Magnetic Properties of Tetravalent Pu in the Perovskites BaPuO₃ and SrPuO₃

BaPuO₃ and SrPuO₃ were synthesized, and their structures were refined in the orthorhombic space group Pbnm, a common distortion from the classic Pm3̅m cubic perovskite. Magnetic-susceptibility measurements, obtained as a function of temperature over the range of 1.8-320 K, exhibit temperature-dependent behavior, with evidence of long-range magnetic order at temperatures higher than their lanthanide and actinide analogues: BaPuO₃ below 164(1) K and SrPuO₃ below 76(1) K. Effective moments of 1.66(10)μ_B for BaPuO₃ and 1.84(8)μ_B for SrPuO₃ were obtained by fitting their paramagnetic susceptibilities using the Curie-Weiss law. Both are below the free-ion value of 2.68 μ_B expected for a Pu⁴⁺⁵I₄ ground level. Ab initio wave function calculations, performed at the relativistic complete active space level including spin-orbit coupling and with an embedded cluster approach that neglects interactions between Pu centers, were used to generate embedded-cluster Pu⁴⁺ magnetic susceptibilities. The calculations agree well with experimental data at higher temperatures, providing evidence that a single-ion representation is sufficient to account for the observed paramagnetic behavior without the need to invoke charge transfer, disproportionation, strong covalent bonding, or other more complex electronic behavior.