The disordered high-temperature structure of (NH4)3H(SO4)2 and its relationship to the room-temperature phase

The structure of letovicite, (NH4)(3)H(SO4)(2) has been redetermined at 293 and 420K. At room temperature the structure crystallizes monoclinic, space group C12/c1 (no. 15), with lattice parameters a = 15.418(5) Angstrom, b = 5.905(5) Angstrom, c = 10.223(5) Angstrom and beta = 102.806(5)degrees. The structure is composed of two symmetrically independent isolated NH4+ tetrahedra and [SO4-H-SO4](3-) dimers with a symmetrical hydrogen bond connecting the two sulfate tetrahedra. At 420 K the structure is rhombohedral, space group R (3) over barm (no. 166) with lattice parameters a = 5.9039(5) and c = 22.5360(6) Angstrom. While the low-temperature phase is completely ordered, this does not hold for the high-temperature phase: the hydrogen and oxygen atoms which constitute the hydrogen bond are disordered with the consequence that the hydrogen bond is now strongly asymmetric. Furthermore, the hydrogen atoms in one of the two symmetrically independent NH4+ tetrahedra are disordered and have probably some share in the high protonic conductivity which has so far been attributed mainly to the hydrogen atom of the hydrogen bond. (C) 2002 Elsevier Science (USA).