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 Ba2.875PuO5.875 was measured between 5 and 270 K. The heat capacity and standard entropy at 298.15 K were found to be Cp = (251 ± 8) J⋅K−1⋅mol−1 and Sₘ⁰=(318± 8)\,J \! · \! K⁻¹ · 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. The exact crystal chemistry of perovskites containing 5f elements has remained a matter of debate. Here, the authors synthesise a BaO-deficient Pu-based perovskite with a composition close to Ba3PuO6 and study the Pu oxidation state, the crystal structure and Ba/Pu ratio, reporting on thermodynamic and magnetic properties and offering new insights into Ba-Pu-O solid-state chemistry.