Lattice, Mossbauer

Other fields benefited, too, from the strong neutron fluxes now accessible for irradiations. New phenomena were observed, such as the annealing of hot-atom effects in inorganic solids by postirradiation heating, leading to an increased retention in the form of the original species (Green and Maddock 1949 Rieder et al. 1950). A new and powerful tool was discovered by Rudolph Mossbauer in 1958 recoilless emission ofy rays with natural or nearly natural line width from nuclei embedded in a solid lattice (Mossbauer 1958, 1962). Resonant absorption of such emissions, e.g., the 14.4 keV transition of Fe fed in the decay of Co (T1/2 = 270 d), provides... 

For example, octahedral quadrupole splitting observed for the cis-octahedral analogs 7,8). More recently, temperature-dependent Mossbauer measurements have been used in conjunction with Raman spectroscopy to determine molecular weights 453) and lattice rigidity 460) of various organotin compounds. 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

The Mossbauer effect can only be detected in the solid state because the absorption and emission events must occur without energy losses due to recoil effects. The fraction of the absorption and emission events without exchange of recoil energy is called the recoilless fraction, f. It depends on temperature and on the energy of the lattice vibrations /is high for a rigid lattice, but low for surface atoms. 

The Bragg scattering of X-rays by a periodic lattice in contrast to a Mossbauer transition is a collective event which is short in time as compared to the typical lattice vibration frequencies. Therefore, the mean-square displacement (x ) in the Debye-Waller factor is obtained from the average over the ensemble, whereas (r4) in the Lamb-Mossbauer factor describes a time average. The results are equivalent. 

The Debye temperature is usually high for metallic systems and low for metal-organic complexes. For metals with simple cubic lattices, for which the model was developed, is found in the range from 300 K to well above 10 K. The other extreme may be found for iron in proteins, which may yield d as low as 100-200 K. Figure 2.5a demonstrates how sharply/(T) drops with temperature for such systems. Since the intensity of a Mossbauer spectrum is proportional to the... 

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

Often the electronic spin states are not stationary with respect to the Mossbauer time scale but fluctuate and show transitions due to coupling to the vibrational states of the chemical environment (the lattice vibrations or phonons). The rate l/Tj of this spin-lattice relaxation depends among other variables on temperature and energy splitting (see also Appendix H). Alternatively, spin transitions can be caused by spin-spin interactions with rates 1/T2 that depend on the distance between the paramagnetic centers. In densely packed solids of inorganic compounds or concentrated solutions, the spin-spin relaxation may dominate the total spin relaxation 1/r = l/Ti + 1/+2 [104]. Whenever the relaxation time is comparable to the nuclear Larmor frequency S)A/h) or the rate of the nuclear decay ( 10 s ), the stationary solutions above do not apply and a dynamic model has to be invoked... 

Herber, R.H. Stmcture, bonding and the Mossbauer lattice temperature. In Berber, R.H. (ed.) Chemical Mossbauer Spectroscopy. Plenum, New York (1984)... 

Griesinger et al. [56] recorded Zn Mossbauer spectra with sources of Zn diffused into ZnO, ZnS (both wurtzite and sphalerite), ZnSe, ZnTe, and Cu, and an enriched ZnO absorber. The isomer shifts extracted from their spectra cover a velocity range of 112 pm s and were found to follow linearly the lattice spacing parameter where p and Mav are the host density and average... 

Potzel and Kalvius were the first to investigate zinc compounds with Zn Mossbauer spectroscopy. The first measurements were carried out with Znp2 powder at 4.2 K [80]. The observed quadrupole splitting could not be explained by a simple lattice sum calculation. More detailed measurements were carried out at... 

About 20 HTe superconducting compounds and copper oxidic systems Correlations of the Cu NQR/NMR data with the Cu( Zn) emission Mossbauer data for HTSC lattices as a tool for the determination of atomic charges... 

RBa2Cu40g (R = Sm, Y, Er) Nuclear-quadrupole coupling parameters at the rare-earth metal and copper sites from Cu ( Zn) and Ga( Zn) Mossbauer emission spectroscopy, EEG tensor in comparison with point charge model, shows that holes in lattices are localized primarily at chain-oxygen sites... 

Ga( Zn), Sn, Te( I) Mossbauer spectroscopy, no modifications of the local symmetry of lattice sites, electronic structure of atoms and intensity of electron-phonon interaction are revealed for Pbi Sn Te solid solutions in the gapless state at 80 and 295 K... 

A Japanese group [297] measured the hyperfine field at Ir nuclei in the alloys Feo.yPto.aJrx (0.03 < x < 0.2) using the Ir (73 keV) Mossbauer transition, in order to understand the mechanism associated with the decrease of the magnetic moment (and negative hyperfine field) in these alloys. The isomer shift was found to increase with increasing x, which could be rationalized by the contraction of the lattice volume. 

Much of the Pt Mossbauer work performed so far has been devoted to studies of platinum metal and alloys in regard to nuclear properties (magnetic moments and lifetimes) of the excited Mossbauer states of Pt, lattice dynamics, electron density, and internal magnetic field at the nuclei of Pt atoms placed in various magnetic hosts. The observed changes in the latter two quantities, li/ (o)P and within a series of platinum alloys are particularly informative about the conduction electron delocalization and polarization. 

---