Refinement of the (NH 4 ) 3 H(SO 4 ) 2 crystal structure: II. X-ray and neutron single-crystal diffraction from phase II at room temperature

The crystal of (NH 4 ) 3 H(SO 4 ) 2 is of special interest due to the possible influence of ammonium ion on the series of phase transitions: I↔II↔ III↔IV↔V↔VII. Earlier, the x-ray single-crystal diffraction study of phase II of the (NH 4 ) 3 H(SO 4 ) 2 crystal showed that the crystal structure of this compound had two crystallographically independent groups of the ammonium ions NH 4 (1) and NH 4 (2) but orientational positions of these ammonium ions were not determined exactly. The refinement of the NH 4 (1) and NH 4 (2) orientational positions in phase II was carried out by the x-ray and neutron single-crystal diffraction study. The analysis of difference Fourier maps of electron charge density and nuclear density suggested the possibility of disordering of NH 4 (2) ammonium ions.