Magnetic properties of trivalent actinides in the octahedral compounds Cs2NaMCl6

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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.