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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>S</mml:mi></mml:mrow> <mml:mrow><mml:mi>m</mml:mi></mml:mrow> <mml:mrow><mml:mn>0</mml:mn></mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mn>318</mml:mn> <mml:mo>±</mml:mo> <mml:mn>8</mml:mn></mml:mrow> <mml:mo>)</mml:mo></mml:mrow> <mml:mspace></mml:mspace> <mml:mi>J</mml:mi> <mml:mspace></mml:mspace> <mml:mo>⋅</mml:mo> <mml:mspace></mml:mspace> <mml:msup><mml:mrow><mml:mi>K</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:mo>⋅</mml:mo> <mml:msup><mml:mrow><mml:mi>mol</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> </mml:math> , 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.