Local Structure and Magnetism of La_1–xM_xPO₄ (M = Sm, ²³⁹Pu, ²⁴¹Am) Explained by Experimental and Computational Analyses

Abstract

With their high chemical and self-irradiation stability, crystalline monazites are among the most promising materials for the encapsulation of nuclear wastes. Yet, the local and magnetic structures of the matrices doped with low-content actinide cation, depicted as most resistant, are still unclear. This limits the development of theoretical approaches predicting their behavior under extreme conditionsself-irradiation and long-term leaching. Here, we characterize the model matrices La 1−x M x PO 4 (0 ≤ x ≤ 0.10)with M = Sm, 239 Pu, 241 Amby X-ray diffraction and solid-state 31 P NMR. As an example, we confirm that La 0.96 241 Am 0.04 PO 4 has higher self-irradiation resistance compared to 241 AmPO 4 . Further, computational analyses show that magnetic properties of the Pu complex are strongly affected by the J-mixing and the paramagnetic NMR shifts are dominated by the Fermi contact contribution, arising from delocalization of the spin density of the cation toward the phosphorus through the bonds.

Bonding and Magnetic Trends in the [AnIII(DPA)3]3– Series Compared to the Ln(III) and An(IV) Analogues

“…The largest effect is obtained for the Pu­(III) derivative with a shrinking of the ground J manifold. This was already observed and discussed for the La 1– x Pu x PO 4 crystal . The J -mixing is around 2% in the lanthanides, whereas it is increased by several orders of magnitude in the actinides due to the strong spin–orbit coupling, especially in the middle of the series (Pu, Bk, Cf, Es), where it reaches around 20–25%, as shown in Table .…”

“…This was already observed and discussed for the La 1−x Pu x PO 4 crystal. 57 The Jmixing is around 2% in the lanthanides, whereas it is increased by several orders of magnitude in the actinides due to the strong spin−orbit coupling, especially in the middle of the series (Pu, Bk, Cf, Es), where it reaches around 20−25%, as shown in Table 1. The effect of electron dynamical correlation on the ITO strength parameters (see Figure 6b) is relatively small, contrary to AILFT (see Figure 6a).…”

Dipolar and Contact Paramagnetic NMR Chemical Shifts in AnIVComplexes with Dipicolinic Acid Derivatives

“…The Fermi contact shift 𝛿 𝐾 𝑐 arises from the spin density at nucleus 𝐾 𝜌 𝑠 (𝒓 𝑲 ) as a consequence of spin delocalization and/or spin polarization. It might be expressed as : 6 𝛿 𝐾 𝑐 = 1 𝑁 𝐴 𝜇 0 𝜇 𝐵 𝑔 𝑒 𝛾 𝐾 𝐴 𝐾 𝑐 ℏ 𝜒 𝑚 𝑆 (2) where 𝑁 𝐴 is the Avogadro constant, 𝜇 0 the magnetic permeability, 𝜇 𝐵 the Bohr magneton, 𝑔 𝑒 the free electron g-factor, 𝛾 𝐾 the gyromagnetic ratio of the observed nucleus. The hyperfine coupling (HFC) constant 𝐴 𝐾 𝑐 /ℏ depends on the spin density at the nuclear position and 𝜒 𝑚 𝑆 is the spin contribution to the magnetic susceptibility.…”