Energy-Level Scheme for Np4+ in PbMoO4

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Abstract
Neptunium-doped (Na-charge-compensated) crystals of PbMoO4 were grown by the Czochralski method. The absorption spectrum between 5000 and 25 000 cm−1 was obtained on a Cary-14 spectrophotometer. The fluorescent spectrum between 4000 and 7000 cm−1 was obtained on a specially modified Beckman DK-2 spectrophotometer. The free-ion parameters obtained from a least-squares fit to the five lowest free-ion levels neglecting configuration mixing are E1 = 2970.0 cm−1, E2 = 14.01 cm−1, E3 = 312.1 cm−1, and ξ5f = 2.89.2 cm−1. The rms deviation for this fit was 17 cm−1. A first-order crystal-field perturbation calculation was carried out. A least-squares fit to the 10 lowest Stark levels yielded the following values for the crystal-field parameters: B02 = 2580.7 cm−1; B04 = − 3534.0 cm−1; B06 = − 1476.6 cm−1; B44 = − 2892.5 cm−1; B′44 = − 918.6 cm−1; B46 = − 663.9 cm−1, and B′46 = − 211.4 cm−1 with an rms deviation of 40 cm−1. A similar least-squares fit, including the effects of J–J mixing, could not be carried out due to rather large computation time requirements. However, it was found that for the lowest state [J = 9/2(I)], the mixing effects are less than 30 cm−1 whereas for the first excited state [J = 11/2(I)], J–J mixing can shift levels by as much as 80 cm−1. The results for Np4+ are compared with those for the “isoelectronic” ion Nd3+.

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