Ni₃Cr₂P₂Q₉ (Q = S, Se): New quaternary transition metal chalcogenides with a unique layered structure

The new transition metal chalcogenides Ni₃Cr₂P ₂S₉ and Ni₃Cr₂P₂Se ₉ have been discovered and characterized. Single-crystal X-ray diffraction studies of the selenide reveal a new layered structure type (space group P6₃/m, a = 6.244(4) Å, c = 18.479(19) Å, Z = 2, R₁ = 0.0235). Powder X-ray diffraction and electron microprobe analysis suggest that the sulfide is isostructural to the selenide and that a solid solution forms between them. The layers are composed of transition metal centered octahedra with chalcogenides at the vertices which are joined in pairs by face sharing along the c-direction. These units share edges to form honeycomb layers in the ab-plane with linear P - Ni - P units in the holes of the honeycomb nets. Transport properties measurements from 80 - 300 K on single crystals with compositions Ni₃Cr₂P₂S _9-xSe_x (x = 0, 3, 6) revealed activated behavior in electrical resistivity (E_a = 0.02-0.03 eV, ρ_300K = 50-220 mΩ·cm) and positive values of the Seebeck coefficient (S_300K = 80-225 μV/K), showing that these compounds behave like p-type semiconductors. Magnetization measurements on Ni₃Cr ₂P₂S₉ single crystals from 5 to 400 K reveal antiferromagnetic interactions between the transition metal ions and an ordering transition near 105 K.