Crystalline electric fields in mixed valent systems (invited)

The inelastic neutron studies of rare-earth-based mixed valent systems have all 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 no transition. A surprising result was the absence of crystalline electric field (CEF) excitations. Recent measurements on the alloy Ce.9−xLaxTh.1 have revealed the existence of CEF excitations. For x = 0, the valence transition is strongly first order and occurs near To?150 K. The inelastic spectra exhibit the typical broad quasielastic scattering. As x increases, To decreases, due to internal pressure effects, and a well-defined, but broad, excitation appears near E = 15 meV. This is interpreted as a CEF excitation between the Γ7 and Γ8 levels of the Ce3+ ion. For x = 0.40, the valence transition is almost completely suppressed and the excitation becomes even sharper.