New families of mixed alkaline-earth nitridomolybdates and nitridotungstates, (Ba,Sr)₃[MN₄] (M = Mo, W)

Two new families of nitrides containing molybdenum(VI) and tungsten(VI) were synthesised by the reaction of barium and strontium nitride with either molybdenum or tungsten foils at high temperature in sealed stainless steel crucibles. The reactions yielded single crystalline products determined by X-ray diffraction to form orthorhombic structures in the space group Pbca (no. 61). The compounds are isostructural with the "low temperature" polymorph of Ba₃MoN₄ and form solid solutions within the limits investigated. The structures contain isolated [Mo(W)N₄]^6- tetrahedra and alkaline-earth cations distributed across three crystallographic sites dependent on cationic radius. The title compounds are the first examples of mixed strontium-barium transition metal nitrides.