Layer structure polytypism among niobium and tantalum selenides

Single crystals of several varieties of tantalum and niobium selenide layer structures have been grown by vapor transport methods.Two two-layer forms have the NbS 2 structure, a three-layer form is like rhombohedral MoS 2, and two four-layer forms are new structures with a selenium stacking sequence of AABBAA CC..., and AABMBBAB .... respectively.These have been refined by least-squares methods.:Refinement adjusts the c coordinates of the selenium atoms 0-1 A away from positions in which each atom sheet is spaced at equal intervals along z.This adjustment moves the selenium atoms away from the interleaving metal atoms.The trigonal prism coordination ~mits in the niobium and tantalum compounds tend to be slightly shorter in the z direction and longer in the x direction than similar molybdenum coordination units.