Structural and physical investigation of ordered BaO-deficient Ba₃PuO₆

Ternary actinide compounds allow for a range of oxidation states and a rich but often not well-known chemistry. Here, a BaO-deficient plutonium-containing perovskite with a composition close to Ba₃PuO₆ has been obtained via solid state synthesis. The compound was characterised using X-Ray Diffraction from room temperature up to 1473 K, revealing two phase transitions and ordered Ba-deficiency. X-ray Absorption Spectroscopy was performed at the Pu L₃- and M₅-edges. The XANES spectrum at the Pu L₃-edge and the XANES in high-energy resolution mode at the Pu M₅-edge both indicate a Pu(VI) oxidation state. Despite the rather complex crystallographic unit cell, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Pu L₃-edge yields Pu-O distances of 2.01(1) and 2.4(3) Å. The low-temperature heat capacity of Ba_2.875PuO_5.875 was measured between 5 and 270 K. The heat capacity and standard entropy at 298.15 K were found to be C_p = (251 ± 8) J⋅K^-1⋅mol^-1 and $S_m^0=\left(318\pm 8\right)J\cdot K^{-1}\cdot {\mathrm{mol}}^{-1}$ , respectively. The magnetic susceptibility was measured from 2 to 320 K and shows a temperature-independent paramagnetism, consistent with the hexavalent oxidation state of Pu. This article reports the first extended low-temperature heat capacity curve and a temperature-dependent magnetic susceptibility measurement in a [Rn]5f² Pu-based system.