Magnetic properties of trivalent actinides in the octahedral compounds Cs2NaMCl6

Abstract

The magnetic susceptibilities of U³⁺, Np³⁺, Pu³⁺, Am³⁺, Cm³⁺, and Bk³⁺ ions in an octahedral environment were measured from 3 to about 50°K with a vibrating‐sample magnetometer. Each actinide ion was in a stoichiometric compound of the cubic Cs₂NaMCl₆ type, except for Cm³⁺ and Bk³⁺, which were doped into diamagnetic Cs₂NaLuCl₆. Curie‐Weiss behavior was found for the uranium, neptunium, and plutonium compounds, with evidence for small distortions from octahedral symmetry in the uranium and plutonium compounds. The americium and berkelium samples displayed temperature‐independent paramagnetism. For the curium sample, the free‐ion moment was observed, but with deviations below 7.5°K that are attributed to crystal‐field splitting of Cm³⁺. The following ground levels were determined: U³⁺, Γ₈; Np³⁺, Γ₅; Pu³⁺, Γ₈; Am³⁺, Γ₁; and Bk³⁺, Γ₁. In Bk³⁺, a Γ₁–Γ₄ separation of 85 cm⁻¹ is derived. Limits on the octahedral crystal‐field parameters were established from the magnetic data and from crystal‐field calculations for U³⁺ and Np³⁺, including intermediate coupling and J mixing from the first excited states. From these parameters, the complete crystal‐field splitting of each actinide ion in Cs₂NaMCl₆ was calculated.