Structure, Phase Transitions, and Isotope Effects in [(CH3)4N]2PuCl6

The single-crystal X-ray diffraction structure of [(CH3)4N]2PuCl6 is presented for the first time, resolving long-standing confusion and speculation regarding the structure of this compound in the literature. A temperature-dependent study of this compound shows that the structure of [(CH3)4N]2PuCl6 undergoes no fewer than two phase transitions between 100 and 360 K. The phase of [(CH3)4N]2PuCl6 at room temperature is Fd3̅c a = 26.012(3) Å. At 360 K, the structure is in space group Fm3̅m, with a = 13.088(1) Å. The plutonium octahedra and tetramethylammonium cations undergo a rotative displacement, and the degree of rotation varies with temperature, giving rise to the phase transition from Fm3̅m to Fd3̅c as the crystal is cooled. Synthesis and structural studies of the deuterated salt [(CD3)4N]2PuCl6 suggest that there is an isotopic effect associated with this phase transition, as revealed by a changing transition temperature in the deuterated versus protonated compound, indicating that the donor–acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are driving the phase transformation.