Stabilization of Pr⁴⁺ in Silicates-High-Pressure Synthesis of PrSi₃O₈ and Pr₂Si₇O₁₈

The reaction between PrO2 and SiO2 was investigated at various pressure points up to 29 GPa in a diamond anvil cell using laser heating and in situ single-crystal structure analysis. The pressure points at 5 and 10 GPa produced Pr-2(III)(Si2O7), whereas (Pr4Si3O12)-Si-III and Pr-2(IV)(O-2)O-3 were obtained at 15 GPa. (Pr4Si3O12)-Si-III can be interpreted as a high-pressure modification of the still unknown orthosilicate Pr-4(III)(SiO4)(3). (PrSi3O8)-Si-IV and (Pr2Si7O18)-Si-IV that contain praseodymium in its rare + IV oxidation state were identified at 29 GPa. After the pressure was released from the reaction chamber, the Pr(IV) silicates could be recovered, indicating that they are metastable at ambient pressure. Density functional theory calculations of the electronic structure corroborate the oxidation state of praseodymium in both (PrSi3O8)-Si-IV and (Pr2Si7O18)-Si-IV. Both silicates are the first structurally characterized representatives of Pr4+-containing salts with oxoanions. All three silicates contain condensed networks of [SiO6] octahedra which is unprecedented in the rich chemistry of lanthanoid silicates.