RARE-EARTH CRYSTAL-FIELD SPECTROSCOPY BY NEUTRON MAGNETIC SCATTERING - FROM XENOTIME TO HIGH-TC SUPERCONDUCTORS

Optical spectroscopy is one of the traditional methods used to determi ne the overall splitting of the rare earth ion energy states within an f(N) configuration in either solution or solid state of transparent m aterials. This technique can provide data over a wide energy range wit h good resolution, and the parameters obtained for the empirical Hamil tonian reflect the ''best fit'' of the observed energies. Absolute int ensity measurements of optical transitions and their comparison with t heory, however, are difficult. Magnetic scattering of thermal neutrons arises from an interaction of the neutron magnetic moment with the co nvection and spin current of the scatterer. Such a weak interaction do es not involve any excited intermediate states of the system and requi res only a first-order perturbation treatment to calculate the scatter ing cross-section. However, neutron spectroscopy probes only states at energies less than approximately 1 eV. It is demonstrated that a comb ined treatment of neutron and optical data can provide the complementa ry information necessary for a proper characterization of the level sp littings and wavefunctions of the rare earth ions in xenotime (RPO4, R = Tb to Yb). Next we discuss the analyses of neutron scattering data for the understanding of the rare earth energy levels and wavefunction s in various high T(c) copper oxide superconductors and related compou nds. The importance of a consistent refinement of the crystal field pa rameters across a series of isostructural rare earth compounds and a s ystematic comparison of the low temperature magnetic properties with m odel calculations are emphasized.