Ni₃Cr₂P₂Q₉ (Q = S, Se):  New Quaternary Transition Metal Chalcogenides with a Unique Layered Structure

Abstract

The new transition metal chalcogenides Ni3Cr2P2S9 and Ni3Cr2P2Se9 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 Ni3Cr2P2S9-xSex (x = 0, 3, 6) revealed activated behavior in electrical resistivity (Ea = 0.02−0.03 eV, ρ300K = 50−220 mΩ·cm) and positive values of the Seebeck coefficient (S300K = 80−225 μV/K), showing that these compounds behave like p-type semiconductors. Magnetization measurements on Ni3Cr2P2S9 single crystals from 5 to 400 K reveal antiferromagnetic interactions between the transition metal ions and an ordering transition near 105 K