Interstitial Incorporation of Plutonium into a Low-Dimensional Potassium Borate

The molten boric acid flux reaction of PuBr3 with KBO2 at 200 °C results in the formation of large light-yellow crystals of K[B5O7(OH)2]·H2O:Pu4+. Single-crystal X-ray diffraction experiments on the Pu-doped K[B5O7(OH)2]·H2O demonstrate two features: (1) K[B5O7(OH)2]·H2O:Pu4+ adopts a one-dimensional borate chain structure with void spaces between the chains. (2) The doping plutonium atoms do not reside on the potassium sites. The latter are not fully occupied. Both laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and energy-dispersive spectrometry analyses indicate that plutonium atoms are uniformly distributed in crystals of K[B5O7(OH)2]·H2O with an atomic K:Pu ratio of approximately 65:1 measured by LA-ICP-MS. UV–vis–NIR spectra taken from both freshly made and one day old crystals show that the plutonium present within the crystals is predominantly characterized by Pu(IV). A small amount of Pu(III) is also present initially, but slowly oxidized to Pu(IV) via interaction with oxygen in the air. X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopic measurements confirm that plutonium is mainly present as a form similar to that of a PuO2 cluster. The combined results suggest that the clusters containing Pu(IV) ions are uniformly distributed in the void spaces between the borate chains.