Mössbauer spectroscopy

Summary

Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer (sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly recoil-free emission and absorption of nuclear gamma rays in solids. The consequent nuclear spectroscopy method is exquisitely sensitive to small changes in the chemical environment of certain nuclei. Typically, three types of nuclear interactions may be observed: the isomer shift due to differences in nearby electron densities (also called the chemical shift in older literature), quadrupole splitting due to atomic-scale electric field gradients; and magnetic Zeeman splitting due to non-nuclear magnetic fields. Due to the high energy and extremely narrow line widths of nuclear gamma rays, Mössbauer spectroscopy is a highly sensitive technique in terms of energy (and hence frequency) resolution, capable of detecting changes of just a few parts in 1011. It is a method completely unrelated to nuclear magnetic resonance spectroscopy. Mössbauer effect Just as a gun recoils when a bullet is fired, conservation of momentum requires a nucleus (such as in a gas) to recoil during emission or absorption of a gamma ray. If a nucleus at rest emits a gamma ray, the energy of the gamma ray is slightly less than the natural energy of the transition, but in order for a nucleus at rest to absorb a gamma ray, the gamma ray's energy must be slightly greater than the natural energy, because in both cases energy is lost to recoil. This means that nuclear resonance (emission and absorption of the same gamma ray by identical nuclei) is unobservable with free nuclei, because the shift in energy is too great and the emission and absorption spectra have no significant overlap. Nuclei in a solid crystal, however, are not free to recoil because they are bound in place in the crystal lattice. When a nucleus in a solid emits or absorbs a gamma ray, some energy can still be lost as recoil energy, but in this case it always occurs in discrete packets called phonons (quantized vibrations of the crystal lattice).

Official source

https://en.wikipedia.org/wiki/Mössbauer_spectroscopy

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The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x similar to 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, Sn-119 Mossbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce4+ and Sn4+. Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50% Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce1-xSnxO2 solid solutions.

Royal Soc Chemistry2016

Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH(2)trz)(3)](NO3)(2): the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH(2)trz)(3)](NO3)(2)center dot H2O

Christine Neuhausen

The thermally induced hysteretic spin transition (ST) that occurs in the polymeric chain compound Fe(NH(2)trz)(3)(2) (1) above room temperature (T-c(sic) = 347 K, T-c(sic) = 314 K) has been tracked by Fe-57 Mossbauer spectroscopy, SQUID magnetometry, differential scanning calorimetry (DSC), and X-ray powder diffraction (XPRD) at variable temperatures. From the XRPD pattern indexation, an orthorhombic primitive cell was observed with the following cell parameters: a = 11.83(2) angstrom, b = 9.72(1) angstrom, c = 6.361(9) angstrom at 298 K (low-spin state) and a = 14.37(2) angstrom, b = 9.61(4) angstrom, c = 6.76(4) angstrom at 380 K (high-spin state). The enthalpy and entropy variation associated to the ST of 1, have been evaluated by DSC as Delta H = 23(1) kJ mol(-1) and Delta S = 69.6(1) J mol(-1) K-1. These thermodynamic data were used within a two-level Ising like model for the statistical analysis of First Order Reversal Curve (FORC) diagram that was recorded for 1, in the cooling mode. Strong intramolecular cooperative effects are witnessed by the derived interaction parameter of J = 496 K. The crystal structure of Cu(NH(2)trz)(3)(2) center dot H2O (2) was obtained thanks to high quality single crystals prepared by slow evaporation after hydrothermal pretreatment. The catena poly[mu-tris(4-amino-1,2,4-triazole-N1,N2) copper.(II)] dinitrate monohydrate (2) crystallizes in the monoclinic space group C2/c, with a = 16.635(6) angstrom, b = 13.223(4) angstrom, c = 7.805(3) angstrom, beta = 102.56(3)degrees, Z = 4. Complex 2 is.a 1D infinite chain containing triple N1,N2-1,2,4-triazole bridges with an intra-chain distance of Cu center dot center dot center dot Cu = 3.903(1) angstrom. A dense H-bonding network with the nitrate counteranion involved in intra-chain and inter-chain interactions is observed. Such a supramolecular network could be at the origin of the unusually large hysteresis loop displayed by 1 (Delta T similar to 33 K), as a result of an efficient propagation of elastic interactions through the network. This hypothesis is strengthened by the crystal structure of 2 and by the absence of crystallographic phase transition for 1 over the whole temperature range of investigation as shown by XRPD.

2010

Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH(2)trz)(3)](NO3)(2): the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH(2)trz)(3)](NO3)(2)center dot H2O

Christine Neuhausen

2010