Verbeekite, the Long-Unknown Crystal Structure of Monoclinic PdSe.

Verbeekite, a monoclinic polymorph of PdSe, 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 PdSe 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) Å, R1 = 0.0368, wR2 = 0.0907 (all data). In contrast to layered PdS-type PdSe, verbeekite exhibits a novel crystal structure type of dichalcogenides of the platinum-group metals with (Se) dimer anions connecting the layers. The possibility of different arrangements of the characteristic (Se) dumbbells is the reason for the various polymorphs of the dichalcogenides, with now five known PdSe representatives.