Insights into the new 3d–5f heterometallic quaternary fluorides: Synthesis, crystal structures, spectroscopic properties, and thermodynamic stability

Two new 3d–5f heterometallic quaternary fluorides, Na₃CrTh₆F₃₀ and Na₄NiTh₆F₃₀, have been synthesized hydrothermally from the reactions between ThF₄, CrF₃ or NiF₂, NaF, and HF. Single crystal X-ray diffraction studies revealed that Na₃CrTh₆F₃₀ and Na₄NiTh₆F₃₀ adopt dense three-dimensional structures with the space group P3¯c1. Surprisingly, these two compounds are isotypic and exhibit a rather unusual case of aliovalent substitution of the 3d metal centers (Cr^III ↔ Ni^II) which is achieved by losing or gaining a Na⁺ cation in their structures. The valences of Cr^III and Ni^II have been confirmed by corroborating evidences from bond valence sum, UV–vis, and X-ray absorption near edge structure studies. Na₃CrTh₆F₃₀ and Na₄NiTh₆F₃₀ exhibit extraordinary thermal stability up to 500 °C and to 800 °C, respectively, and they retain their original structures in strong acids including 14.9 M HNO₃ and aqua regia. First-principle calculations based on density functional theory indicat that the formation enthalpy of Na₃CrTh₆F₃₀ is −118.945 kJ/mol; suggesting that Na₃CrTh₆F₃₀ is thermodynamically stable under the simulated condition. This finding helps shed light on the potential products generated from corrosion or during the thorium nuclear fuel cycle.