Formation of Heterotypic Substitutional Solid Solutions (NH 4 ) 10–x K x [H 2 W 12 O 42 ] · n H 2 O in the Ammonium Paratungstate ‘Z'/Potassium Paratungstate ‘Z' System

The synthesis of four solid solutions (NH 4 ) 10–x K x [H 2 W 12 O 42 ] · n H 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 [H 2 W 12 O 42 ] · 10 H 2 O (I) and two solid solutions (x/n: 3.1/4.0, and 2.6/4.0) with the structure of monoclinic (NH 4 ) 10 [H 2 W 12 O 42 ] · 4 H 2 O (II), was accomplished by a new method from ammoniacal monotungstate solution by adding the appropriate amount of potassium hydroxide and the release of ammonia during evaporative crystallization. The preparation of corresponding single 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 ammonium cations, crystal water molecules, and the paratungstate ‘Z' anion revealed that the predominance of the triclinic structure (2.6 ≤ x ≤ 10.0) in the system is effected by the bulkiness of NH 4 + . The transition area (2.6 ≤ x ≤ 3.1; 4 ≤ n ≤ 8.5) with coexisting triclinic and monoclinic mixed crystals represents the miscibility gap, typical for a heterotypic substitutional solid solution. The ‘resistance' of three specific K + positions, to be substituted by NH 4 + , is caused by peculiarities of bond lengths, coordination numbers, and character of coordinated neighbors.