowlying empty 4/ state, 8 * 9 but the excitation energy is ~ 1 eV, making it less likely to have a sponta-Theoretical treatment of Mossbauer hyperfine spectra in the presence of electronic relaxation characterizes, by a correlation time r c , the fluctuation of the electromagnetic field acting on the Mossbauer atom. These fluctuations then cause relaxation processes in the atoms electronic shell which are described by the relaxation time neous surface-valence transition. Other exame pies may be found among the intermetallic compounds of trivalent samarium and ytterbium.( a )Permanent address: Institut fur Festkorperforschung, to be published. r r . Most present theories 1 " 4 deal with the limiting case where T C is much shorter than either r r or the "precession time" r L of the nuclear spin in the hyperfine field (T L = H/A, with A = hyperfine coupling constant). Both conditions, are safely fulfilled in spin-lattice processes 1 where, for example, r L~H /k B Q (0=Debye temperature) andThe frequency dependence of the relaxation parameter which determines the line shape of the Mossbauer spectrum in spin-spin relaxation is derived directly from the experimental spectrum for the case of the 84-keV resonance of 170 Yb in Cs 2 NaYbCl 6 . The scale of the frequency variation of its relaxation parameter is in agreement with a theoretical estimate.816
The influence of low molecular weight cryoprotectants (CPs) such as glycerol (GL), 1,2-propanediol (PD) and dimethylsulfoxide (DMSO) on the structure of rat liver microsomal membranes on the stages of equilibration and upon freezing up to -196 degrees C was studied using a multiparametric fluorescent probe of flavonol nature. It was estimated that the studied CPs have individual concentration ranges defining low amplitude of their action on biomembranes. An exceeding of these ranges strongly increases the violation of membrane native structure already at the stage of incubation with CPs, strengthening it during the freezing procedure. According to the perturbation effect on microsomal membranes the studied CPs can be arranged in a sequence: DMSO>PD>GL.
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