Sr₁₀[Mo₂N₆][MoN₄]₂ and β-Sr₃MoN₄

Two new phases in the strontium-molybdenum-nitrogen phase diagram have been synthesized. Single crystals of Sr₁₀[Mo₂N ₆][MoN₄]₂ were grown from a mixed sodium-gallium melt at 900°C in a sealed niobium tube. It crystallizes in the triclinic space group P1 (Z=1) with a = 6.421(1), b = 9.190(2), c = 9.263(2) Å and α = 64.939(5), β = 82.961(6) and γ = 89.692(6)°. The new, orthorhombic polymorph of Sr₃MoN₄ was synthesized as a polycrystalline powder from an intimate mixture of Sr ₂N and molybdenum metal in the presence of N₂. Rietveld analysis shows it to crystallize in Pbca (Z=8) with a = 10.4835(2), b = 9.7374(2) and c = 11.5114(2) Å. Studies suggest that excess nitrogen is essential to the formation of this polymorph. Sr₁₀[Mo ₂N₆][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.