Thorium copper phosphides: more diverse metal–phosphorus and phosphorus–phosphorus interactions than U analogues

Abstract

To explore the chemical analogy between thorium and heavier actinides in soft anionic environments, three new thorium phosphides (ThCuP₂, β-ThCu₂P₂, and ThCu₅P₃) have been prepared through solid-state reactions using CuI as a reaction promoter. The structure of ThCuP₂ can be described as a filled UTe₂-type with both dimeric P₂⁴⁻ and monomeric P³⁻ anions, in which Th is coordinated by eight P atoms in a bicapped trigonal prismatic arrangement and Cu is tetrahedrally coordinated by four P atoms. β-ThCu₂P₂ contains only P³⁻ anions and is isostructural with BaCu₂S₂. In this structure, Th is coordinated by seven P atoms in monocapped trigonal prismatic geometry and Cu is tetrahedrally coordinated by four P atoms. ThCu₅P₃ adopts the YCo₅P₃-type structure consisting of P³⁻ anions. This structure contains Th atoms coordinated by six P atoms in a trigonal prismatic arrangement and Cu atoms that are either tetrahedrally coordinated by four P atoms or square pyramidally coordinated by five P atoms. Electric resistivity measurements and electronic structure calculations on β-ThCu₂P₂ indicate a metal. These new compounds may be charge-balanced and formulated as Th⁴⁺Cu⁺(P₂⁴⁻)_1/2P³⁻, Th⁴⁺(Cu⁺)₂(P³⁻)₂, and Th⁴⁺(Cu⁺)₅(P³⁻)₃, respectively. The structural, bonding, and property relationships between these Th compounds and related actinide and rare-earth phases are discussed. Titled compounds display more diverse ion–ion interactions and different electronic structures from those in UCuP₂ and UCu₂P₂ that were synthesized under similar experimental conditions, suggesting divergence of thorium-phosphide chemistry from uranium-phosphide chemistry.