Preparation, structures, and properties of niobium chalcogenide halides, NbX2Y2 (X = S, Se; Y = Cl, Br, I)

The preparation of the compounds NbX₂Y₂ (X = S, Se; Y = Cl, Br, I). including that of large single crystals, by chemical transport techniques is reported. Most of these compounds exist in a triclinic low-temperature form and a monoclinic high-temperature form. The crystal structures of triclinic low-temperature NbSe₂Cl₂ and monoclinic NbS₂Cl₂ are reported; both structures consist of cage-shaped units of Nb₂X₄ which are linked together by the Cl atoms to form sheets parallel to the a, b planes. Both the Nb and the X atoms are present in pairs which indicates that the compounds can be formulated as Nb⁴⁺₂(X₂)²⁻₂₂Y⁻₄, in agreement with XPS spectra. The presence of Nb⁴⁺ in pairs explains the observed diamagnetic and semiconducting properties of NbX₂Y₂. The preparation and some properties of MoS₂Cl₂ (orthorhombic) are also given.