Future directions in Mössbauer spectroscopy

Since the discovery of the Mössbauer effect more than 40 years ago, a wide range of applications in a number of different disciplines has been described. One great advantage of the Mössbauer effect is its high energy resolution, which enables its use as a highly sensitive probe of the atomic environment, providing information on valence state, site occupancies, site co-ordination and distortion and magnetic structure, and processes such as phase transitions, relaxation, lattice dynamics and diffusion. Properties that have not been fully exploited include spatial and time resolution. The milliprobe, for example, enables a two orders of magnitude increase in spatial resolution. Further improvement may be anticipated with developments in synchrotron studies of nuclear forward scattering. Other possibilities on the horizon include development of a Mössbauer electron microscope which would focus conversion electrons using conventional electron optics, and development of focusing lenses for gamma rays on a laboratory scale. Time resolution has also not been widely exploited. Studies with conventional techniques are possible over a wide range of time scales, from in situ investigations of phase transitions, oxidation and other chemical reactions, to diffusion studies, to studies at the intrinsic time scale of the Mössbauer effect to investigate processes such as electron transfer. Existing facilities for forward nuclear scattering at synchrotron installations reduce this timescale even further. Mössbauer spectroscopy can be performed under a wide range of P,T conditions, and the limits have not yet been reached. Developments in nuclear forward scattering could expand the pressure and temperature limits, raising the tantalising possibility of measuring Mössbauer parameters at mantle temperatures and pressures.