Sr10[Mo2N6][MoN4]2 and β-Sr3MoN4

Two new phases in the strontium-molybdenum-nitrogen phase diagram have been synthesized. Single crystals of Sr-10[Mo2N6][MoN4](2) were grown from a mixed sodium-gallium melt at 900degreesC in a sealed niobium tube. It crystallizes in the triclinic space group P (1) over bar (Z = 1) with a = 6.421 (1), b = 9.190(2), c = 9.263(2) Angstrom and alpha = 64.939(5), beta = 82.961(6) and gamma = 89.692(6)degrees. The new, orthorhombic polymorph of Sr3MoN4 was synthesized as a polycrystalline powder from an intimate mixture of Sr2N and molybdenum metal in the presence of N-2. Rietveld analysis shows it to crystallize in Pbca (Z = 8) with a = 10.4835(2), b = 9.7374(2) and c = 11.5114(2) Angstrom. Studies suggest that excess nitrogen is essential to the formation of this polymorph. Sr-10[Mo2N6][MoN4](2) is the First molybdenum nitride to contain the anionic unit [Mo2N6](8-), which appears as isolated, edge-sharing pairs of MoN4 that facilitate Mo-V-Mo-V bonding. This compound also contains isolated [MoN4](6-) tetrahedra and the presence of these two units results in the first mixed valent molybdenum nitride. Resistivity measurements show it to be insulating.