Verbeekite, the Long-Unknown Crystal Structure of Monoclinic PdSe₂

Abstract

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.

Two‐Dimensional Noble‐Metal Chalcogenides and Phosphochalcogenides

“…Under pressure, the PdS 2 ‐type structure reversibly transforms to the pyrite‐type structure . The pyrite‐phase competes with the marcasite‐type structure, as it is close in energy (Δ E =6.2 kJ mol −1 for PdSe 2 ).…”

“…An indirect approach to obtain 1 T ‐PdSe 2 via a high‐energy phase has been suggested by Lei et al. : At a temperature of 1600 K and a pressure of 11.5 GPa, 2 O ‐PdSe 2 undergoes a phase transition to the verbeekite structure (Figures f and c) . Verbeekite‐PdSe 2 was first found as a mineral and appears to be a stable polymorph of PdSe 2 (Δ E ≈1.4 kJ mol −1 ), which is closer to the structure of PdP 2 (Figure i) .…”

“…To provide insight into the stability and properties of these phases, we show that the structure of PdS 2 and PdSe 2 (Figure a) is related to the pyrite‐type structure (Figure e) . We then connect the pyrite structure to its high‐pressure phases (marcasite and verbeekite in Figure d,f) and show its transformation back to the 1 T phase (Figure b).…”

“…Verbeekite‐PdSe 2 was first found as a mineral and appears to be a stable polymorph of PdSe 2 (Δ E ≈1.4 kJ mol −1 ), which is closer to the structure of PdP 2 (Figure i) . Lei et al . predict that verbeekite‐PdSe 2 may transform under pressure to the slightly distorted 1 T structure known as AuTe 2 ‐type (Figure c) and few‐layer 1 T ‐PdSe 2 may then be cleaved from this phase.…”

Zweidimensionale Edelmetallchalkogenide und ‐phosphochalkogenide

“…Um einen Einblick in die Stabilitäten und Eigenschaften dieser Phasen zu bekommen, zeigen wir, dass die Struktur von PdS 2 und PdSe 2 (Abbildung a) mit dem Pyrit‐Strukturtyp (Abbildung e) verwandt ist . Anschließend verbinden wir den Pyrit‐Strukturtyp mit dessen Hochdruckphasen (Markasit und Verbeekit in Abbildung d,f) und zeigen die Transformation zurück zur 1 T ‐Phase (Abbildung b).…”