Verbeekite, the Long-Unknown Crystal Structure of Monoclinic PdSe₂

Verbeekite, a monoclinic polymorph of PdSe₂, was reported for the first time in 2002 by Roberts et al. The mineral has been discovered in the Musonoi Cu-Co-Mn-U mine, Democratic Republic of Congo, and was named after Dr. Théodore Verbeek, the first geoscientist who studied the palladium mineralization there (1955-1967). Until today, the crystal structure of this very rare mineral has been unknown. By syntheses via multianvil high-pressure/high-temperature methods at 11.5 GPa and 1300 °C, synthetic verbeekite could be obtained in a high degree of purity and comparatively good crystal quality, which made it possible to determine the full crystal structure for PdSe₂ verbeekite from single-crystal X-ray diffractometer data: I2/a, a = 671.0(2) pm, b = 415.42(8) pm, c = 891.4(2) pm, β = 92.42(3)°, V = 248.24(4) ų, R1 = 0.0368, wR2 = 0.0907 (all data). In contrast to layered PdS₂-type PdSe₂, verbeekite exhibits a novel crystal structure type of dichalcogenides of the platinum-group metals with (Se₂)^2- dimer anions connecting the layers. The possibility of different arrangements of the characteristic (Se₂)^2- dumbbells is the reason for the various polymorphs of the dichalcogenides, with now five known PdSe₂ representatives.