Absorption Spectrum of Cs2NpCl6

The absorption spectrum of single crystal Cs2NpCl6 has been investigated at 4.2, 77, and 300°K. The crystal field-split 5f3[SL]-J levels of tetravalent neptunium below 13 000 cm−1 in energy are sufficiently separated by spin–orbit interaction to permit a first-order analysis of the crystal field splitting. The crystal field splitting of the I4 and F4 states based on the octahedral symmetry of the hexachloro complex is calculated using parameters B4 = 520 and B6 = 150 cm−1. The center of gravity of the crystal field-split J levels can be compared with a theoretical “free-ion” calculation for intermediately coupled J levels using the Racah parameters E1 = 2955, E2 = 17.4, E3 = 313.8 in cm−1 and a spin–orbit parameter ζ = 1985 cm−1. In comparison with the crystal spectra of hexachloro tetravalent protactinium and uranium, the neptunium crystal-field splitting is smaller by roughly a factor of 2. This observation leads one to suspect an actinide contraction since the crystal structure is similar for all three systems.