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 ${\text{5f}}^3\mathrm{[SL]‐J}$ 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 ${}^{4}I$ and ${}^{4}F$ states based on the octahedral symmetry of the hexachloro complex is calculated using parameters $B_4\text{\hspace{0.17em}=\hspace{0.17em}520}$ and $B_6\text{\hspace{0.17em}=\hspace{0.17em}150}{\mathrm{cm}}^{\text{−1}}$ . 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 $E^1{\text{\hspace{0.17em}=\hspace{0.17em}2955, E}}^2{\text{\hspace{0.17em}=\hspace{0.17em}17.4, E}}^3\text{\hspace{0.17em}=\hspace{0.17em}313.8}{\mathrm{in\; cm}}^{\text{−1}}$ and a spin–orbit parameter $\text{ζ\hspace{0.17em}=\hspace{0.17em}1985}{\mathrm{cm}}^{\text{−1}}$ . 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.