Crystal structure (at T = 163 K) and phase transitions of NH4HgI3·H2O

NH4HgI3·H2O crystallizes in the monoclinic system, space group Cc (No. 9), with: a = 10.9865(11) A; b = 9.7683(10) A; c = 8.6759(9) A; β = 90.395(8) deg.; V = 931.07(16) A3 and Z = 4 (T = 163 K). Refinement of single crystal X-ray data lead to R(F) = 0.0271. The structure consists of tetrahedra [HgI4] linked by common corners to infinite chains, running parallel to [0 0 1]. The ammonium groups and water molecules are located between the chains ensuring the stability of the structure by hydrogen bonding contacts: N-H...I, O-H...I and N-H...O. Differential scanning calorimetry reveals three anomalies at 219, 345 and 375 K. The peak at 219 K, according to the single crystal X-ray diffraction, was attributed to an orientational disorder of ammonium groups. IR-spectroscopy shows that the NH4HgI3·H2O compound loses crystallization water gradually in the temperature range (350-540 K)