Formation of heterotypic substitutional solid solutions (NH4)(10-x)K-x[H2W12O42]center dot nH(2)O in the ammonium paratungstate 'Z' potassium paratungstate 'Z' system

The synthesis of four solid solutions (NH4)(10-x)K-x[H2W12O42] . nH(2)O (x/ n: 6.9/9.7, 5.9/9.5, 3.3/8.5, and 2.6/9.0) with the structure of triclinic K-10[H2W12O42] . 10 H2O (I) and two solid solutions (x/n: 3.1/4.0, and 2.6/ 4.0) with the structure of monoclinic (NH4)(10)[H2W12O42] . 4 H2O (II), was accomplished by a new method from ammoniacal monotungstate solution by add ing the appropriate amount of potassium hydroxide and the release of ammoni a during evaporative crystallization. The preparation of corresponding sing le crystals was achieved by slow evaporation of saturated solutions of the corresponding polycrystalline samples according to the method of isothermal evaporation. The study of coordination and space filling behavior of the potassium and a mmonium cations, crystal water molecules, and the paratungstate 'Z' anion r evealed that the predominance of the triclinic structure (2.6 less than or equal to x less than or equal to 10.0) in the system is effected by the bul kiness of NH4+. The transition area (2.6 less than or equal to x less than or equal to 3.1; 4 less than or equal to n less than or equal to 8.5) with coexisting triclinic and monoclinic mixed crystals represents the miscibili ty gap, typical for a heterotypic substitutional solid solution. The 'resis tance' of three specific K+ positions, to be substituted by NH4+, is caused by peculiarities of bond lengths, coordination numbers, and character of c oordinated neighbors.