PHASE TRANSITIONS IN THE MIXED COMPOUND NH₄(HSO₄)_0.45(H₂PO₄)_0.55

S. Bouattour, T. Mhiri, A.W. Kolsi and A. Daoud

Laboratoire de L'Etat solide, Faculté des sciences de Sfax-3038 TUNISIE

The present work intends to study the solid state phase diagram with the border compounds NH₄HSO₄ and NH₄H₂PO₄.

Colourless and transparent single crystals of NH₄(H₅O₄)_0.45(H₂PO₄)_0.55 were grown by slow evaporation at room temperature of stoichiometric NH₄HSO₄ and NH₄H₂PO₄ aqueous solution:

x NH₄HSO₄ + (1-x)NH₄H₂PO₄ ---- > NH₄(HSO₄)_x(H₂PO₄)_1-x

Measurements of the dielectric constants of NH₄(H₅O₄)_0.45(H₂PO₄)_0.55 revealed two phase transitions at T=117 and T=157K. These transitions were confirmed by Raman Scattering on polycrystalline salnples. The room temperature phase is disordered and exhibits orthorhombic symmetry (a= 7.477(2) A, b= 7.813(3) A, c= 7.340(2) A).