DSC, X-Ray diffraction and Raman studies in the compound K-2.51(NH4)(1.49)Hg3Cl10 center dot 2H(2)O

The mixed compound potassium-ammonium mercurate exhibits two phase transiti ons at T-1=395 K and T-2=424 K: phase I - (395 K) --> phase II - (424 K) -- > phase III These transitions have been detected by differential scanning c alorimetry, X-ray diffraction and Raman scattering on polycrystalline sampl es of K-2.51(NH4)(1.49)Hg3Cl10.2H(2)O. The room temperature phase has space group Pmcm (a=4.511(4) Angstrom, b=14.489(9) Angstrom and c=16.074(12) Ang strom). Phase II is disordered and exhibits orthorhombic symmetry (a=4.500( 1) Angstrom, b=17.758(5) Angstrom and c=22.895(5) Angstrom). Hydrogen bondi ng, the nature and the degree of structure (dis)order and the mechanisms of the phase transitions are discussed. The NH; ions show an important orient ation disorder at the high temperatures. (C) 1999 Published by Elsevier Sci ence S.A. All rights reserved.