Two solid solution series exist in the system MgMoO 4 -NiMoO 4 . The α-Ni 1-y Mg y MoO 4 solution series, isostructural to α-NiMoO 4 , is thermodynamically stable at ambient conditions for compositions between 0 % and about 75 % magnesium content. The solution series β-Mg 1-x Ni x MoO 4 , isostructural to MgMoO 4 and the high temperature β modification of NiMoO 4 , is thermodynamically stable at ambient conditions for compositions with Ͻ 25 % nickel content. A complete solid solution series β-Mg 1-x Ni x MoO 4 exists at higher temperatures (Ͼ 823 K). The transition temperature for the α Ǟ β transition decreases with increasing magnesium content. The coexist-