Synthesis and structure of new mixed alkaline-earth nitridomolybdates and nitridotungstates, (Ba,Ca)3[MN4](M = Mo, W)

New molybdenum(VI) and tungsten(VI) nitrides were synthesised by the reaction of barium nitride and calcium 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 (LT) form of Ba3Mo (W)N4. The structures contain isolated [Mo(W)N4]6– tetrahedra and partially disordered alkaline earth cations whose distribution across three crystallographic sites is dependent on cationic radius.