MAGF · Magnetochemical studies of high valent silver fluorides

Magnetochemical studies of high valent silver fluorides

Project objectives Silver ions in the unusual and rare oxidation state of +2, Ag(II) are electronic analogues of the copper ions found in high-temperature ceramic oxide superconductors. In contrast to copper however, Ag(II) is unstable in an oxygen environment, reacting to either a mixed valent Ag(I)/Ag(III) oxide or liberating oxygen gas through oxidation of the oxide ions. In a fluoride context, Ag(II) is stable and compounds such as AgF2 and Cs2AgF4 are known, if reactive; in the latter example, the coupling between the two Ag ions is ferromagnetic, which is unusual. Work performed The parent compound AgF2 has an unusual structure of puckered layers of square planar Ag(II) ions and the magnetism of AgF2 is unusual. Muon spin resonance experiments, carried out at the ISIS on the MuSR instrument at zero field, tracked the magnetisation of the sample through the known transition temperature of 163 °K, below which analysis of the data suggests that this system is a 3-D Heisenberg system. Additionally, the broader theme of angular momentum, which is the source of the electronic magnetic moments in the silver systems, has been explored with respect to energy transfer in non-equilibrium thermodynamic ensembles. Collisional energy transfer was analysed previously within the frame of energy conservation and an essentially energy-based analysis, usually through a potential energy surface. Main results In collaboration with Prof. Anthony McCaffery, we have developed the application of the conservation of angular momentum and its conversion to linear momentum to describe the transfer of energy between atmospherically relevant molecules, such as N2, O2 and OH. Significantly, the method can be applied quantitatively to very large ensembles and can track collisional outcomes per ensemble member with quantum-state resolution. Results from this analysis, published in Chemical Physics Letters, the Journal of Chemical Physics and the Journal of Physical Chemistry A show that equilibration between the energetic modes of translation, rotation and vibration occur at very different rates, with equilibrium temperatures being slowly achieved and with significant undercooling of rotational modes in systems with a large rotational constant. In collaboration with Los Alamos National Laboratory, we have begun the extension of this approach to very large ensembles of polyatomic molecules, especially CO2 and CH4, with full rotational and vibrational quantum-state resolution. In this way, we hope to be able to analyse quantitatively the molecular processes for energy transfer that are directly relevant to changes in atmospheric composition caused by the release of CO2 from the burning of fossil fuels - the molecular mechanism of anthropogenic climate change.

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