PHASE TRANSITIONS IN THE MIXED COMPOUND: K_2.54(NH₄)_1.46Hg₃C₁₀.2H₂0

A. Kabadou, R. Ben Hassen and A. Ben Salah

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

The present work intends to study the effect of partial substitution of NH₄⁺ by K⁺ in NH₄HgCl₃.

Single crystals of K_2.54(NH₄)_1.46Hg₃Cl₁₀·2H₂O were grown by slow evaporation from a mixture of water and acetone containing stoichiometric KCl- NH₄Cl-HgCl₂ at room temperature in the ratio 1 / 1 /2.

The formula is determined by refinement of the crystal structure and confirmed by chemical analysis.

Potassium-ammonium mercurate exhibits two phase transitions at T₁= 395K and T₂=424K:

phase I - (395K) -> phase II - (424K) -> phase III

These transitions have been detected by differential scanning calorimetry, X-ray diffraction, Raman Scattering on polycrystalline samples of K_2.54(NH₄)_1.46Hg₃Cl₁₀·2H₂O. The room temperature phase is ordered and exhibits orthorhombic symmetry { space group Pmcm with a= 4. 511(1 )Ä, b=14.489(2)Ä and c=16.074(4)Ä ). By heating across the I -> II transition, the cell parameters become a= 4.500(2), b=17.758(5) and c= 22.894(5)Ä.

Hydrogen bonding, the nature and the degree of structure (dis)order and the mechanisms of the phase transitions are discussed. The NH₄⁺ ions show an important orientation disorder at the high temperatures.