Crystalline electric fields in mixed valent systems

Inelastic neutron studies of rare-earth-based mixed valent systems shown remarkably similar results: a broad quasielastic line with half width on the order of 10 meV. This width exhibits a strong temperature dependence in those systems which undergo a valence transition and is only weakly temperature dependent in those systems which show on transition. A surprising result was the absence of crystalline electric field (CEF) excitations. Recent measurements on the CE (.9-x)La(x)Th (.1) revealed the existence of CEF excitations. For x=0, the valence transition is strongly first order and occurs near T - or approximately 150 K. The inelastic spectra exhibit the typical broad quasielastic scattering. As x increases, T decreases, due to internal pressure effects and a well-defined, but broad, excitation appears near E=15 meV. This is interpreted as CEF excitation between the gamma 7 and gamma 8 levels of the Ce3(+) ion. For x=0.40, the valence transition is almost completely suppressed and the excitation becomes even sharper.