Crystalline electric fields in mixed valent systems (invited)

Abstract

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−xLa_xTh_.1 have revealed the existence of CEF excitations. For x = 0, the valence transition is strongly first order and occurs near T_o?150 K. The inelastic spectra exhibit the typical broad quasielastic scattering. As x increases, T_o 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.