Synthesis and structure of two forms of ammonium monomolybdate (NH4)2MoO4

Ammonium monomolybdate (NH4)(2)MoO4 exists in two different polymorphic forms which differ in their lattice constants and in the arrangement of the ammonium cations relative to the molybdate anions. The ammonium molybdates (NH4)(2)MoO4(mS60)(1)) and (NH4)(2)MoO4(mP60)(2)) are synthesized by the reaction of ammonia and (NH4)(6)[MO7O24]. 4H(2)O (NH4)(2)MoO4(mS60) crystallizes isostructural to the potassium compound in space group C2/m (Nr. 12) and lattice constants a=1263.6(3), b= 652.2(1) pm, c = 776.4(2) pm and beta= 117.36(1)degrees (V = 568.3(2).10(6) pm(3)) containig four formula units per unit cell (R = 0.0250). (NH4)(2)MoO4(mP60) crystallizes monoclinic in space group P2(1)/n (Nr. 14) and lattice constants a = 622.8(2), b=777.0(1) pm, c=1118.8(4)pm and beta=98.09(2)degrees (V= 536.0(3).10(6) pm(3)) (R = 0.0205). The different arrangements of the polyhedra within the unit cell is caused by hydrogen bridges. A transition point was not yet determined.