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Proton ordering in (NH 4 ) 3 H(SO 4 ) 2 at low‐temperature phase transitions
Author(s) -
Sohn Yoo Jung,
Sparta Karine M.,
Prinz Sebastian,
Meven Martin,
Roth Georg,
Heger Gernot
Publication year - 2013
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
eISSN - 2052-5206
pISSN - 2052-5192
DOI - 10.1107/s2052519213013092
Subject(s) - monoclinic crystal system , triclinic crystal system , crystallography , neutron diffraction , superstructure , crystal structure , phase transition , chemistry , diffraction , single crystal , space group , x ray crystallography , phase (matter) , proton , crystal (programming language) , materials science , condensed matter physics , physics , thermodynamics , optics , programming language , computer science , organic chemistry , quantum mechanics
Single‐crystal neutron diffraction was used to investigate the H‐atom disorder in triammonium hydrogen disulfate (TAHS), (NH 4 ) 3 H(SO 4 ) 2 , below room temperature. Crystal structure analysis of the monoclinic phase III shows an increase of proton ordering with decreasing temperature in the (SO 4 )H(SO 4 ) dimer. Moreover, the NH 4 + groups on a general position begin ordering in this phase. The monoclinic unit cell of TAHS‐IV doubles in the b direction and a slight distortion of SO 4 2− and NH 4 + tetrahedra is observed. The order parameter introduced by Landau was determined for the second‐order II/III and III/IV phase transitions from the intensities of the superstructure reflections. TAHS‐V has a triclinic space group and the crystal structure seems to be completely ordered according to a structure analysis by single‐crystal X‐ray diffraction measurements. In addition, the decisive role of the dynamical disorder of different ammonium groups on successive phase transitions is discussed. Additional peaks were observed by X‐ray powder diffraction measurements at ∼ 70 K on cooling, which refers to the V/VII phase transition. These additional peaks remained up to ∼ 85 K on heating. They were described with a doubling of the unit cell along all three principal crystallographic directions.