SYNTHESIS AND STRUCTURE OF SOME TERNARY MOLYBDENUM SULFIDE (CHEVREL PHASE) ANALOGS; A STUDY OF THE STRUCTURE OF LANTHANUM NICKELATE

Attempts to synthesize niobium and mixed-metal niobium ana- logs of ternary molybdenum sulfides (Chevrel phases) were not suc- cessful. The Cr, Cu, and Zn derivatives of the Chevrel phase analog Mo(,6)S(,6)I(,2) have been prepared by reaction of the metal with Mo(,6)S(,6)I(,2). The large-ion derivatives of Pb and Sn would not form. The deriva- tives formed are members of the space group R(\u27)3, isomorphous with M(,x)Mo(,6)S(,8). Single crystal structure determinations were carried out on Mo(,6)S(,6)I(,2) and the Zn and Cu derivatives using a Enraf-Nonius CAD4 diffractometer and MoK(alpha) radiation. The crystals used were small cubes approximately 0.05 mm on an edge. Refinement of the Mo(,6)S(,6)I(,2) structure using 291 averaged reflections and cell dimensions of a(,H) = 9.617(3) and c(,H) = 10.421(5) (ANGSTROM) gave residuals of R = 0.050, Rw = 0.059. The occupancy of I was refined giving a formula of Mo(,6)S(,6)I(,1.83). For the Cu compound 270 averaged reflections were used during refinement with cell dimensions of a(,H) = 9.732(6) and c(,H) = 10.454(5) (ANGSTROM). The stoichiometry was found to be Cu(,0.91)Mo(,6)S(,6)I(,1.83). The residuals were R = 0.042, Rw = 0.051. For the Zn compound 264 averaged reflections were used with cell dimensions of a(,H) = 9.723(1) and c(,H) = 10.454(3) (ANGSTROM), giving stoichiometry of Zn(,0.94)Mo(,6)S(,6)I(,1.84), and residuals of R = 0.050, Rw = 0.059. Structural behavior is analogous to the sulfide systems, M(,x)Mo(,6)S(,8), except the cations are located exclusively in the outer tetrahedral sites. Lanthanum nickelate, La(,2)NiO(,4), was studied to determine the structure of a non-stoichiometric sample. A K(,2)NiF(,4) structure, space group I4/mmm, was observed but the axial oxygen (0,0,z position) formed a four-leaf clover pattern in the plane perpendicular to the ternary axis. This is attributed to delocalization of the oxygen into 4 positions off the z axis. Ordering of the delocalized oxygen would lead to the orthorhombic cell observed by others in stoichiometric La(,2)NiO(,4)