New ternary and quaternary transition-metal selenides: Syntheses and characterization

Abstract

New ternary and quaternary transition-metal selenides have resulted from substitutional chemistry of the phase Nb2Pd3Se8 and examination of the system Nb/Pd/Se. The structures of the phases Ta2Pd3Se8, Co2Ta4PdSe12, and Nb2Pd0.71Se5 have been established through single-crystal X-ray methods. The compound Ta2Pd3Se8 forms in space group Pbam with two formula units in a cell of dimensions a=15.152(2), b=10.631(2), and c=3.540(1) Å. This material is isostructural with Nb2Pd3Se8. The compound Co2Ta4PdSe12 forms in space group C2/m with two formula units in a cell of dimensions a=12.951(3), b=3.413(1), c=19.277(5) Å, and β=110.37(2)°. This material forms with a new laminar structural type; each layer is composed of an unusual association of metal-centered Se polyhedra. The Pd atom occupies a square plane, the Co atom occupies a square pyramid and there are two types of Ta atoms—octahedral and trigonal prismatic. The layers pack in a pseudo-closest-packed manner with the formation of several vacant octahedral and tetrahedral sites between the layers. Like the phase Co2Ta4PdSe12 the compound Nb2Pd0.71Se5 also forms in a new layered structural type. This compound crystallizes in space group C2/m with four formula units in a cell of dimensions a=12.788(6), b=3.391(1), c=15.416(6) Å, and β=101.48(2)°. The material consists of layers of composition ∞2[Nb4PdSe10] that result from association of Pd-centered Se square planes and Nb-occupied Se trigonal prisms. Between these layers a rhombic site is 42% occupied by an additional Pd atom that bonds to four coplanar Se atoms. The results of electrical conductivity measurements indicate that this material is a metallic conductor.